Vocab v62

Phenylketonuria (PKU)

A disorder caused by deficiency of an enzyme needed to convert the essential amino acid phenylalanine to the normally nonessential amino acid tyrosine; as a result, tyrosine cannot be made and becomes conditionally essential.

medium chain fatty acids

A fatty acid with 8 to 12 carbon atoms.

short chain fatty acids

A fatty acid with fewer than eight carbon atoms.

long chain fatty acids

A fatty acid with more than 12 carbon atoms.

ghrelin

A hunger-arousing hormone secreted by an empty stomach "hunger hormone"

micelle

A small droplet of fat formed, via emulsification, in the small intestine.

Solvent, solute, solution

A solvent is a substance that can dissolve other substances (solutes) to form a solution.

homeostasis

A state of balance or equilibrium. A tendency to maintain a balanced or constant internal state; the regulation of any aspect of body chemistry, such as blood glucose, around a particular level

sterols

A sterol is different from other lipids in that it consists of a distinct multi-ring structure. There are many types of sterols, but the most abundant and widely discussed is cholesterol (see Figure 6.11). You may have heard of the potentially unhealthy relationship between elevated blood cholesterol and heart disease. However, you may not be as familiar with the many important functions that cholesterol serves in your body. For instance, cholesterol is used to synthesize bile acids. A bile acid, which plays a critical role in digestion and absorption of lipids, consists of a cholesterol molecule attached to a very hydrophilic subunit, making a bile acid molecule amphipathic (much like a phospholipid). This structure allows bile acids to help break up dietary lipids in the intestine, preparing them for chemical digestion and subsequent absorption. Cholesterol is also a component of cell membranes, where it helps maintain flexibility. In addition to these functions, cholesterol is integral to the synthesis of many of the reproductive hormones (such as testosterone and estrogen), vitamin D energy metabolism, calcium homeostasis, and electrolyte (salt) balance. Almost every tissue in your body can make cholesterol—this is especially true for liver tissue. As such, cholesterol is not an essential nutrient. Many dietary factors influence how much cholesterol a person synthesizes, however. For example, eating a low-calorie or low-carbohydrate diet decreases cholesterol synthesis in some people.8 This is not the case for everyone, however, because carbohydrate intake may only affect cholesterol synthesis in those with certain genetic make-ups. Cholesterol production can also be lowered by certain medications (commonly referred to as statins), such as those taken by people at elevated risk for heart disease. In addition to the cholesterol made by cells, the body obtains cholesterol from animal-derived foods such as shellfish, meat, butter, eggs, and liver (see Figure 6.12). Plants do not produce substantial amounts of cholesterol. So, exclusively plant-based diets are relatively very low in this substance. Because cholesterol is synthesized by the body, vegans who do not eat animal-based products are not at risk of cholesterol deficiency. Although plants make very small amounts of cholesterol, some contain relatively large amounts of sterol-like compounds called phytosterols. An interesting group of phytosterols occurs naturally in corn, wheat, rye, and other plants. Similar sterols are produced commercially and marketed under various brand names, such as Benecol®. These products are often found in butter substitutes, yogurt drinks, salad dressings, and even dietary supplements. Some studies suggest that consuming roughly 500 to 2,000 milligrams of phytosterols a day may decrease blood cholesterol, lowering one's risk for cardiovascular disease.10 Because a typical serving of a plant sterol-fortified table spread contains about 1,000 milligrams of the sterol, you would need to consume one or two servings daily to reach this goal. The mechanisms by which plant sterols decrease blood cholesterol are not well understood. However, most plant sterols are not easily absorbed and appear to bind cholesterol in the intestine. This may increase cholesterol elimination in the feces. Similarly, some plant sterols can cause loss of fat in the feces, leading to diarrhea and possibly decreased absorption of fat-soluble vitamins.

lingual lipase

An enzyme, produced by the salivary glands, that cleaves fatty acids from glycerol molecules.

Protein synthesis

As a result, protein synthesis within a particular cell is neither consistent nor random; it is tightly regulated by the amounts and types of proteins needed by the cell and the entire body at a given time. For example, synthesis of the proteins needed for calcium absorption in your small intestine is turned on or off depending on your body's need for calcium, ensuring that calcium availability is maintained at optimal levels. Because protein synthesis is not an ongoing process, a cell must be notified when to make a particular protein. This communicative process, cell signaling, conveys physiological conditions or cellular needs to the nucleus of the cell. Cell signaling initiates the second step of protein synthesis, transcription, whereby a specific type of ribonucleic acid (RNA), called messenger RNA (mRNA), is constructed using DNA as a template. One way to think about this process is that it is like reading a cookbook. A chemical called deoxyribonucleic acid (%NA) provides the fundamental instructions for protein synthesis. Found in a cell's nucleus, coiled strands of DNA combine with special proteins to form chromosomes, which serve as the complete, organized cookbook. There are 23 different chromosomes in human cells. However, in most cells each chromosome has a matched pair. In total, there are 46 chromosomes in each cell except for egg and sperm cells. Each chromosome is subdivided into thousands of units, each of which is like an individual recipe. Each recipe, or gene, provides the instructions and list of ingredients needed to make a protein. In other words, a gene (the recipe) tells a cell (the cook) which amino acids are needed and in what order they must be arranged to synthesize a protein (the food). Transcription: In order for protein synthesis to take place, the information contained in the DNA's genetic code must be communicated from inside the nucleus to the cytoplasm of the cell where proteins are made. To accomplish this process, the instructions contained in the targeted portion of the DNA strand are converted to a chemical called messenger ribonucleic acid (mRNA). A series of mRNA subunits bind to the targeted gene coding for the protein that needs to be synthesized. These mRNA subunits then join, forming a strand of mRNA that is essentially a mirror image of the part of the DNA molecule that carries the instructions for protein synthesis. The newly formed strand of mRNA separates from the DNA, exits the nucleus, and enters the cytoplasm where it participates in the next step of protein synthesis: translation. Once the mRNA strand is outside the nucleus, the strand binds to a cytoplasmic organelle called a ribosome, on which the third step of protein synthesis occurs. Translation, the process whereby amino acids are joined via peptide bonds, requires another form of RNA called transfer ribonucleic acid (tRNA). tRNA units carry amino acids to the ribosome to be assembled into a peptide chain. For translation to proceed, the ribosome moves along the mRNA strand, reading its sequence. The sequence of the mRNA, in turn, instructs specific tRNAs to transfer the amino acids they are carrying to the ribosome. One by one, amino acids join together via peptide bonds to form a growing peptide chain. When translation is complete, the newly formed protein separates from the ribosome. This is not the final step in the formation of a new protein, however; the final structure and shape of the protein are yet to be determined.

Alternative sweeteners

For individuals who want to reduce their caloric intake, choosing a specific artificial sweetener may not be as simple as you might think. Some artificial sweeteners lose their sweetness when heated and therefore cannot be used for baking or cooking. Others are not chemically stable and become bitter over time. In order to use alternative sweeteners safely and effectively, it is important to understand some of their properties. Saccharin: There was once concern that saccharin might cause cancer, but recent studies support its safety for human consumption. Saccharin is extremely sweet, very stable, and inexpensive to produce. Commercial products with saccharin include Sweet 'N Low® and Sugar Twin®. Aspartame: Although it has the same energy content as sucrose (4 kcal per gram), aspartame is almost 200 times sweeter. The food industry uses aspartame in many sugar-free beverages, but because it is not heat stable, aspartame cannot be used in products that require cooking. Although the U.S. Food and Drug Administration (FDA) has ruled aspartame safe for most people, some individuals claim that it causes adverse effects such as headaches, dizziness, nausea, and seizures. While several studies have reported that aspartame increases the occurrence of cancer in laboratory animals, hundreds of studies have failed to find similar effects in humans. People with the genetic disorder phenylketonuria (PKU) should not consume products sweetened with aspartame because they are not able to metabolize it properly. Commercial products with aspartame include NutraSweet® and Equal® Acesulfame K: Used extensively throughout Europe, acesulfame K was approved in 1998 for use in the United States, where it is sold under the trade name Sweet One®. Unlike aspartame, this artificial sweetener is heat stable and is used in a wide variety of prebaked commercial products. Sucralose: Even though sucralose is roughly 600 times sweeter than sucrose, it provides minimal calories because it is difficult for the body to digest and absorb. Since it is water soluble and stable, sucralose is used in a broad range of foods and beverages. Commercial products with sucralose include Splenda® and Altern® Sugar alcohols: Sugar alcohols occur naturally in plants (particularly fruits) and have half the sweetness and calories of sucrose (roughly 2.5 kcal per gram). Sorbitol, mannitol, and xylitol (the most common sugar alcohols) are often found in sugar-free products such as chewing gums, breath mints, candies, toothpastes, mouthwashes, and cough syrups. One advantage of sugar alcohols is that, unlike sucrose, they do not promote tooth decay. When consumed in excessive amounts however, sugar alcohols can cause diarrhea. Stevia: The term stevia is used to describe a group of related plants that grow in some regions of Central and South America, as well as in the southwestern region of the United States. The leaves of stevia plants are particularly sweet, a fact long recognized by the native peoples of these semi-arid regions. Stevia is essentially free of calories and is considerably sweeter than table sugar. Commercial products with stevia include Truvia® and NuStevia™. Agave: The Agave plant grows in warm, dry regions of the United States as well as in parts of South America. It is the same plant that is used to make tequila. Although there may be some health benefits associated with Agave in its natural, native state, these benefits are lost after the sugars are extracted to produce a thick, sweet syrup. In fact, you may be surprised to learn that agave has more calories per tablespoon than table sugar (60 vs. 48 kcal, respectively). Because Agave is derived from plants, some people believe it is a "natural" healthier alternative compared to other sweeteners. However, there is not much research to support this claim, and like all other sweeteners, it is best to limit your intake.

condensation reaction

In contrast to hydrolytic reactions, a chemical reaction that results in the formation of a water molecule is called a condensation reaction. A chemical reaction whereby a chemical bond joins two molecules together, releasing water in the process. The typical adult produces about approximately 1 cup (or 200-300 mL) of water every day from condensation and other metabolic reactions.

what are lipids soluble in

Lipids are all insoluble in polar solvents like water but highly soluble in the non-polar or weakly polar organic solvents, including ether, chloroform, benzene, and acetone. In fact, these four solvents are often referred to as "lipid-solvents" or "fat-solvents". Because they are nonpolar and water is polar, lipids are not soluble in water. That means the lipid molecules and water molecules do not bond or share electrons in any way. The lipids just float in the water without blending into it.

where does the body make amino acids

Nonessential amino acids are mainly synthesized from glucose (alanine, arginine [from the urea cycle in hepatic cells], asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, and serine), except for tyrosine, which is synthesized from phenylalanine.

Protein function in body

Once amino acids are circulated away from the GI tract, your body uses them to make the thousands of proteins it needs via protein synthesis. Using the previous analogy, this is the stage at which you would use all of the disassembled materials from someone else's house to build one of your own. The deconstructed materials could be used to build walls, cabinets, stairways, or furniture—each of these household items has its own function. Similarly, the proteins that your body makes can be classified into general categories based on their functions (see Table 5.2). Some proteins, such as those in your muscles, are used for movement. Others, such as the hormone insulin, are used to regulate blood glucose. Some proteins can be broken down and used for energy, and some amino acids can be converted to glucose. In the following sections, you will learn more about the various types of proteins and amino acids your body needs, as well as other ways that they are used to promote functionality and health. Proteins Provide Structure Being constituents of muscles, skin, bones, hair, and fingernails, proteins comprise most of the structural materials in your body. Collagen, for instance, is a structural protein that forms a supporting matrix in bones, teeth, ligaments, and tendons. Proteins are also important structural components of cell membranes and organelles. The synthesis of structural proteins is especially important during periods of active growth and development, such as infancy and adolescence. Enzymes Catalyze Chemical Reactions The myriad chemical reactions in your body are driven by a class of protein molecules called enzymes. These biological catalysts speed up chemical reactions without being consumed or altered in the process. Without the catalytic functions of enzymes, the thousands of chemical reactions needed by your body to function would simply not occur or, at best, would occur at very slow rates. Examples of enzymes you have already learned about are amylase and pepsin, which catalyze reactions needed to digest carbohydrates and proteins, respectively. Muscle Proteins Facilitate Movement Protein is also necessary for movement, which results from the contraction and relaxation of the many muscle fibers in the body. Muscles are involved in both voluntary and involuntary movements such as those needed for cardiovascular function and physical activity, respectively. Nearly half of the body's protein is present in skeletal muscle, and adequate protein intake is required to form and maintain muscle mass and function throughout life. Although there are many proteins related to movement, perhaps the most important are actin and myosin, which make up much of the machinery needed for muscles to contract and relax. This is why protein deficiency can cause muscle wasting and weakness. Some Proteins Serve as Transporters Amino acids are also used to make transport proteins, which are responsible for escorting substances into and around the body, as well as across cell membranes. For example, absorption of many nutrients (such as calcium) requires one or more transport proteins to help the nutrients cross the lumen-facing portion of membranes surrounding enterocytes. Protein deficiency can decrease the body's production of intestinal transport proteins, resulting in secondary malnutrition. In addition to facilitating the transport of substances across cell membranes, many proteins are critical for the transport of nutrients and other substances in the blood. Examples of circulating transport proteins include hemoglobin, which transports gases (oxygen and carbon dioxide), and a variety of binding proteins that transport hormones and fat-soluble vitamins in the blood. Hormones and Cell-Signaling Proteins Are Critical Communicators Tissues and organs have a variety of ways to communicate with each other, and most of these methods involve proteins. Although not all hormones are proteins, most are, including secretin, gastrin, insulin, and glucagon are proteins. Beyond hormonal communicators, there are also specialized proteins embedded in cell membranes that communicate information about the extracellular environment to the intracellular space. Some of these proteins are involved in the cell-signaling process that initiates protein synthesis itself. Others regulate cellular metabolism. Together, hormones and cell-signaling proteins make up part of the body's critical communication network. Thus, protein deficiency can have profound effects on your body's ability to coordinate all of its functions. Proteins Protect the Body One of the most vital and basic functions of the proteins in your body is protecting it from physical danger and infection. For instance, skin is mainly made of proteins that form a barrier between the outside world and your internal environment. If your skin gets cut, blood clots, which are produced by a series of clotting proteins, close off the possible entry point to infection. And, if a bacterium or other foreign substance does enter the body, your immune system responds by producing antibody (or immunoglobulin) proteins to help fight the infection. Antibodies bind to foreign substances so they can be destroyed. Protein deficiency can make it difficult for the body to prevent and fight certain diseases because its natural defense systems become weakened. This is why infection and illness often accompany protein deficiency. Fluid Balance Is Regulated in Part by Proteins Another function of proteins is regulating how fluids are distributed in the body. As you might know, most of your body is made of water. This important fluid is found both inside and outside of cells. The fluid outside of cells can be subdivided into that found in blood and lymph vessels (intravascular fluid) and that found between cells (extravascular fluid). The amount of fluid in these spaces is tightly regulated by a variety of means, some of which involve proteins. Albumin, a protein present in relatively high quantities in the blood plays such a role. As the heart beats, blood is pumped out of the heart and into blood vessels that become increasingly narrower. As the pressure builds, the fluid portion of the blood is squeezed out of the tiny capillaries. Albumin, which remains in the blood vessels, becomes more concentrated as more fluid is lost. The high concentration of albumin draws the fluid that was once squeezed out of the blood vessel back into it (see Figure 5.7). Severe protein deficiency can impair albumin synthesis, resulting in low levels of albumin in the blood and causing fluid to accumulate in the space surrounding tissues. This condition, called edema, can sometimes be observed as swelling in the hands, feet, and abdominal cavity. Although edema is commonly seen in severely malnourished individuals, it can be caused by other factors as well, such as congestive heart failure. Proteins Help Regulate pH Proteins are involved in regulating the acidity or alkalinity of your body fluids, referred to as the fluids' pH. Your body must maintain certain pH levels in each of its fluids to ensure optimal health. One way that blood pH is maintained is through the action of certain proteins such as hemoglobin, which act to increase and decrease the blood's pH as needed. As such, the body can have difficulty maintaining optimal pH balance during periods of severe protein deficiency.

triglyceride digestion

Once ingested, lipids must be digested, absorbed, and circulated away from the small intestine. The basic goal of triglyceride digestion is to separate (or cleave) most of the fatty acids from their glycerol backbones—in other words, lipolysis. This process, which involves several enzymes and other secretions produced by the gastrointestinal tract and accessory organs, is relatively more complicated than the digestion of the other macronutrients (carbohydrates and proteins). This is because it is more difficult for your body, which is made principally of water-soluble compounds, to digest water-insoluble molecules such as lipids than water-soluble molecules such as sugars and proteins (see Figure 6.13). Still, as you will soon understand, the digestion of dietary lipids occurs in your mouth, stomach, and small intestine every time you eat a meal containing fats or oils. A small portion of triglyceride digestion occurs in your mouth. As chewing breaks food apart, lingual lipase, an enzyme produced by your salivary glands, begins to remove fatty acids from the glycerol molecules. After the food is swallowed, lingual lipase accompanies the bolus into your stomach, where the enzyme continues to cleave additional fatty acids from glycerol. The second stage of triglyceride digestion begins when food enters your stomach, stimulating the release of gastrin from specialized stomach cells. Recall from Chapter 3 that gastrin is a hormone that stimulates the release of gastric juices in the stomach. Gastrin also causes the muscular wall of the stomach to vigorously contract. Circulating in the blood, gastrin quickly stimulates the release of gastric lipase, an enzyme produced in the stomach. Gastric lipase, a component of the gastric juices, picks up where lingual lipase left off by further cleaving fatty acids from the glycerol molecules. Although some fatty acids are cleaved from the glycerol backbones in the mouth and stomach by lingual and gastric lipases, respectively, triglyceride digestion is not complete until the chyme interacts with bile and the enzyme pancreatic lipase in your small intestine. The mixing of chyme with bile, an emulsifier, is an important step in lipid digestion because the watery environment of the gastrointestinal tract can cause lipids to clump together in large lipid globules that are difficult to digest. To overcome this problem, the final stage of triglyceride digestion occurs in two complementary and consecutive phases in the small intestine. In the first phase of intestinal triglyceride digestion, bile disperses large lipid globules into smaller lipid droplets, making the lipids more accessible to the intestine's digestive enzymes (see Figure 6.14). In response to the hormone cholecystokin (CCK) secreted by enterocytes lining the small intestine in the presence of chyme, the gallbladder contracts and releases bile into the duodenum. Recall from Chapter 3 that bile is made by the liver and stored in the gallbladder until needed. Bile is comprised of a mixture of bile acids, cholesterol, and phospholipids. Both bile acids and phospholipids are amphipathic. Because the hydrophilic and hydrophobic components of bile are attracted to water and lipids, respectively, the large lipid globules are pulled apart into smaller droplets when they mix with the bile and phospholipids. Bile then surrounds each newly formed droplet, or micelle, stabilizing it in the intestine. The process whereby large lipid globules are broken down and stabilized into micelles is called emulsification. When a person's bile contains an excess amount of cholesterol in relation to its other components, gallbladder disease may develop. Recall from Chapter 3 that the accumulation of cholesterol, calcium, and cellular debris in bile can lead to the formation of gallstones. Surgical removal of the gallbladder is the most common treatment for persistent gallstone-related problems. Because some people may have difficulty emulsifying—and therefore digesting—fat after the gallbladder is removed, doctors often recommend initially avoiding high-fat meals after undergoing gallbladder surgery. Emulsification by itself does not complete lipid digestion; fatty acids remaining attached to glycerol molecules still need to be chemically cleaved. To accomplish this, lipid-containing chyme stimulates cells in the small intestine to release the hormone secretin, which in turn stimulates the pancreas to release pancreatic juice containing the enzyme pancreatic lipase. Recall from Chapter 5 that secretin, a hormone made by enterocytes lining the small intestine, also signals the release of sodium bicarbonate and proteases from the pancreas. Pancreatic lipase completes triglyceride digestion by cleaving the remaining fatty acids from their glycerol molecules. In general, two of the three fatty acids are removed from the triglyceride molecules, producing a monoglyceride and two free (unbound) fatty acids. The final products of lipid digestion (fatty acids, glycerol, and monoglycerides) are then taken up into the intestinal cells and circulated to the rest of the body. This process requires special handling because many of these substances are hydrophobic, while both the interiors of the intestinal cells and the circulatory system are hydrophilic. Once again, it is the amphipathic property of phospholipids that makes lipid absorption possible. Lipid absorption is accomplished in one of two ways, depending on how hydrophilic the lipid is (see Figure 6.15). Because they are relatively water soluble (hydrophilic), short- and medium-chain fatty acids can be transported into intestinal cells unassisted. More hydrophobic compounds (such as long-chain fatty acids, monoglycerides, and cholesterol) must first be repackaged into micelles within the intestinal lumen before they can move into the intestinal cells. Once a lipid-containing micelle comes into contact with the lumenal surface of an intestinal cell, the micelle's contents are released into the interior of the enterocyte. Because short- and medium-chain fatty acids are relatively water soluble, they can be put directly into the blood. Attached to a protein called albumin, these fatty acids are circulated directly from the small intestine to the liver. Once in the liver, short and medium-chain fatty acids can be metabolized or rerouted for delivery to other cells in the body. Compared to what happens with the short- and medium-chain fatty acids, circulation of larger, less hydrophilic lipids away from the gastrointestinal tract is more involved. First, longchain fatty acids and monoglycerides must be reassembled into triglycerides inside the intestinal cell. These hydrophobic lipids, along with cholesterol and phospholipids, are then incorporated into chylomicron particles, which are released into the lymphatic system for initial circulation. Chylomicra (the plural form of the word chylomicron) package their hydrophobic lipids, such as triglycerides, within a hydrophilic exterior shell formed mainly from phospholipids and proteins (see Figure 6.16). The lymphatic system eventually delivers the chylomicra into the blood, where they travel to cells that take up their contents. The chylomicron is an example of a lipoprotein, a type of particle that your body makes to transport lipids. In this particular case, chylomicra circulate dietary lipids (exogenous lipids), whereas other lipoproteins circulate lipids made by the liver (endogenous lipids). You will learn more about other lipoproteins in the following section. The enzyme lipoprotein lipase enables chylomicra to deliver their dietary fatty acids to cells. Lipoprotein lipase is produced in many tissues (especially adipose and muscle tissues) and resides within the lumens of the blood vessels surrounding the tissues that produce this specialized enzyme. As chylomicra circulate in the blood, they are attacked by lipoprotein lipase, releasing fatty acids that are then taken up by the surrounding cells. After delivering dietary fatty acids to various tissues throughout the body, the residual fragments of the chylomicra remain in the blood. These chylomicron remnant fragments are taken up by the liver where they are broken down and their contents reused or recycled.

Protein digestion

Protein digestion requires the splitting of peptide bonds that hold the amino acids together. Once this occurs, the released amino acids are absorbed and circulated in the blood to all the body's cells, where the amino acids are used for protein synthesis. This is somewhat like disassembling someone else's house and then using the materials to build another house that perfectly fits your own personal needs. The body also efficiently and systematically breaks down and recycles its own proteins when they become old and nonfunctional. In fact, you can think of your body as having its own protein-recycling center. The stages of protein digestion, absorption, and circulation are shown in Figure 5.6 and described next. Before you can use the proteins in the foods you eat, they must be broken down into their component amino acids. Although a small amount of mechanical digestion of protein occurs as you chew your food, chemical digestion of protein does not begin until the food comes in contact with specialized cells in the stomach. The presence of food causes these stomach cells to release the hormone gastrin, which, in turn, triggers other cells in the stomach lining to release hydrochloric acid (HCl), mucus, and pepsinogen. Pepsinogen is an inactive form of pepsin, an enzyme needed for protein digestion. In general, an inactive precursor of an enzyme is called a proenzyme (sometimes called a zymogen). Once the components of the gastric juices are released, the chemical digestion of proteins can begin via the two-step process described next. First, hydrochloric acid disrupts the weak chemical bonds responsible for the protein's secondary, tertiary, and quaternary structures. This process of denaturation straightens out the complex protein structure, helping to expose the peptide bonds to the digestive enzymes present in the stomach and small intestine. It is important to note that the peptide bonds present in the primary structure of the protein are still intact. Second, hydrochloric acid converts the proenzyme pepsinogen into its active form, pepsin. Pepsin is an example of a protease, an enzyme that breaks peptide bonds between amino acids. Note that the stomach does not produce the active protease enzyme pepsin. Instead, it produces and stores the inactive (or safe) proenzyme pepsinogen. This protects the stomach from the active enzyme's protein-digesting function until it is needed. As a result of the actions of stomach acid and enzymes, proteins are partially digested to shorter polypeptide chains and some free amino acids. The partially broken-down proteins are now ready to leave the stomach and enter the small intestine to be digested further. Protein digestion in the small intestine takes place both in the lumen and within the cells that line it. Initiating this cascading series of digestive events, the arrival of amino acids and smaller polypeptides in the small intestine stimulates the release of the hormones secretin and cholecystokinin (CCK) from the intestinal cells. These hormones then enter the blood, where they travel to the pancreas. Secretin signals the pancreas to release bicarbonate (the same substance that makes up baking soda) into the lumen of the small intestine. Bicarbonate neutralizes the acid from the stomach and inactivates pepsin. Secretin and CCK also signal the release of proenzymes from the pancreas. Upon entering the small intestine, each of these proenzymes is activated and is then able to break peptide bonds holding specific sequences of amino acids together. The resulting di- and tripeptides are further broken down by a multitude of proteases produced in the cells that make up the absorptive surface of the small intestine. This usually results in the complete breakdown of proteins into their amino acid constituents. When protein digestion is complete, amino acids can enter the cells (enterocytes) of the small intestine. Most amino acids are absorbed in the duodenum, where they enter the blood and circulate to the liver for further processing. The breakdown of proteins into amino acids is a relatively complete process—it typically results in the absorption and circulation of amino acids (not proteins). Sometimes, however, partially digested proteins are absorbed or enter the circulation by squeezing through the gap junctions between enterocytes. When this happens, the body's immune system might respond as if these partial-breakdown products were dangerous. In such cases, the person is said to have an allergic response, or what is more commonly called a food allergy. 10 The majority of food allergies are caused by proteins present in eggs, milk, peanuts, soy, and wheat. Researchers estimate that approximately 2 percent of adults and 5 percent of infants and young children in the United States have food allergies.11 Note, however, that all adverse reactions to foods are not true food allergies. A negative physiological response to a substance in a food that does not trigger an immune response is called a food intolerance (or food sensitivity). An example of a food intolerance is lactose intolerance, which was discussed in Chapter 4.

ergogenic

intended to enhance physical performance, stamina, or recovery

Racemic mixture

is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule.

Hypokalemia

low potassium in the blood

hyperchromic

more than normal color; vivid; highly colorful

adipokines

protein hormones made and released by adipose tissue (fat) cells Signaling proteins secreted by adipocytes that cause a cellular response in another organ.

beriberi

the thiamin-deficiency disease; characterized by loss of sensation in the hands and feet, muscular weakness, advancing paralysis, and abnormal heart action In the United States, cerebral beriberi is typically associated with alcoholism both because alcohol decreases thiamin absorption and because some alcoholics have very poor diets.

trace mineral

vital to health, a mineral required in the diet in amounts less than 100 milligrams per day only need to consume small amounts each day; iron, copper, iodine, selenium, chromium, zinc.

Large intestine

The last section of the digestive system, where water is absorbed from food and the remaining material is eliminated from the body by forming feces. Before discussing nutrient absorption, it is important to understand the events that take place in the last region of the GI tract—the large intestine. Not all foods are completely digested, which means that undigested food residue (mostly the remains of plant-based foods) exit the small intestine and enter the large intestine. The large intestine is shaped like an inverted letter U( ) ù and is approximately 5 feet long. The first portion of the large intestine, the colon, receives approximately 2 liters of undigested material from the small intestine every day. A sphincter separating the small from the large intestine serves two main functions (1) to prevent the contents of the large intestine from flowing backward into the small intestine and (2) to prevent the premature flow of undigested food from the small intestine into the colon. Following the colon is the rectum, the segment of the large intestine that leads to the anal canal, which opens to the outside of the body (see Figure 3.15). Materials entering the large intestine consist mostly of undigested remains from plant-based foods, water, bile, and electrolytes. Muscles embedded within the wall of the colon squeeze the undigested food residue, slowly propelling the material forward. As material moves through the various regions of the colon, massive amounts of water and electrolytes are absorbed and returned to the blood for reuse by the body. This exemplifies the body's ability to reclaim its important resources. The slow, propulsive movement of the large intestine also provides an ideal environment for bacteria to grow and flourish. Although bacteria reside throughout the entire GI tract, their density is greatest in the colon. The number and variety of bacteria residing in the large intestine is astronomical— more than 400 different species call your large intestine home. This natural microbial population, also referred to as the (* microbiota, helps maintain a healthy environment throughout the GI tract. Some bacteria residing in your colon break down undigested food residue; others produce nutrients such as vitamin K, certain B vitamins, and some lipids. A healthy and diverse GI microbiota can help inhibit the growth of disease-causing bacteria. There is also evidence that the microbial population residing in your colon may reduce risk to certain diseases such as colon cancer. To help establish and maintain a healthy intestinal microbiota, it is important to consume both probiotic and prebiotic foods. A probiotic food is one that contains live microorganisms that are beneficial in the body. Fermented foods such as yogurt, sauerkraut, sour pickles, and certain soy products are examples of probiotic foods. Some dietary supplements also supply probiotic bacteria. A prebiotic food is typically fiber-rich and may stimulate the proliferation of the microbial population in the large intestine by providing a source of nourishment. Some health care professionals believe that consuming both probiotic and prebiotic foods provides a powerful defense against disease-causing bacteria. Once the remaining material completes its journey through the large intestine, it is ready to be eliminated from the body. This solid waste, now called feces, consists mainly of undigested and unabsorbed matter, dead cells, secretions from the GI tract, water, and bacteria. As the feces approaches the end of the colon, it passes into the rectum, which serves as a holding chamber. An accumulation of feces causes the walls of the rectum to stretch, signaling the need to defecate. Unlike other sphincters in the GI tract, the sphincter located between the rectum and the anal canal is under voluntary control. This enables a person to determine whether the time is right for waste elimination. When the sphincter relaxes, the feces move into the anal canal and are expelled from the body. The consistency of feces depends mainly on its water content. If undigested food residue moves too quickly through the colon, a sufficient amount of water cannot be extracted from it. This results in loose, watery feces referred to as diarrhea. Prolonged diarrhea can cause excessive water loss from the body, which can lead to dehydration. Conversely, when the contents in the large intestine move too slowly, too much water may be absorbed. This can cause the fecal matter to become hard and dry, resulting in a condition called constipation. Constipation can make elimination difficult and can put excessive strain on the muscles in the colon wall.

chain length

the number of carbon atoms in a fatty acid's backbone

Biotin (B7)

#iotin (vitamin B7) is yet another water-soluble vitamin involved in energy metabolism. However, biotin is somewhat unique among the water-soluble vitamins because it is obtained from both the diet and biotin-producing bacteria in the large intestine. As illustrated in Figure 7.7, sources of dietary biotin include peanuts, tree nuts (such as almonds and cashews), mushrooms, eggs, and tomatoes. Biotin is often bound to proteins in food, and it is released during digestion. Sometimes, biotin is bound too tightly to food proteins, making it difficult to absorb. In fact, biotin was first discovered because of its role in egg white injury, a condition whereby biotin's bioavailability is reduced severely because it is consumed with avidin, a protein present in large quantities in raw egg whites. Do not let the threat of egg white injury dissuade you from eating cooked eggs, however—heat denatures avidin, allowing biotin to be absorbed normally. Alcohol can also decrease biotin absorption, and high cooking temperatures can destroy biotin in foods

Adults excrete approximately 1½ quarts (1500 mL) of fluid every day as urine. Water is also expelled as water vapor in expired air (300 mL/day), as perspiration (500 mL/day), and in feces (200 mL/day). In total, the body loses approximately 10.6 cups (about 2,500 mL) of fluid every day.

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Do vegetarians necessarily have any special nutritional risks? The answer to this question depends on what kind of vegetarian a person is. In general, a well-balanced lacto-ovo- or lactovegetarian diet can easily provide adequate protein, energy, and micronutrients. Dairy products and eggs are convenient sources of high-quality protein and many vitamins and minerals. However, because meat is often the primary source of bioavailable iron, eliminating it can make it difficult to meet your iron requirements. Furthermore, vegans may be at increased risk of being deficient in several micronutrients, including calcium, zinc, iron, and vitamin B12. This risk is increased further during pregnancy, lactation, and periods of growth and development such as infancy and adolescence.21 It is especially important that vegetarians consume sufficient amounts of plant-based foods rich in these micronutrients.

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Even foods with the same amounts of total protein can contain different combinations of amino acids.

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Fluid located inside of a cell is referred to as intracellular fluid, whereas extracellular fluid is located outside of a cell. Extracellular fluid that fills spaces between or surrounding cells is referred to as intercellular fluid. Extracellular fluid, a component of your blood and lymph, is referred to as intravascular fluid. You may be surprised to learn that the majority of bodily fluid (roughly 6 gallons, or 23 liters) is located inside cells.

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Nerve impulses and muscle contractions occur when balances of ions shift across cell membranes. When a nerve cell is at rest, positively charged potassium ions are concentrated inside of the cell while positively charged sodium ions and negatively charged chloride ions are concentrated outside of the cell. Upon stimulation, sodium ions rush into the cell, and potassium ions rush out. This exchange of ions across the cell membrane produces an electrical signal, or a nerve impulse. Muscle fibers function in much the same manner, but their activation causes calcium (instead of sodium) to flow inside the cell, signaling the muscle to contract. Once calcium has been pumped back out of the muscle cell, it can again relax.

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Neurotransmitters are endogenous chemicals that enable neurotransmission. They are a type of chemical messenger which transmits signals across a chemical synapse from one neuron (nerve cell) to another 'target' neuron, muscle cell, or gland cell.[1] Neurotransmitters are released from synaptic vesicles in synapses into the synaptic cleft, where they are received by neurotransmitter receptors on the target cell. Many neurotransmitters are synthesized from simple and plentiful precursors such as amino acids, which are readily available and only require a small number of biosynthetic steps for conversion. Neurotransmitters are essential to the function of complex neural systems. The exact number of unique neurotransmitters in humans is unknown, but more than 200 have been identified. Most neurotransmitters are about the size of a single amino acid; however, some neurotransmitters may be the size of larger proteins or peptides. A released neurotransmitter is typically available in the synaptic cleft for a short time before it is metabolized by enzymes, pulled back into the presynaptic neuron through reuptake, or bound to a postsynaptic receptor. Nevertheless, short-term exposure of the receptor to a neurotransmitter is typically sufficient for causing a postsynaptic response by way of synaptic transmission.

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amphipathic

A characteristic of a substance that contains both hydrophilic and hydrophobic portions.

cofactor

A cofactor is a non-protein chemical compound or metallic ion that is required for an enzyme's activity as a catalyst, a substance that increases the rate of a chemical reaction. Cofactors can be considered "helper molecules" that assist in biochemical transformations. Zinc functions primarily as a cofactor. Because enzymes needed for gene expression rely on zinc, this mineral is essential to protein synthesis, cell maturation, growth, and proper immune function. Zinc also acts as a potent antioxidant, and it appears to play a role in the stabilization of cell membranes.

acrodermatitis enteropathica

A condition called acrodermatitis enteropathica is caused by a genetic defect in the protein that transports zinc into intestinal cells. Because zinc is not absorbed, excessive amounts are lost in the feces. Babies born with acrodermatitis enteropathica fail to grow properly and usually develop severely red and scaly skin, especially around the scalp, eyes, and feet. While fatal if it goes untreated, acrodermatitis enteropathica can be controlled through lifelong zinc supplementation. When a high enough level of zinc is consumed, adequate amounts can be absorbed—even with a faulty transport protein.

chylomicron

A particle, composed of relatively hydrophobic lipids, cholesterol, and phospholipids, that transports dietary lipids in the lymph for circulation. Note that the plural form of chylomicron is chylomicra.

atherosclerosis

A narrowing and stiffening of the blood vessels due to plaque buildup, causing the restriction of blood flow. A blood clot (a small, insoluble particle made of clotted blood and clotting factors) or aneurysm (the outward bulging of the vessel due to weakness) may also restrict blood flow

protease

A type of enzyme that breaks peptide bonds between amino acids.

Emulsion

An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable). Emulsions are part of a more general class of two-phase systems of matter called colloids. An emulsifier (also known as an "emulgent") is a substance that stabilizes an emulsion by increasing its kinetic stability. One class of emulsifiers is known as "surface active agents", or surfactants. Emulsifiers are compounds that typically have a polar or hydrophilic (i.e. water-soluble) part and a non-polar (i.e. hydrophobic or lipophilic) part. Because of this, emulsifiers tend to have more or less solubility either in water or in oil. Emulsifiers that are more soluble in water (and conversely, less soluble in oil) will generally form oil-in-water emulsions, while emulsifiers that are more soluble in oil will form water-in-oil emulsions. Examples of food emulsifiers are: Egg yolk - in which the main emulsifying and thickening agent is lecithin. In fact, lecithos is the Greek word for egg yolk. Mustard[19] - where a variety of chemicals in the mucilage surrounding the seed hull act as emulsifiers Soy lecithin is another emulsifier and thickener Pickering stabilization - uses particles under certain circumstances Sodium phosphates

Bone tissue

Bone tissue is complex, and is composed of two different kinds of bone cells: osteoblasts and osteoclasts. An osteoblast promotes bone formation, whereas an osteoclast promotes the breakdown of older bone by a process called bone resorption. These cells work in concert to keep your bones healthy and strong, largely by synthesizing and breaking down calcium-containing hydroxyapatite as needed. To facilitate bone resorption, osteoclasts break down small pockets of old and damaged bone, releasing calcium, phosphorus, and other substances into the blood. In order to maintain stable bone mass, osteoblasts fill the dissolved pockets with new bone. This continuous process referred to as bone remodeling (or bone turnover) is illustrated in Figure 8.5. Bone remodeling makes it possible for bones to grow and adapt to mechanical stress (exercise) by stimulating bone remodeling in areas with the greatest load. Like muscles, bones also respond to regular exercise by becoming stronger. Equally important, exercise-induced microscopic changes in bone may also help improve bone architecture, the pattern of trabeculae and associated structures.

Cellular differentiation

Cellular differentiation is the process in which a cell changes from one cell type to another.[2][3] Usually, the cell changes to a more specialized type. Differentiation occurs numerous times during the development of a multicellular organism as it changes from a simple zygote to a complex system of tissues and cell types. Differentiation continues in adulthood as adult stem cells divide and create fully differentiated daughter cells during tissue repair and during normal cell turnover. Some differentiation occurs in response to antigen exposure. Differentiation dramatically changes a cell's size, shape, membrane potential, metabolic activity, and responsiveness to signals. These changes are largely due to highly controlled modifications in gene expression and are the study of epigenetics. With a few exceptions, cellular differentiation almost never involves a change in the DNA sequence itself. Thus, different cells can have very different physical characteristics despite having the same genome. Among dividing cells, there are multiple levels of cell potency, the cell's ability to differentiate into other cell types. A greater potency indicates a larger number of cell types that can be derived. A cell that can differentiate into all cell types, including the placental tissue, is known as totipotent. In mammals, only the zygote and subsequent blastomeres are totipotent, while in plants, many differentiated cells can become totipotent with simple laboratory techniques. A cell that can differentiate into all cell types of the adult organism is known as pluripotent. Such cells are called meristematic cells in higher plants and embryonic stem cells in animals, though some groups report the presence of adult pluripotent cells. Virally induced expression of four transcription factors Oct4, Sox2, c-Myc, and Klf4 (Yamanaka factors) is sufficient to create pluripotent (iPS) cells from adult fibroblasts.[4] A multipotent cell is one that can differentiate into multiple different, but closely related cell types. Oligopotent cells are more restricted than multipotent, but can still differentiate into a few closely related cell types.[5] Finally, unipotent cells can differentiate into only one cell type, but are capable of self-renewal. Cellular differentiation is often controlled by cell signaling. Many of the signal molecules that convey information from cell to cell during the control of cellular differentiation are called growth factors. Although the details of specific signal transduction pathways vary, these pathways often share the following general steps. A ligand produced by one cell binds to a receptor in the extracellular region of another cell, inducing a conformational change in the receptor. The shape of the cytoplasmic domain of the receptor changes, and the receptor acquires enzymatic activity. The receptor then catalyzes reactions that phosphorylate other proteins, activating them. A cascade of phosphorylation reactions eventually activates a dormant transcription factor or cytoskeletal protein, thus contributing to the differentiation process in the target cell.[21] Cells and tissues can vary in competence, their ability to respond to external signals.

eicosanoid

Eicosanoids are involved in regulating the immune and cardiovascular systems and act as chemical messengers that influence a variety of physiologic functions. Your body produces both ω-3 and ω-6 eicosanoids, which have somewhat opposing actions. The ω-6 eicosanoids (produced from linoleic acid) tend to cause inflammation, stimulate blood clot formation, and induce the constriction of blood vessels, whereas the ω-3 eicosanoids (produced from linolenic acid) tend to reduce inflammation, stimulate dilation (or relaxation) of the blood vessel walls, and inhibit blood clotting. Both ω-3 and ω-6 eicosanoids are important to health, and the body can shift its relative production in response to its needs and relative availability of the parent compounds (linoleic and linolenic fatty acids) in the body. Dietary choices can influence the amount and types of eicosanoids that you make. For example, Alaska natives who consume high amounts of ω-3 fatty acids from fish and marine mammals (such as whales and seals) have enhanced physiologic responses that are stimulated by ω-3 eicosanoids. Consequently, Alaska natives who consume traditional diets tend to take more time to form blood clots than people who consume fewer ω-3 fatty acids. Research suggests that alterations in the balance of ω-3 to ω-6 eicosanoids may influence a person's risk of conditions related to inflammation, such as heart disease and cancer. This is why experts often recommend that people regularly consume fish.

Folate (B9)

Folate (vitamin #9) refers to a group of related water-soluble vitamins involved in single-carbon transfers, amino acid metabolism, and DNA synthesis. Good sources of folate include organ meats, legumes (such as lentils and pinto beans), okra, and many green leafy vegetables such as spinach (see Figure 7.8). A common form of folate called folic acid is rarely found in foods, but is often included in vitamin supplements and is added during the fortification of food.

Enterocyte

Epithelial cells that make up villus. Produce maltase which breaks down maltose. Maltose is made from starch digestion. Also produce lactase and sucrase. How do these cells know to produce specific enzymes? At what point in mitosis are different parts of the gene activated or deactivated? villi - Small finger-like projections that cover the inner lining of the small intestine (note that villus is the singular form of villi). Villi are specialized for absorption in the small intestine as they have a thin wall, one cell thick, which enables a shorter diffusion path. They have a large surface area so there will be more efficient absorption of fatty acids and glycerol into the bloodstream. Another way to think about the inner lining of the small intestine is to imagine a bathroom rug folded like an accordion. The folds in the rug represent the large rounded folds, whereas each tiny loop that covers the surface of the rug represents a villus. Villi (the plural form of villus) consist of specialized epithelial cells called enterocytes. Lined up closely, side-by-side, enterocytes are held together by interlocking proteins, referred to as tight junctions. Sometimes tight junctions develop gaps, allowing partially digested food particles or other substances to leak through. This can trigger an immunological response, which is known as a food allergy. Enterocytes, or intestinal absorptive cells, are simple columnar epithelial cells which line the inner surface of the small and large intestines. A glycocalyx surface coat contains digestive enzymes. Microvilli on the apical surface increase its surface area. This facilitates transport of numerous small molecules into the enterocyte from the intestinal lumen. These include broken down proteins, fats, and sugars, as well as water, electrolytes, vitamins, and bile salts. Enterocytes also have an endocrine role, secreting hormones such as leptin.

fatty acids

Fatty acids are made entirely of carbon, hydrogen, and oxygen atoms, and comprise the most abundant type of lipid in your body and the foods you eat (see Figure 6.1). A chain of carbon atoms forms the backbone of each fatty acid. One end of this carbon chain, called the alpha (α) end, contains a carboxylic acid group (-COOH). The other end, called the omega (ω) end, contains a methyl group (-CH3 ). Both in the body and in foods, most fatty acids do not exist in their free (unbound) form. Instead, they are components of larger molecules, such as triglycerides and phospholipids.

complex carbohydrates (polysaccharides)

In contrast to the simple carbohydrates that contain one or two monosaccharides, complex carbohydrates (or polysaccharides) are comprised of many monosaccharides bonded together. The types and arrangements of sugar molecules determine the shape and form of the polysaccharide. For example, some polysaccharides have an orderly linear appearance, whereas others are highly branched like branches of a tree. Three of the most common polysaccharides—starch, glycogen, and fiber—are discussed next.

Roton

In theoretical physics, a roton is an elementary excitation, or quasiparticle, in superfluid helium-4. The dispersion relation of elementary excitations in this superfluid shows a linear increase from the origin, but exhibits first a maximum and then a minimum in energy as the momentum increases. Excitations with momenta in the linear region are called phonons; those with momenta close to the minimum are called rotons. Excitations with momenta near the maximum are sometimes called maxons.

Muonium

Muonium is an exotic atom made up of an antimuon and an electron,[1] which was discovered in 1960 by Vernon W. Hughes [2] and is given the chemical symbol Mu. During the muon's 2.2 µs lifetime, muonium can enter into compounds such as muonium chloride (MuCl) or sodium muonide (NaMu).[3] Due to the mass difference between the antimuon and the electron, muonium (μ+e−) is more similar to atomic hydrogen (p+e−) than positronium (e+e−). Its Bohr radius and ionization energy are within 0.5% of hydrogen, deuterium, and tritium, and thus it can usefully be considered as an exotic light isotope of hydrogen. Although muonium is short-lived, physical chemists study it using muon spin spectroscopy (μSR),[5] a magnetic resonance technique analogous to nuclear magnetic resonance (NMR) or electron spin resonance (ESR) spectroscopy. Like ESR, μSR is useful for the analysis of chemical transformations and the structure of compounds with novel or potentially valuable electronic properties. Muonium is usually studied by muon spin rotation, in which the Mu atom's spin precesses in a magnetic field applied transverse to the muon spin direction (since muons are typically produced in a spin-polarized state from the decay of pions), and by avoided level crossing (ALC), which is also called level crossing resonance (LCR).[5] The latter employs a magnetic field applied longitudinally to the polarization direction, and monitors the relaxation of muon spins caused by "flip/flop" transitions with other magnetic nuclei.

pancreatic amylase

Once the partially digested starch enters the stomach, the acidic environment stops the enzymatic activity of salivary amylase. The short chains of glucose enter the small intestine, where they encounter pancreatic amylase, an enzyme made in the pancreas and delivered to the small intestine as part of the pancreatic juice. Pancreatic amylase picks up where salivary amylase left off, breaking the chemical bonds that hold the glucose molecules together. The glucose chains get shorter and shorter, eventually forming the disaccharide maltose. The final enzyme in the sequence of starch digestion is maltase, which is made in the cells (enterocytes) that line the small intestine. Once maltase is released into the intestinal lumen, the last remaining chemical bond in maltose is split. This results in two free (unbound) glucose molecules that are readily absorbed into the blood (see Figure 4.11).

Denaturation

One way a protein's three-dimensional shape can be altered is by denaturation. Denaturation is akin to flattening out one of the pieces of paper that makes up a folded paper fan protein. Flattening out the paper results in a fan that probably does not work, just as denaturation can cause a protein to lose its function. Compounds and conditions that denature proteins are called denaturing agents. These include physical agitation (e.g., shaking), heat, detergents, acids, alkaline (basic) solutions, salts, alcohol, and heavy metals (e.g., lead and mercury). Heat denatures the proteins in egg white, causing it to transform from a clear gel-like substance into a soft opaque solid.

Optical rotation

Optical rotation or optical activity (sometimes referred to as rotary polarization) is the rotation of the orientation of the plane of polarization about the optical axis of linearly polarized light as it travels through certain materials. Optical activity occurs only in chiral materials, those lacking microscopic mirror symmetry. Unlike other sources of birefringence which alter a beam's state of polarization, optical activity can be observed in fluids. This can include gases or solutions of chiral molecules such as sugars, molecules with helical secondary structure such as some proteins, and also chiral liquid crystals. It can also be observed in chiral solids such as certain crystals with a rotation between adjacent crystal planes (such as quartz) or metamaterials. Rotation of light's plane of polarization may also occur through the Faraday effect which involves a static magnetic field, however this is a distinct phenomenon that is not usually classified under "optical activity." In order to display optical activity, a fluid must contain only one, or a preponderance of one, stereoisomer. If two enantiomers are present in equal proportions then their effects cancel out and no optical activity is observed; this is termed a racemic mixture.

osteopenia

Osteopenia is a condition that begins as you lose bone mass and your bones get weaker. This happens when the inside of your bones become brittle from a loss of calcium. It is very common as you age. Total bone mass peaks around age 35. People who have osteopenia are at a higher risk of having osteoporosis.

Lactose intolerance

People who do not produce enough of the enzyme lactase have difficulty digesting lactose—a condition called lactose intolerance. Lactose intolerance is more common among certain ethnic and racial populations, such as African Americans, Native Americans, and Asian Americans. In addition, the ability to produce lactase can decline with age, making lactose intolerance somewhat more common in older adults. When people with lactose intolerance consume lactose-containing foods, much of the undigested lactose enters the large intestine. Bacteria in the large intestine break down the lactose, which produces gas and sometimes causes other symptoms such as abdominal cramping and bloating. -- The gas probably results from lactose breaking down where it usually doesn't break down causing a build up in pressure. The availability of lactose-free products and lactase-containing preparations makes it relatively easy for those with lactose intolerance to enjoy dairy products and obtain important nutrients such as calcium.

Secondary structure

Polypeptide chains are relatively linear molecules. However, functional proteins are anything but linear— most have three-dimensional shapes. Because the backbone of the polypeptide chain is made of a series of weakly charged amino and carboxylic acid groups, these charges attract and repel each other like magnets. This causes portions of the polypeptide to fold into an organized and predictable pattern—the secondary structure of the protein. The two most common folding patterns are the alpha-helix, similar in shape to a spiral staircase, and the beta-folded sheet, similar in shape to a folded paper fan. Both folding patterns are illustrated in Figure 5.4.

Positronium hydride

Positronium hydride, or hydrogen positride[1] is an exotic molecule consisting of a hydrogen atom bound to an exotic atom of positronium (that is a combination of an electron and a positron). Its formula is PsH. It was predicted to exist in 1951 by A Ore,[2] and subsequently studied theoretically, but was not observed until 1990. R. Pareja, R. Gonzalez from Madrid trapped positronium in hydrogen laden magnesia crystals. The trap was prepared by Yok Chen from the Oak Ridge National Laboratory. In this experiment the positrons were thermalized so that they were not traveling at high speed, and they then reacted with H− ions in the crystal.[4] In 1992 it was created in an experiment done by David M. Schrader and F.M. Jacobsen and others at the Aarhus University in Denmark. The researchers made the positronium hydride molecules by firing intense bursts of positrons into methane, which has the highest density of hydrogen atoms. Upon slowing down, the positrons were captured by ordinary electrons to form positronium atoms which then reacted with hydrogen atoms from the methane.[5] PsH is constructed from one proton, two electrons, and one positron. The binding energy is 1.1±0.2 eV. The lifetime of the molecule is 0.65 nanoseconds. The lifetime of positronium deuteride is indistinguishable from the hydride. The decay of positronium is easily observed by detecting the two 511 keV gamma ray photons emitted in the decay. The energy of the photons from positronium should differ slightly by the binding energy of the molecule. However this has not yet been detected.[1]

Proteins

Proteins are large molecules composed of one or more polypeptide chains. A polypeptide chain is made up of building blocks called amino acids, which are joined together by chemical bonds referred to as peptide bonds. Chemically distinct from the other energy- yielding macronutrients (carbohydrates and lipids), all proteins contain not only carbon and hydrogen but also nitrogen. Proteins come in a variety of sizes: some are very simple, comprised of only a few amino acids, while others contain thousands of amino-acid building blocks. Most proteins are of intermediate size, containing 250 to 300 amino acids. Because they vary so greatly in size, proteins can be classified based on their number of amino acids: dipeptides have two amino acids, tripeptides have three, and so forth.

Adipose Tissue

Recall from Chapter 6 that adipose tissue is a type of connective tissue comprised largely of specialized cells called adipocytes. Each adipocyte contains a lipid-filled core that consists primarily of triglycerides. The number and size of adipocytes determine the amount of adipose tissue in your body. Only recently have researchers fully recognized the complexity of adipose tissue and its relationship to health. Once thought of as a mere passive site for energy storage, scientists now know that adipose tissue is a source of many different hormones and other signaling molecules that link obesity with several chronic, weight-related diseases. Furthermore, where adipose tissue is found in the body (subcutaneous adipose tissue vs. visceral adipose tissue) and types of adipose tissue (white adipose tissue vs. brown adipose tissue) are now known to be associated with overall health and risk for disease.

where does the body make blood

Red blood cells, most white blood cells, and platelets are produced in the bone marrow, the soft fatty tissue inside bone cavities. Two types of white blood cells, T and B cells (lymphocytes), are also produced in the lymph nodes and spleen, and T cells are produced and mature in the thymus gland.

Starch Digestion

Salivary glands release salivary amylase, which breaks bonds in starch. Acidity of gastric juice halts the enzymatic activity of salivary amylase. The polysaccharides pass unchanged into the small intestine. The pancreas releases pancreatic amylase into the small intestine, which converts polysaccharides to the disaccharide maltose. The enzyme maltase, which resides on the absorptive surface of the small intestine (enterocytes), completes starch digestion by splitting maltose, forming two glucose molecules which are readily absorbed into the blood.

Do adults have stem cells?

Scientists are discovering that many tissues and organs contain a small number of adult stem cells that help maintain them. Adult stem cells have been found in the brain, bone marrow, blood vessels, skeletal muscle, skin, teeth, heart, gut, liver, and other (although not all) organs and tissues. Why do adults have stem cells? How is their production process different than regular cells?

anaphylaxis

Severe allergic reaction A rapid immune response that causes a sudden drop in blood pressure, rapid pulse, dizziness, and a narrowing of the airways. Infrequently, seizures have been reported during anaphylaxis. Death due to anaphylaxis usually occurs as a result of respiratory obstruction or cardiovascular collapse, or both.

Cholesterol needed for vitamin D

Several forms (vitamers) of vitamin D exist. The two major forms are vitamin D2 or ergocalciferol, and vitamin D3 or cholecalciferol; vitamin D without a subscript refers to either D2 or D3 or both. These are known collectively as calciferol.[17] Vitamin D2 was chemically characterized in 1931. In 1935, the chemical structure of vitamin D3 was established and proven to result from the ultraviolet irradiation of 7-dehydrocholesterol. The transformation that converts 7-dehydrocholesterol to vitamin D3 occurs in two steps.[162][163] First, 7-dehydrocholesterol is photolyzed by ultraviolet light in a 6-electron conrotatory ring-opening electrocyclic reaction; the product is previtamin D3. Second, previtamin D3 spontaneously isomerizes to vitamin D3 (cholecalciferol) in an antarafacial sigmatropic [1,7] hydride shift. At room temperature, the transformation of previtamin D3 to vitamin D3 in an organic solvent takes about 12 days to complete. The conversion of previtamin D3 to vitamin D3 in the skin is about 10 times faster than in an organic solvent. Vitamin D3 is produced photochemically from 7-dehydrocholesterol in the skin of most vertebrate animals, including humans.[166] The precursor of vitamin D3, 7-dehydrocholesterol is produced in relatively large quantities. 7-Dehydrocholesterol reacts with UVB light at wavelengths of 290-315 nm.[167] These wavelengths are present in sunlight, as well as in the light emitted by the UV lamps in tanning beds (which produce ultraviolet primarily in the UVA spectrum, but typically produce 4% to 10% of the total UV emissions as UVB). Exposure to light through windows is insufficient because glass almost completely blocks UVB light. Vitamin D3 (cholecalciferol) is produced industrially by exposing 7-dehydrocholesterol to UVB light, followed by purification.[178] The 7-dehydrocholesterol is a natural substance in fish organs, especially the liver,[179] or in wool grease (lanolin) from sheep. Vitamin D2 (ergocalciferol) is produced in a similar way using ergosterol from yeast or mushrooms as a starting material.

Gallstones

The formation and accumulation of hard, pebble-like deposits inside the gallbladder, referred to as gallstones, can interfere with the normal flow of bile. Some people with gallstones have no symptoms, while others experience extreme pain. When gallstones block the passage of bile on its way from the gallbladder to the small intestine, the gallbladder can become enlarged and inflamed, causing pain. Although the exact cause of gallstone formation is not clear, it is more common in women than men, and the risk of developing gallstones increases with age. Other risk factors associated with gallstone formation include obesity, rapid weight loss, and pregnancy. Often, surgical removal of the gallbladder is the only way to treat this painful condition. Because bile is necessary for fat digestion, you may wonder how people get by without their gallbladders. Although a person may initially experience difficulty with fat digestion after gallbladder removal, the liver continues to produce bile, which subsequently can be released into the small intestine.

HDL cholesterol

The liver and to a lesser extent the small intestine, also make a type of lipoprotein called high-density lipoprotein (HDL). Compared to other lipoproteins, HDLs have the lowest lipid-to-protein ratios, and therefore they have the highest densities (hence, high-density lipoprotein). HDLs circulate in the blood to collect excess cholesterol from cells and transport it back to the liver, a process referred to as reverse cholesterol transport. Because a high level of HDL cholesterol in the blood is generally associated with a lower risk of cardiovascular disease, HDL cholesterol is often referred to as "good cholesterol." There are several types of HDL, however, and not all forms are equally effective in removing excess cholesterol. Different HDLs have different proteins (apoproteins) on their surfaces, resulting in somewhat different functions. The presence of particular apoproteins makes some HDLs more or less efficient at cholesterol removal, and researchers continue to investigate why different people have different apoproteins associated with their HDL particles. In conclusion, there are three overarching lipid transport pathways, each involving different lipoproteins, as summarized here. • One pathway exclusively transports dietary (exogenous) lipids and involves chylomicra. • Another primarily transports endogenous lipids originating in the liver and adipose tissues and exogenous lipids and involves VLDLs, IDLs, and LDLs. • Another transports excess cholesterol back to the liver and involves HDL (reverse cholesterol transport).

head group

The polar, water-soluble region of a phospholipid that consists of a phosphate group linked to one of several small, hydrophilic molecules. A phosphate containing, hydrophilic chemical structure that serves as a component of a phospholipid. There are many different types of head groups, but the most common are choline, ethanolamine, inositol, and serine.

Alpha Tocopherol (vitamin E)

The term vitamin E refers collectively to eight different compounds that have similar chemical structures. Of these, alpha-tocopherol is the most biologically active. As described in Chapter 6, biological membranes, such as cell membranes, are composed of a bilayer of phospholipid molecules. Naturally, maintenance of these membranes is vital to the stability and function of cells and their contents. Vitamin E, an antioxidant, plays a major role in this maintenance by protecting the fatty acids embedded in biological membranes from free radical-induced oxidative damage. This protection is especially important for cells that are exposed to high levels of oxygen, such as those in the lungs and red blood cells. The ability of vitamin E to act as an antioxidant is enhanced by the presence of other antioxidant micronutrients, such as vitamin C and the mineral selenium. Because antioxidants protect DNA from cancer-causing free radical damage, some researchers believe that vitamin E might prevent or cure certain types of cancer. Indeed, diets high in vitamin E are associated with decreased cancer risk, but there is little experimental evidence that vitamin E itself is responsible.26 In other words, eating foods rich in vitamin E is likely better for you than taking supplementary vitamin E.

Whole-grain food

To ensure adequate fiber intake, it is important to eat a variety of fruits and vegetables every day. It is likewise important to eat whole-grain foods. The nutritional value of grains is greatest when all three components of the grain—bran, germ, and endosperm are present (see Figure 4.10). Whereas the bran contains most of the grain's fiber, the germ contains much of its vitamins and minerals. The endosperm contains mostly starch. Sometimes the bran layer of the wheat kernel is removed during the milling process to produce a product known as refined flour. As a result, foods made with refined flour have very little fiber. When reading food labels, it is important to look for the phrases "whole-grain cereals" and "whole-wheat flour" because foods made with simply "wheat flour" are not necessarily good sources of fiber.

partial hydrogenation

Trans fatty acids are found naturally in some foods, such as dairy and beef products. However, most dietary trans fatty acids are produced commercially via a process called partial hydrogenation. Partial hydrogenation converts oils, such as corn oil, into solid fats, such as margarine or shortening, by converting many of the carbon-carbon double bonds into carbon-carbon single bonds. This is achieved through the chemical addition of hydrogen atoms; hence the term hydrogenation. Aside from decreasing the number of double bonds (and thus increasing the number of carbon-carbon single bonds), the process of partial hydrogenation converts some of the remaining cis double bonds to trans double bonds, causing the lipid to become high in trans fatty acids. Partial hydrogenation has long been used in food manufacturing because adding partially hydrogenated lipids imparts desirable food texture and reduces spoilage. Crackers, pastries, bakery products, shortening, and margarine have long been the main sources of the trans fatty acids in the diet.1 However, a recent trend toward decreasing trans fatty acid intake has resulted in new food preparation and processing methods that reduce or eliminate trans fatty acids in many foods. For example, many fast-food chains have switched from frying their foods in high-trans fatty acid shortening to trans fat-free vegetable oils.

universal solvent

Water- due to its polarity and ability to dissolve many different solutes From alchemy: Alkahest is a hypothetical "universal solvent": able to dissolve every other substance, including gold.

Body heat is generated when energy-yielding nutrients are metabolized, such as during physical activity.

We really are just fancy bioreactors.

vitamin K deficiency bleeding

What is Vitamin K Deficiency Bleeding or VKDB? Vitamin K deficiency bleeding or VKDB, occurs when babies cannot stop bleeding because their blood does not have enough Vitamin K to form a clot. The bleeding can occur anywhere on the inside or outside of the body. Because a newborn's large intestine completely lacks vitamin K-producing bacteria at birth and human milk often contains very low levels of this essential nutrient, babies receive only minimal amounts of vitamin K during their first few weeks of life. To counter the possibility of this condition, the American Academy of Pediatrics recommends that all newborns be given vitamin K injections.

calcitonin

When blood calcium levels are too high, the parathyroid glands produce less PTH. This decreases the formation of calcitriol, which in turn reduces calcium absorption in the small intestine. Elevated blood calcium also stimulates the thyroid gland to produce a hormone called calcitonin, which decreases calcium loss in bone, decreases calcium absorption in the small intestine, and increases calcium levels in the urine. Together, these processes help lower blood calcium levels back to normal. Calcitonin is a hormone that is produced in humans by the parafollicular cells (commonly known as C-cells) of the thyroid gland. Calcitonin is involved in helping to regulate levels of calcium and phosphate in the blood, opposing the action of parathyroid hormone (PTH).

Food allergies

When protein digestion is complete, amino acids can enter the cells (enterocytes) of the small intestine. Most amino acids are absorbed in the duodenum, where they enter the blood and circulate to the liver for further processing. The breakdown of proteins into amino acids is a relatively complete process—it typically results in the absorption and circulation of amino acids (not proteins). Sometimes, however, partially digested proteins are absorbed or enter the circulation by squeezing through the gap junctions between enterocytes. When this happens, the body's immune system might respond as if these partial-breakdown products were dangerous. In such cases, the person is said to have an allergic response, or what is more commonly called a food allergy. 10 The majority of food allergies are caused by proteins present in eggs, milk, peanuts, soy, and wheat. Researchers estimate that approximately 2 percent of adults and 5 percent of infants and young children in the United States have food allergies.11 Note, however, that all adverse reactions to foods are not true food allergies. A negative physiological response to a substance in a food that does not trigger an immune response is called a food intolerance (or food sensitivity). An example of a food intolerance is lactose intolerance, which was discussed in Chapter 4.

White Adipose Tissue (WAT) vs Black adipose tissue (BAT)

Whereas WAT serves as the body's primary depot for energy storage, BAT plays an important role in body temperature regulation and basal metabolism. Found primarily in the neck and upper back, BAT contains numerous iron-containing mitochondria that not only give BAT its characteristic brownish appearance, but are also metabolically active, releasing heat (thermogenesis) in the process. Until recently, it was believed that, at least in humans, only newborns had significant amounts of brown adipose tissue. However, more recent studies have demonstrated that this is not the case. Not only do adults have BAT, but both exposure to cold and physical activity, particularly in cold temperatures, appear to increase its metabolic rate. This has important implications in terms of body weight regulation because BAT-related metabolism increases basal energy expenditure.

zinc smelting

Zinc smelting is the process of converting zinc concentrates (ores that contain zinc) into pure zinc. Zinc smelting has historically been more difficult than the smelting of other metals, e.g. iron, because in contrast, zinc has a low boiling point. At temperatures typically used for smelting metals, zinc is a gas that will escape from a furnace with the flue gas and be lost, unless specific measures are taken to prevent it. The most common zinc concentrate processed is zinc sulfide,[1] which is obtained by concentrating sphalerite using the froth flotation method. Secondary (recycled) zinc material, such as zinc oxide, is also processed with the zinc sulfide.[2] Approximately 30% of all zinc produced is from recycled sources.[3]

protein-energy malnutrition (PEM)

a condition characterized by loss of muscle and fat mass and an increased susceptibility to infection that results from the long-term consumption of insufficient amounts of energy and/or protein to meet the body's needs A condition whereby protein deficiency is accompanied by a deficiency in energy, and usually, one or more micronutrients. PEM can also occur in adults. Unlike children, however, adults with PEM rarely experience kwashiorkor. Instead, they generally develop marasmus. There are many causes of PEM in adulthood, including inadequate dietary intake, such as sometimes occurs in alcoholics and those with eating disorders; protein malabsorption, such as occurs with some gastrointestinal disorders such as celiac disease; excessive and chronic blood loss; cancer; infection; and injury (especially burns). Adults with PEM sometimes experience extreme muscle loss because the body's muscles are broken down to provide glucose and energy. Fat accumulation in the liver and edema are common, and adults with severe PEM experience decreased function of many vital physiological systems, including the cardiovascular, renal (kidneys), digestive, endocrine, and immune systems. Treatment of PEM in adults is often long and difficult. For example, if the cause is infection, treatment may involve both dietary intervention and the use of antibiotics. By contrast, if protein deficiency is a result of an eating disorder, psychological counseling becomes a key component of the health care plan. Regardless of its cause, effective treatment of adult PEM presents a special challenge to any medical team.

Lipolysis

breakdown of fat The metabolic process by which a triglyceride's fatty acids are removed from the glycerol backbone. Compared to other energy-yielding macronutrients, triglycerides represent the body's richest source of energy. As you may recall from Chapter 1, the complete breakdown of 1 gram of fatty acids yields approximately 9 kilocalories which is more than twice the yield from 1 gram of carbohydrate or protein (4 kilocalories). Therefore, gram for gram, high-fat foods yield more calories than do other foods. Note that, before a triglyceride can be used for energy, its fatty acids must be removed from the glycerol backbone. This metabolic process is called lipolysis. The subsequent breakdown of fatty acids to produce ATP is called beta-oxidation, which splits the carbon-carbon bonds in the fatty acid chain into several 2-carbon units. It is these 2-carbon molecules that are metabolized further to yield numerous molecules of ATP. In addition to using fatty acids as an immediate energy source, the body can convert them to other energy-yielding organic compounds called ketones. Recall from Chapter 4 that the production of ketones from fatty acids, called ketogenesis, occurs when glucose availability is low. Ketogenesis is important because some tissues (such as brain, heart, skeletal muscle, and kidney tissue) can metabolize ketones to synthesize ATP, thus slowing down the mobilization and metabolism of muscle protein for energy during periods of low calorie or low carbohydrate intake.

lanugo

fine, soft hair, especially that which covers the body and limbs of a human fetus or newborn. In addition to providing energy and protecting internal organs from injury, adipose tissue also insulates the body. Humans rely on subcutaneous adipose tissue to keep warm, and people with very little body fat often have difficulty regulating body temperature. In fact, a physiological response to insufficient body fat is the growth of very fine hair on the body. This hair, called lanugo, makes up for the absence of subcutaneous adipose tissue to some extent by providing a layer of external insulation. The presence of lanugo is common in very malnourished and underweight people, such as those with the eating disorder anorexia nervosa.7 For this reason, the appearance of lanugo can serve as an important sign of malnutrition during a clinical assessment.

adiposity

obesity

adipose

(especially of body tissue) used for the storage of fat.

Enzymes are like antimolecules. How did enzymes come about? Does every molecule have a corresponding enzyme? Do some molecules have multiple enzymes?

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Excessive fluoride intake can cause dental fluorosis, pitting and mottling (discoloration) of the teeth, and skeletal fluorosis, weakening of the skeleton. Fluorosis is a special concern in small children, who sometimes swallow large amounts of toothpaste on a daily basis. Thus, parents should carefully monitor toothbrushing routines.

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In some instances, the presence of one mineral can interfere with the absorption of another. This is particularly true for minerals that have similar charges. For example, calcium, iron, copper, magnesium, and zinc all carry a positive (2+) charge. High intake of any one of these nutrients may reduce absorption of the others.

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Saliva serves another useful purpose—it helps you to taste your food. Taste is a complex process that occurs when food dissolves in saliva. When this happens, taste molecules react with receptors found in taste buds that reside on the surface of your tongue. Once stimulated, taste receptors send neural signals to the brain, which allows you to experience basic taste sensations such as sweet, sour, salty, bitter, and savory. However, the ability to experience the thousands of different flavors that you enjoy in your food is also the result of aroma molecules. Aroma molecules stimulate receptors located in the nasal cavity. Just like taste, the brain also interprets these signals as well. As a result of taste and aroma molecules, your food delivers a rich and flavorful experience. When you are congested, aroma molecules are not able to reach receptors in the nasal cavity, which makes your food taste bland and flavorless.

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The body can synthesize all the phospholipids and sterols it needs, there are no dietary requirements for either of them.

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The main excretory organs involved in the removal of metabolic waste products include the liver, kidneys, lungs, and skin. Collectively, these organs help prevent the accumulation of toxic waste products by aiding in their removal. For example, the liver converts ammonia— a nitrogen-containing by-product of protein breakdown—into a less toxic substance called urea, which is released into the blood. The kidneys then filter the urea out of the blood so it can be excreted from the body in urine. This is why people with impaired kidney function often undergo a treatment called dialysis. Dialysis involves a special machine that performs similar functions of healthy kidneys. Without dialysis, toxic metabolic waste products would accumulate in the blood, eventually resulting in death.

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The tongue is a powerful muscle that assists in chewing and swallowing. As food mixes with saliva, the tongue manipulates the food mass and pushes it up against the hard, bony palate that makes up the roof of your mouth. As illustrated in Figure 3.10, swallowing takes place in two phases. First, as you prepare to swallow, your tongue directs the soft, moist mass of chewed food, now referred to as a bolus, to the region at the back of your mouth, an area known as the pharynx. The pharynx is the shared space between the oral and nasal cavities. This first phase of swallowing occurs under voluntary control, but once the bolus reaches the pharynx, the second (involuntary) phase of swallowing begins. At this point, the bolus is ready to enter the esophagus, a narrow muscular tube that ends at the stomach. During the involuntary phase of swallowing, the upper-back portion of the mouth (called the soft palate) lifts upward, blocking the entrance to the nasal cavity. This helps guide the bolus into the correct passageway—the esophagus. This movement also causes the epiglottis, a cartilage flap, to cover the trachea, the airway leading to the lungs. If it were not for the epiglottis, food would readily lodge in the trachea causing us to choke. Once the bolus moves past this dangerous intersection, the voluntary and involuntary phases of swallowing are ready for the next bite of food. The esophagus is lubricated and protected by a thin layer of mucus, which facilitates the passage of food. Peristalsis propels the food through the esophagus toward the stomach, where the bolus encounters the first of several sphincters in the GI tract, the gastroesophageal sphincter. As the bolus approaches the end of the esophagus, the gastroesophageal sphincter relaxes long enough for it to pass into the stomach. Once this occurs, the sphincter closes, preventing the contents of the stomach from re-entering the esophagus. The entire trip from the pharynx to the stomach takes only 6 to 10 seconds.

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When a person becomes dehydrated, several homeostatic control mechanisms work together to restore fluid balance. This stimulates the pituitary gland to release antidiuretic hormone (ADH), or vasopressin) into the blood. ADH circulates to the kidneys, where it stimulates water conservation by decreasing the amount of water excreted in the urine. The increased water retention helps restore blood volume to healthy levels. As the pituitary gland releases ADH in response to low blood volume, low blood pressure (also caused by dehydration) stimulates the adrenal glands to release the hormone aldosterone. Aldosterone signals the kidneys to reduce the amount of solutes (sodium) excreted in the urine and instead return the solutes to the blood. Remember that where solutes go, water follows via osmosis. Together, the complementary actions of ADH and aldosterone restore blood volume to a healthy level.

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When table salt (sodium chloride), one of the most familiar electrolytes, is added to water, it separates into its charged components: sodium (Na+) and chloride (Cl-) . These, along with potassium (K+) are the three most abundant ions in the human body. As you previously learned, it is the concentrations of dissolved sodium, chloride, and potassium ions in fluids that direct the flow of water during osmosis. In this way, electrolytes play important roles in regulating fluid balance. A proper distribution of positively and negatively charged ions inside and outside of cells orchestrates many important physiological functions, including nerve cell transmission and muscle contraction.

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total energy expenditure (TEE) - The collective sum of energy used by the body. adaptive thermogenesis - A temporary expenditure of energy that enables the body to adapt to temperature changes in the environment and physiological conditions. nonexercise activity thermogenesis - An expenditure of energy associated with spontaneous movement such as fidgeting and maintenance of posture.

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alpha-keto acid

A compound similar to an amino acid that does not have an amino group; used to synthesize nonessential amino acids. For most people, the remaining 11 amino acids are nutritionally nonessential because they can be synthesized from other compounds. To do this, the body transfers an amino group from an essential amino acid to an alpha-keto acid, which is, in essence, an amino acid without an amino group. This process, called transamination, results in the synthesis of the nonessential amino acids. Under some conditions, however, the body is unable to synthesize one or more of the nonessential amino acids. Some infants (especially those born prematurely) cannot synthesize several of the traditionally nonessential amino acids, for example. These amino acids are therefore considered conditionally essential because they must be obtained from the diet until the baby matures. Fortunately, sufficient amounts of the conditionally essential amino acids can typically be obtained from human milk or regular infant formula. Because the milk produced by women who deliver prematurely does not contain some essential amino acids in amounts sufficient to meet the needs of their infants, premature infants' diets are usually supplemented with the proper amino acids.

marasmus

A form of PEM characterized by extreme wasting of muscle and loss of adipose tissue. Severe PEM actually encompasses a spectrum of malnutrition. At the extremes are two distinct types of severe PEM, and between them are conditions that combine features of both.22 At one end of the spectrum is a condition called marasmus, which results from severe, chronic, overall malnutrition. When a child develops marasmus, his fat and muscle tissue are depleted, and the skin hangs in loose folds, with the bones clearly visible beneath the skin. Children with marasmus tend, at first, to be alert and ravenously hungry, but, with increasing severity, they become apathetic and lose their appetites. Clinicians often say that marasmus represents the body's survival response to long-term, chronic dietary insufficiency

Hadronic atoms

A hadronic atom is an atom in which one or more of the orbital electrons are replaced by a negatively charged hadron.[5] Possible hadrons include mesons such as the pion or kaon, yielding a pionic atom or a kaonic atom (see Kaonic hydrogen), collectively called mesonic atoms; antiprotons, yielding an antiprotonic atom; and the Σ− particle, yielding a Σ− or sigmaonic atom. Unlike leptons, hadrons can interact via the strong force, so the orbitals of hadronic atoms are influenced by nuclear forces between the nucleus and the hadron. Since the strong force is a short-range interaction, these effects are strongest if the atomic orbital involved is close to the nucleus, when the energy levels involved may broaden or disappear because of the absorption of the hadron by the nucleus.[2][7] Hadronic atoms, such as pionic hydrogen and kaonic hydrogen, thus provide experimental probes of the theory of strong interactions, quantum chromodynamics.[9]

Epinephrine

A hormone released from the adrenal glands that stimulates glycogenolysis in emergency situations. Whereas glucagon is involved in the day-to-day regulation of blood glucose, another hormone, epinephrine, can also stimulate glycogenolysis. Under stressful conditions, the adrenal glands release epinephrine, which acts on both the liver and skeletal muscles. As a result, glycogen is quickly broken down, and glucose is released. Sometimes called the fight-or flight reaction, this response helps ensure that glucose is available during extreme circumstances. However, recall that only the liver can release glucose into the blood, whereas glucose resulting from glycogen breakdown in skeletal muscle is used strictly within the muscle cells.

Hypernucleus

A hypernucleus is a nucleus which contains at least one hyperon (a baryon carrying the strangeness quantum number) in addition to the normal protons and neutrons. The first was discovered by Marian Danysz and Jerzy Pniewski in 1952 using the nuclear emulsion technique, based on their energetic but delayed decay. They have also been studied by measuring the momenta of the K and pi mesons in the direct strangeness exchange reactions. In particle physics, a hyperon is any baryon containing one or more strange quarks, but no charm, bottom, or top quark.[1] This form of matter may exist in a stable form within the core of some neutron stars.[2]

lacteal

A lacteal is a lymphatic capillary that absorbs dietary fats in the villi of the small intestine. The lymphatic system also plays an important role in the circulation of fat-soluble nutrients (mostly lipids and some fat-soluble vitamins) away from the small intestine. Each villus contains a lymphatic vessel—a lacteal— through which the nutrients are absorbed. Each lacteal connects to a larger network of lymphatic vessels that circulate a translucent liquid called lymph. Though the circulatory route of the lymphatic system initially bypasses the liver, it eventually delivers the nutrients to the blood. Once in the blood, nutrients can be taken up and used by cells. Triglycerides are emulsified by bile and hydrolyzed by the enzyme lipase, resulting in a mixture of fatty acids, di- and monoglycerides. These then pass from the intestinal lumen into the enterocyte, where they are re-esterified to form triglyceride. The triglyceride is then combined with phospholipids, cholesterol ester, and apolipoprotein B48 to form chylomicrons. These chylomicrons then pass into the lacteals, forming a milky substance known as chyle. The lacteals merge to form larger lymphatic vessels that transport the chyle to the thoracic duct where it is emptied into the bloodstream at the subclavian vein. At this point, the fats are in the bloodstream in the form of chylomicrons. Once in the blood, chylomicrons are subject to delipidation by lipoprotein lipase. Eventually, enough lipid has been lost and additional apolipoproteins gained, that the resulting particle (now referred to as a chylomicron remnant) can be taken up by the liver. From the liver, the fat released from chylomicron remnants can be re-exported to the blood as the triglyceride component of very low-density lipoproteins. Very low-density lipoproteins are also subject to delipidation by vascular lipoprotein lipase, and deliver fats to tissues throughout the body. In particular, the released fatty acids can be stored in adipose cells as triglycerides. As triglycerides are lost from very low-density lipoproteins, the lipoprotein particles become smaller and denser (since protein is denser than lipid) and ultimately become low-density lipoproteins. Lipoproteins they are thought to be atherogenic.

Fats and oils

A lipid that is liquid at room temperature is called an oil, and one that is solid at room temperature is called a fat. There are many different types of lipids, but the most common ones in your body and the foods you eat include the fatty acids, triglycerides, phospholipids, sterols, and fat-soluble vitamins. The chain length of a fatty acid affects its chemical properties and physiological functions. For example, chain length influences the temperature at which a fatty acid melts (its melting point), and lipids constructed predominantly of short-chain fatty acids are generally oils or even gases. Chain length also affects solubility in water: short-chain fatty acids are generally more water soluble than long-chain fatty acids. Because the human body is mostly water, it is relatively easy to absorb and transport water-soluble substances such as short-chain fatty acids. Conversely, the body needs more complex processes to absorb, transport, and use dietary lipids that are more water insoluble, such as the long-chain fatty acids.

Decline in calcium absorption

A person's risk of developing bone disease depends in part on the peak bone mass achieved in early adulthood. After bone mass peaks, the process of age-related bone loss begins. There are many reasons for this decline.8 First, calcium absorption often decreases with age. This is, in part, due to age-related changes in vitamin D synthesis and metabolism, both of which contribute to vitamin D insufficiency. To maintain healthy blood calcium concentrations, bone resorption increases, releasing calcium and other bone-related minerals into the blood. Second, reproductive hormone concentrations naturally decline with age. For example, estrogen, a reproductive hormone produced in the ovaries, is important to the maintenance of bone strength in women. As women reach menopause, declining estrogen levels accelerate bone loss. Men also lose bone mass as they age, but the loss is more gradual (see Figure 8.7).

phospholipids

A phospholipid is similar to a triglyceride in that both contain a glycerol molecule bonded to fatty acids (see Figure 6.9). However, instead of having three fatty acids (like a triglyceride), a phospholipid has only two. Replacing the third fatty acid is a phosphate-containing hydrophilic (or water loving, meaning that it mixes easily with water) head group. There are many different types of head groups, but the most common are choline, ethanolamine, inositol, and serine. Phospholipids are amphipathic, meaning they contain both hydrophilic and hydrophobic regions. Having both properties is very advantageous in the body. While the head group (the hydrophilic region) of each phospholipid molecule is attracted to water, the fatty acids (the hydrophobic region) repel water. This structure allows phospholipids to both act as major components of cell membranes and play important roles in the digestion, absorption, and transport of lipids throughout your body. Their amphipathic structure is also helpful in terms of how phospholipids can be used in the food industry. For example, the phospholipid lecithin is often used in foods such as mayonnaise and salad dressings as a stabilizer. Lecithin prevents the lipid- and water-soluble components of these foods from separating from each other. Phospholipids such as lecithin are added to some salad dressings to keep their hydrophilic and hydrophobic components mixed.

apoprotein

A protein embedded within the outer shell of a lipoprotein that enables it to circulate in the blood and interact with the cells that require its contents. Embedded within the outer shell of a lipoprotein, apoprotein (or apolipoprotein) molecules enable a lipoprotein to circulate in the blood and interact with the cells that require its contents. Like the chylomicra made in the small intestine, the lipoproteins made in the liver are constructed so that their hydrophilic components (proteins and phospholipids) face outward and their hydrophobic components (such as triglycerides) face inward (recall Figure 6.16). Because lipid is less dense than protein, the density of a lipoprotein depends on its relative amounts (or percentages) of lipids and proteins. Lipoproteins with more lipid relative to the amount of protein have lower densities than those with more protein relative to the amount of lipid. With the exception of chylomicra, lipoproteins are named according to their densities, so the relative densities give each lipoprotein its name.

Glucose as an energy source

A rich source of energy, glucose can be used by every cell in the body to make ATP. This conversion is accomplished by means of a catabolic pathway (see Chapter 3). The first step in the metabolic breakdown of glucose is glycolysis, a series of chemical reactions that splits glucose, a six-carbon molecule, into two three-carbon molecules called pyruvate. Because oxygen is not required for any of the steps in this pathway, glycolysis is considered to be an anaerobic metabolic pathway. The amount of energy released through glycolysis is small— enough to make two ATPs per glucose molecule—but there is much more energy yet to be harvested. When conditions are right, the pyruvate molecules can be broken down further to yield additional energy. This requires another metabolic pathway called the tricarboxylic acid (TCA) cycle, or also the Krebs cycle, that functions under oxygen-rich conditions. This series of oxygen-requiring chemical reactions, referred to as aerobic metabolism, releases considerably more energy. Thus, the total net yield from one molecule of glucose can range between 36-38 ATPs. In addition to ATP, the metabolic breakdown of glucose also generates waste products such as water and carbon dioxide. Recall that metabolic waste products such as these are eliminated from the body via expired air, urine, and sweat.

goitrogen

A type of dietary compound called a goitrogen can decrease the ability of the thyroid gland to utilize iodine. Goitrogens are found in soybeans, cassava (a root eaten worldwide), and cruciferous vegetables such as cabbage, cauliflower, and Brussels sprouts. Goitrogens were named as such because they can potentially cause goiter, a disease characterized by enlargement of the thyroid gland. The consumption of goitrogens does not typically pose a problem, except in conditions of very low iodine intake or in people who have thyroid dysfunction.

lipoprotein

A type of particle that transports lipids throughout the body.

Peristalsis

A vigorous, wave-like muscular contraction called peristalsis propels the food from one region of the GI tract to the next (see Figure 3.8). The rate of peristalsis increases and decreases to ensure that the food mass moves along the GI tract at the appropriate rate. At certain points throughout the GI tract, a circular band of muscle called a sphincter regulates the flow of food. Sphincters act like one-way valves, opening and closing in response to neural and hormonal signals. Relaxation of the sphincter allows the food to flow forward. Once the food reaches the next organ, the sphincter closes to prevent the food from flowing backward (see Figure 3.9)

adipocyte

Adipocytes can accumulate large amounts of triglycerides, thus serving as a reservoir of energy. Adipose tissue is located in many parts of your body, including directly under your skin (subcutaneous adipose tissue) and around the vital organs in your abdomen (visceral adipose tissue). Many of your body's organs and tissues (such as the kidneys and breasts) also have adipose tissue associated with them. Not only does this help protect these organs, it also provides them with a readily available energy source. Compared to glycogen, the storage of excess energy as triglycerides has two major advantages. First, because triglycerides are not stored with water which takes up space, a large amount can fit into a small space. Second, as previously discussed, a gram of pure triglyceride stores over twice the energy as does a gram of carbohydrate. Consequently, your body can store much more energy in 1 pound of adipose tissue than in 1 pound of liver glycogen. Indeed, the body has a seemingly infinite ability to store excess energy in adipose tissue, whereas its capacity to store glycogen is limited. The hormone insulin stimulates the storage of triglycerides during times of energy excess, such as after a meal, by causing adipocytes (and to a lesser extent skeletal muscle) to take up glucose and fatty acids from the blood. Insulin also stimulates the conversion of excess glucose to fatty acids (another form of lipogenesis), which are in turn incorporated into triglycerides. Finally, insulin inhibits lipolysis. The increased lipogenesis and decreased lipolysis experienced after a meal help direct excess glucose and fatty acids to adipose tissue, where they are stored as triglycerides for later use. __________ Adipocytes, also known as lipocytes and fat cells, are the cells that primarily compose adipose tissue, specialized in storing energy as fat.[1] Adipocytes are derived from mesenchymal stem cells which give rise to adipocytes through adipogenesis. In cell culture, adipocytes can also form osteoblasts, myocytes and other cell types. There are two types of adipose tissue, white adipose tissue (WAT) and brown adipose tissue (BAT), which are also known as white and brown fat, respectively, and comprise two types of fat cells.

Insulin function

After a person eats carbohydrate-rich foods, blood glucose levels quickly rise, leading to hyperglycemia. This in turn prompts the pancreas to increase its release of insulin. Insulin has several important effects, all of which help to lower the level of glucose in the blood. Insulin is capable of communicating only with cells (e.g., those in adipose tissue and skeletal muscle) that have built-in "receivers" located on the outer surface of their cell membranes. These receivers, referred to as insulin receptors, bind insulin, in turn signaling the cell to take up glucose from the blood. Once glucose is inside the cell, it can be used as a source of energy (ATP). For example, insulin promotes the storage of excess glucose as glycogen in liver and skeletal muscle cells. This is often important after a meal when there may be more glucose available than what is needed by the body. However, only small amounts of glucose can be stored this way. Once this limit is reached, glucose is redirected to metabolic pathways that convert it to fat. Unlike glycogen storage, the body has a seemingly endless capacity to store body fat. In addition to stimulating the storage of excess glucose as glycogen and fat, insulin also plays an important role in the preservation of skeletal muscle. All of these actions work together to help bring blood glucose levels back down to normal—an excellent example of how the body maintains glucose homeostasis. The hormonal regulation of blood glucose by insulin is illustrated in Figure 4.15.

protein turnover

Although proteins serve many functions within the body, they eventually—inevitably—wear out. Fortunately, human bodies can recycle and reuse most of the amino acids from retired proteins to synthesize new ones. The continual coordinated process of breaking down and resynthesizing protein is known as protein turnover. By regulating protein turnover, the body can adapt to periods of growth and development during childhood and maintain relatively stable amounts of protein during adulthood without requiring enormous amounts of protein from food. As you have learned, amino acids can be converted to glucose or used as a source of energy. For this to occur, the nitrogen-containing amino group must first be removed. This process (deamination) produces ammonia (NH3), which is toxic to cells. In response to its production, the liver quickly converts ammonia to urea, a less toxic nitrogen-containing substance. The urea is then released into the blood, filtered out of the blood by the kidneys, and excreted in the urine. Protein turnover results in a somewhat complex flux (or remodeling) of amino acids in your body every day. Measuring protein turnover can provide health professionals with important information about overall protein status. One's protein status can be assessed by comparing protein intake to nitrogen loss in body secretions such as urine, sweat, and feces.13 When nitrogen loss equals nitrogen intake, the body is in neutral nitrogen balance. When nitrogen loss exceeds intake, as can occur during starvation, illness, or stress, a person is in negative nitrogen balance. When nitrogen intake exceeds loss, as can occur during childhood or recovery from an illness, a person is in positive nitrogen balance. Knowing whether a person is in neutral, positive, or negative nitrogen balance can help clinicians diagnose and treat certain disease states and physiologic conditions. For example, people who are on dialysis because of kidney failure often experience negative nitrogen balance and therefore require specialized nutritional support. Conversely, growing children should be in a state of positive nitrogen balance. If this is not the case, protein intake may need to be increased.

Amino acid

Amino acids have three common components: • A central carbon atom bonded to a hydrogen atom, • A nitrogen-containing amino group, and • A carboxylic acid group. Collectively, these three components of amino acids are referred to as the common structure. In the body, the amino and carboxylic acid groups almost always exist in weakly charged states. These charges cause proteins to twist and bend, ultimately giving rise to the protein's shape. As you will learn later in the chapter, it is the shape of a protein that imparts its function. In addition to the common structure, each amino acid contains a unique side-chain group called an R-group; it is the structure of the R-group that distinguishes each amino acid from the others, and the subtle differences in the R-groups give each amino acid its distinctive chemical and physical nature. For example, some of the R-groups are negatively charged, some are positively charged, and some have no charge at all. Figure 5.1 illustrates a generic amino acid and some examples of R-groups.

sickle cell anemia

An example of a disease caused by an inherited genetic variation is sickle cell anemia (or sickle cell disease). Sickle cell anemia is caused by a small alteration in the DNA code that ultimately results in the production of defective, misshapen molecules of the protein hemoglobin within red blood cells. Because hemoglobin is responsible for carrying oxygen and carbon dioxide in the blood, complications such as fatigue and increased risk of infections sometimes occur and can be serious. When sickle-shaped red blood cells accumulate, they can damage the delicate lining of blood vessels and cause pain. Often, a person with sickle cell anemia will need to have a blood transfusion in order to have sufficient ability to deliver oxygen to their cells.

Exotic atom

An exotic atom is an otherwise normal atom in which one or more sub-atomic particles have been replaced by other particles of the same charge. For example, electrons may be replaced by other negatively charged particles such as muons (muonic atoms) or pions (pionic atoms). Because these substitute particles are usually unstable, exotic atoms typically have very short lifetimes and no exotic atom observed so far can persist under normal conditions. In a muonic atom an electron is replaced by a muon, which, like the electron, is a lepton. Since leptons are only sensitive to weak, electromagnetic and gravitational forces, muonic atoms are governed to very high precision by the electromagnetic interaction. Since a muon is more massive than an electron, the Bohr orbits are closer to the nucleus in a muonic atom than in an ordinary atom, and corrections due to quantum electrodynamics are more important. Study of muonic atoms' energy levels as well as transition rates from excited states to the ground state therefore provide experimental tests of quantum electrodynamics.

metallothionein

An intestinal protein that regulates the amount of zinc released into the blood. Metallothionein (MT) is a family of cysteine-rich, low molecular weight (MW ranging from 500 to 14000 Da) proteins. They are localized to the membrane of the Golgi apparatus. MTs have the capacity to bind both physiological (such as zinc, copper, selenium) and xenobiotic (such as cadmium, mercury, silver, arsenic) heavy metals through the thiol group of its cysteine residues, which represent nearly 30% of its constituent amino acid residues.[2]

Onium

An onium (plural: onia) is the bound state of a particle and its antiparticle. The classic onium is positronium, which consists of an electron and a positron bound together as a metastable state, with a relatively long lifetime of 142 ns in the triplet state.[10] Positronium has been studied since the 1950s to understand bound states in quantum field theory. A recent development called non-relativistic quantum electrodynamics (NRQED) used this system as a proving ground. Pionium, a bound state of two oppositely-charged pions, is useful for exploring the strong interaction. This should also be true of protonium, which is a proton-antiproton bound state. Understanding bound states of pionium and protonium is important in order to clarify notions related to exotic hadrons such as mesonic molecules and pentaquark states. Kaonium, which is a bound state of two oppositely charged kaons, has not been observed experimentally yet. The true analogs of positronium in the theory of strong interactions, however, are not exotic atoms but certain mesons, the quarkonium states, which are made of a heavy quark such as the charm or bottom quark and its antiquark. (Top quarks are so heavy that they decay through the weak force before they can form bound states.) Exploration of these states through non-relativistic quantum chromodynamics (NRQCD) and lattice QCD are increasingly important tests of quantum chromodynamics.

phenotype

An organism's physical appearance, or visible traits. the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.

Fluid Balance Is Regulated in Part by Proteins

Another function of proteins is regulating how fluids are distributed in the body. As you might know, most of your body is made of water. This important fluid is found both inside and outside of cells. The fluid outside of cells can be subdivided into that found in blood and lymph vessels (intravascular fluid) and that found between cells (extravascular fluid). The amount of fluid in these spaces is tightly regulated by a variety of means, some of which involve proteins. Albumin, a protein present in relatively high quantities in the blood plays such a role. As the heart beats, blood is pumped out of the heart and into blood vessels that become increasingly narrower. As the pressure builds, the fluid portion of the blood is squeezed out of the tiny capillaries. Albumin, which remains in the blood vessels, becomes more concentrated as more fluid is lost. The high concentration of albumin draws the fluid that was once squeezed out of the blood vessel back into it (see Figure 5.7). Severe protein deficiency can impair albumin synthesis, resulting in low levels of albumin in the blood and causing fluid to accumulate in the space surrounding tissues. This condition, called edema, can sometimes be observed as swelling in the hands, feet, and abdominal cavity. Although edema is commonly seen in severely malnourished individuals, it can be caused by other factors as well, such as congestive heart failure. - Blood is pumped out of the heart and into blood vessels. - The narrow diameter of the blood vessels surrounding organs and tissues causes blood pressure to increase. This forces fluid and nutrients (but not albumin) out of the capillary vessels. - The increased concentration of albumin causes water to be drawn back into the blood. When this does not happen, edema can occur. Edema is common in protein deficiency when albumin synthesis is limited.

Antioxidant

Antioxidants are compounds that inhibit oxidation. Oxidation is a chemical reaction that can produce free radicals, thereby leading to chain reactions that may damage the cells of organisms. Antioxidants such as thiols or ascorbic acid (vitamin C) terminate these chain reactions. To balance the oxidative stress, plants and animals maintain complex systems of overlapping antioxidants, such as glutathione and enzymes (e.g., catalase and superoxide dismutase), produced internally, or the dietary antioxidants vitamin C and vitamin E.

renin

As illustrated in Figure 8.9, the kidneys respond to low blood pressure by releasing an enzyme called renin. Once in the blood, renin converts the liver-derived protein angiotensinogen into angiotensin I, which in turn is converted to angiotensin II in the lungs. Angiotensin II stimulates the adrenal glands to release aldosterone, which, then, signals the kidneys to retain sodium and return it to the blood. This process assists in the restoration of healthy blood volume by drawing water into the blood via osmosis. Aldosterone also causes blood vessels to constrict, which, along with restored blood volume, re-establishes normal blood pressure. This helps explain why some people with high blood pressure are advised to restrict their sodium intakes. In fact, some drugs control high blood pressure by disrupting this system.

monoglyceride vs diglyceride vs triglyceride

As previously mentioned, most fatty acids do not exist in their free (unbound) form. Instead, they comprise various parts of larger, more complex molecules called monoglycerides, diglycerides, and triglycerides. As implied by their names, these substances are defined by the number of fatty acids present in their chemical structures. Whereas a monoglyceride molecule has only one fatty acid attached to a molecule of glycerol, a diglyceride molecule consists of one glycerol molecule with two fatty acids attached, and a triglyceride is made up of one glycerol molecule with three fatty acids attached. The fatty acids can be saturated, monounsaturated, polyunsaturated, or a mixture thereof. In each case, the fatty acid molecule (or molecules) are attached to a glycerol backbone, which by itself is made up of three carbon atoms (see Figure 6.8). The metabolic process by which fatty acids combine with glycerol to form triglycerides is called lipogenesis.

gastric stretching

As the stomach fills with food, stretch receptors in the stomach relay information to the brain via neural signals, inhibiting further intake of food. High-volume foods, such as those with large amounts of water and/or fiber, increase gastric stretching, which in turn helps people feel full and satisfied. Although gastric stretching helps relieve the sensation of hunger, it is not the only factor associated with satiety. The sensation of fullness is also triggered by the presence of certain nutrients in the blood following a meal. For example, when food intake causes blood glucose levels to increase, the brain responds by releasing neurotransmitters that stimulate satiety, providing a signal to terminate eating. Some studies also show that high-fat meals suppress feelings of hunger longer than low-fat meals with the same number of calories. The signaling of satiation brought on by the presence of certain nutrients in the blood following a meal is an example of a post-absorptive mechanism of food regulation.

Legumes

As with carbohydrates, some foods generally contain more protein than do others. Meat, poultry, fish, eggs, dairy products, and nuts contain more protein (per gram) than grains, fruits, and vegetables. Leguminous plants such as soybeans, dried beans, lentils, peas, and peanuts are unique in that they are associated with bacteria that can take nitrogen from the air and incorporate it into amino acids, which legumes use to make their own proteins. This is why legumes tend to contain more protein than most other plants and why the 2015 Dietary Guidelines for Americans states that legumes can "count" as either a vegetable or protein food. Nitrogen fixation is carried out naturally in soil by microorganisms termed diazotrophs that include bacteria such as Azotobacter and archaea. Some nitrogen-fixing bacteria have symbiotic relationships with plant groups, especially legumes.[2] Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation on rice roots. Nitrogen fixation occurs between some termites and fungi.[3] It occurs naturally in the air by means of NOx production by lightning. Plants that contribute to nitrogen fixation include those of the legume family—Fabaceae— with taxa such as kudzu, clover, soybean, alfalfa, lupin, peanut and rooibos. They contain symbiotic rhizobia bacteria within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants.[20] When the plant dies, the fixed nitrogen is released, making it available to other plants; this helps to fertilize the soil.[1][21] The great majority of legumes have this association, but a few genera (e.g., Styphnolobium) do not. In many traditional and organic farming practices, fields are rotated through various types of crops, which usually include one consisting mainly or entirely of clover or buckwheat (non-legume family Polygonaceae), often referred to as "green manure".

Diverticulitis

Because humans do not have the enzymes needed to digest fiber, it passes through the GI tract relatively intact, increasing fecal mass. As a result, feces move through the colon more quickly, which may help prevent or alleviate constipation. Not only can hard, dry feces make elimination more difficult, but it can also contribute to diverticular disease (or diverticulosis), a condition associated with persistent constipation. Constipation can cause undue straining during a bowel movement, which in turn can lead to the formation of pouches called diverticula that protrude along the colon wall (see Figure 4.9). Diverticulitis occurs when the diverticula become infected or inflamed (note that the suffix-itis refers to inflammation). Symptoms of diverticulitis include cramping, diarrhea, fever, and, occasionally, bleeding from the anus. Preventing conditions such as diverticular disease is another reason why a diet high in fiber may be beneficial to one's health.

Chromium in body

Chromium (Cr) is a trace mineral that may be critical to proper insulin function. Chromium was first designated as an essential nutrient because scientists discovered that its deficiency caused a diabetic-like state in certain animals. It was discovered later that chromium might be critical to glucose regulation and insulin function in humans as well. While there is considerable debate as to whether chromium is a required nutrient for humans, it is still classified by some experts as essential. Chromium bioavailability is increased by the presence of vitamin C and acidic medications such as aspirin (acetylsalicylic acid). Conversely, chromium absorption is decreased by the presence of antacids. Regardless of how much is consumed, however, very little (less than 2 percent) chromium is absorbed—most is excreted in the feces. Absorbed chromium is circulated in the blood to the liver, and excess is excreted in the urine. Scientists do not understand why consuming large amounts of simple sugars can increase urinary chromium excretion.37 When the body is unable to excrete excess chromium in the urine, it is deposited mainly in the liver. Some studies suggest that chromium is required for the hormone insulin to function properly—especially in people with type 2 diabetes.38 Chromium may also be essential to normal growth and development in children. At the very least, it appears to increase muscle mass and decrease fat mass in laboratory animals.39 Because of this, a form of chromium called chromium picolinate has been widely marketed as an ergogenic aid for athletes. Other types of chromium supplements are promoted as products that help regulate blood glucose. Toxic levels of chromium have been observed in people exposed to high levels of industrially released chromium, however. For example, when stainless steel is heated to very high temperatures, such as during welding, chromium is released into the air. Environmental exposure of this kind causes skin irritations and may increase the risk of lung cancer.40 These complications are never attributed to dietary chromium.

Copper in the body

Copper is an essential trace mineral that acts as a cofactor for nine enzymes involved in reduction-oxidation (redox) reactions. Once absorbed, copper circulates in the blood to the liver, where it is bound to its primary transport protein, ceruloplasmin. The majority of copper in the body does not circulate freely, but is bound to various transport proteins, storage proteins, and copper-containing enzymes. Excess copper is not stored in the body—it is instead incorporated into bile and eliminated in the feces. As a cofactor for at least nine metalloenzymes, copper plays an important role in energy metabolism, iron metabolism, neural activity, antioxidant defense, and connective tissue synthesis. Superoxide dismutase, a particularly important copper-containing enzyme, acts as an antioxidant, stabilizing the free radical molecules that cause cell and tissue damage in the body. Copper is also required for the syntheses of collagen and norepinephrine, a neurotransmitter that is essential to brain function.

Mineral deficiency

For instance, a plant grown in selenium-deficient soil will have a lower selenium content than one grown in selenium-rich soil. Not surprisingly, selenium deficiency is more common in people living in geographic regions with low-selenium soil. Deficiency can come as a result of soil and water mineral content.

fructose

Fructose is a naturally occurring monosaccharide found primarily in honey, fruits, and vegetables. Although it is the most abundant sugar in fruits and vegetables, the majority of fructose consumed in the Western diet comes from foods and beverages made with high-fructose corn syrup. High-fructose corn syrup (HFCS) is a widely used sweetener consisting of glucose and fructose that is manufactured from cornstarch. It is found in soft drinks, cereals, fruit juice beverages, soups, and a multitude of other foods. In the early 1970s, food manufacturers began using HFCS as an inexpensive replacement for sucrose. As illustrated in Figure 4.4, the consumption of HFCS (per capita) steadily increased, reaching an all-time high in 1999. Perhaps due to health concerns associated with HFCS, the average per capita consumption of HFCS has declined in recent years. The U.S. Department of Agriculture estimates that, on average, Americans now consume approximately 127 kcal from HFCS each day, a 30 percent decrease since 2000. The liver converts the majority of fructose to glucose, which is then used as a source of energy. However, an abundance of fructose in the diet, regardless if it comes from sucrose or HFCS, may be potentially harmful to your health. Of particular concern is a condition called nonalcoholic fatty liver disease. This can occur when the liver converts excess fructose into fat rather than glucose. As a result, fat accumulates in the liver. Researchers have a long way to go before they fully understand the relationship between excess fructose consumption and nonalcoholic fatty liver disease. Nonetheless, most health experts would agree that excessive amounts of added sugar in your diet can be detrimental to your health.

Ketones

Gluconeogenesis increases glucose availability, but too much can have negative consequences; the body cannot rely on the use of amino acids from the breakdown of muscle protein for very long. To minimize loss of muscle, the body reduces its dependence on glucose by using an alternative energy source called ketones. These organic compounds are made from fatty acids under conditions of limited glucose availability. Ketones produced from the metabolism of body fat are released into the blood and used primarily by the brain for an alternate energy source. This glucose-sparing response helps minimize the loss of muscle protein by lessening the cellular demand for glucose. Ketone synthesis is not without its own consequences, however. When excessive ketones accumulate in the blood, a condition called ketosis can occur, causing a variety of complications, including loss of appetite. In fact, this is one reason why many popular low-carbohydrate diets help some people lose weight. ketogenic diet

Glactose

Glucose and galactose look rather similar. However, slight differences in their chemical structures result in important differences in their physiological functions. Few foods contain galactose in its free state. Rather, the majority of dietary galactose comes from a naturally occurring disaccharide in dairy products. Like fructose, the majority of galactose in the body is converted to glucose and used for energy. Disaccharides consist of two monosaccharides bonded together. Present in a wide variety of foods, the most common disaccharides are lactose, maltose, and sucrose. Note that, in all of these disaccharides, at least one of the monosaccharides in the pair is glucose (see Figure 4.5). Lactose: A disaccharide composed of galactose joined with glucose, lactose is the most abundant carbohydrate in milk and many other dairy products (e.g., yogurt, cheese, and ice cream)

where is glycogen stored

Glycogen functions as one of two forms of energy reserves, glycogen being for short-term and the other form being triglyceride stores in adipose tissue (i.e., body fat) for long-term storage. In humans, glycogen is made and stored primarily in the cells of the liver and skeletal muscle. Glycogen is stored glucose. Glycogenesis is the process of glycogen synthesis, in which glucose molecules are added to chains of glycogen for storage. This process is activated during rest periods following the Cori cycle, in the liver, and also activated by insulin in response to high glucose levels.

Hydrogen-4.1

Hydrogen-4.1 looks like helium, it has 2 protons and 2 neutrons. One of its electrons was changed by a muon. Since the orbital of the muon is special and very near the atomic nucleus, that muon can see as a part of the atomic nucleus. The whole atomic nucleus can look as: "The atomic nucleus is form by 1 muon, 2 protons and 2 neutrons, and only one electron outside", so it can look as one of the isotopes of hydrogen, and an exotic atom, too. A muon's weight is 0.1U，so its name was Hydrogen-4.1(4.1H). The Hydrogen-4.1 atom can react with other atoms. Its behavior looks like a hydrogen atom and not a noble helium atom.[4]

glucagon function

Increases the blood sugar level by stimulating the liver Within several hours after eating, blood glucose levels begin to decline. This shifts the hormonal balance away from insulin and toward glucagon. The primary function of glucagon is to increase the level of glucose in the blood. The brain is particularly sensitive to low blood glucose levels, and even a relatively small drop in blood glucose (hypoglycemia) can make a person feel nauseous, dizzy, anxious, lethargic, and irritable. This is one reason why it is hard to concentrate when you have not eaten for a long period of time. To increase glucose availability during these times, glucagon stimulates the breakdown of energystoring liver glycogen into glucose. These glucose molecules are released into the blood, increasing glucose availability to the rest of the body. The term for this metabolic process—glycogenolysis—literally means the breakdown ("lysis") of glycogen. Liver glycogen can supply glucose for approximately 24 hours before being depleted. The breakdown of liver glycogen is an effective short-term solution for providing cells with glucose. However, because this reserve can be quickly depleted, the body must soon find an alternative glucose source. As glycogen stores dwindle, glucagon stimulates another metabolic process called gluconeogenesis. Gluconeogenesis is the synthesis of glucose from noncarbohydrate sources. Taking place mostly in the liver, gluconeogenesis synthesizes glucose mainly from amino acids derived from protein associated with skeletal muscle. You may be wondering why cells do not convert fat into glucose. Although fat can provide cells with plentiful amounts of energy, the carbon atoms contained in fat cannot be utilized to make glucose.

tight junctions

Interlocking proteins between adjacent enterocytes that create an impermeable barrier. Sometimes tight junctions develop gaps, allowing partially digested food particles or other substances to leak through. This can trigger an immunological response, which is known as a food allergy

Iodine in the body

Iodine (I) is an essential trace mineral that although affects the body in many ways, has only one essential function—the production of hormones by the thyroid gland. These hormones regulate growth, reproduction, and energy metabolism. Thyroid hormones also influence the immune system and neural development. Technically, most of the iodine in your body exists in the form of iodide (I- ). Consistent with much of the literature, however, this mineral is referred to herein and commonly as iodine. Iodine is highly bioavailable, and is absorbed mostly in the small intestine. Once absorbed, iodine is circulated to the thyroid gland and used to synthesize the thyroid hormone thyroxine (T4). Thyroxine, which contains four iodine atoms, can be converted to triiodothyronine (T3), a more active form that contains three iodine atoms. T3, and T4 help regulate energy metabolism, growth, and development, and are critical to proper brain, spinal cord, and skeletal development during fetal growth. Because thyroid hormones are involved in the regulation of energy metabolism, low levels can cause severe fatigue. Thyroid-stimulating hormone (TSH), a hormone produced by the pituitary gland, regulates iodine uptake by the thyroid gland (see Figure 8.16). During periods of iodine deficiency, TSH production increases, in turn increasing the percentage of iodine taken up by the thyroid gland. As you might expect, TSH production decreases when excess iodine is consumed. Iodine not needed by the body is excreted in the urine. A type of dietary compound called a goitrogen can decrease the ability of the thyroid gland to utilize iodine. Goitrogens are found in soybeans, cassava (a root eaten worldwide), and cruciferous vegetables such as cabbage, cauliflower, and Brussels sprouts. Goitrogens were named as such because they can potentially cause goiter, a disease characterized by enlargement of the thyroid gland. The consumption of goitrogens does not typically pose a problem, except in conditions of very low iodine intake or in people who have thyroid dysfunction.

Magnesium

Magnesium (Mg) is a major mineral that is important to many physiological processes, such as energy metabolism and enzyme function. Blood magnesium concentrations are regulated primarily by the small intestine and, to a lesser extent, by the kidneys. As with most minerals, magnesium absorption occurs in the small intestine. Absorption increases when blood levels are low and decreases when blood levels are high. The majority of magnesium in the body is associated with bones, where it helps provide structure. Magnesium typically exists as a positively charged ion (Mg ) 21 , which stabilizes enzymes and neutralizes negatively charged ions. For instance, magnesium helps stabilize high-energy compounds such as ATP, and is therefore vital to energy metabolism. All told, magnesium participates in more than 300 chemical reactions. Though perhaps most notable for its role in DNA and RNA synthesis, magnesium also influences nerve and muscle function—especially in heart tissue.

Malt

Malt is germinated cereal grain that has been dried in a process known as "malting". The grain is made to germinate by soaking in water and is then halted from germinating further by drying with hot air. Malting grain develops the enzymes (α-amylase, β-amylase) required for modifying the grains' starches into various types of sugar, including monosaccharide glucose, disaccharide maltose, trisaccharide maltotriose, and higher sugars called maltodextrines. Malts contains maltose which is composed of glucose joined with glucose

Niacin (B3)

Niacin (vitamin B3) is an essential water-soluble vitamin involved in energy and vitamin C metabolism and synthesis of fatty acids and proteins. Niacin is usually obtained through the diet, but can also be synthesized in the body from tryptophan, an essential amino acid. As such, both niacin and tryptophan are considered dietary sources of niacin. The niacin equivalent (NE) is a unit of measure for the combined amounts of niacin and tryptophan in a given food. The NEs of selected foods are illustrated in Figure 7.3. Both niacin and tryptophan are found in a variety of foods, such as liver, poultry, fish, tomatoes, beef, and mushrooms.

Nonheme iron

Nonheme iron is not part of hemoglobin or myoglobin, and is found mainly in plant-based foods such as green leafy vegetables, mushrooms, and legumes. Many dietary factors influence the bioavailability of nonheme iron. For instance, nonheme iron exists in two charged forms: ferric iron(Fe3+) and ferrous iron(Fe2+). The latter is more bioavailable than the former. The following factors can also influence the bioavailability of nonheme iron: • Vitamin C (ascorbic acid) is one of the best-known enhancers of nonheme iron absorption. Because vitamin C converts ferric iron to ferrous iron in the small intestine, consuming it with nonheme iron in a meal enhances iron absorption. • Meat factor is a compound found in meat that increases the bioavailability of nonheme iron. Consuming even a small amount of meat factor along with nonheme iron-containing grains or vegetables will increase iron bioavailability. • Chelator compounds such as phytates and polyphenols bind to nonheme iron in the intestine, making the iron unavailable for absorption. Phytates are found in many vegetables, grains, and seeds, whereas polyphenols are found primarily in plant-based foods such as spinach, tea, coffee, and red wine. If you were to consume one cup of coffee or tea with a nonheme iron-containing meal, iron absorption might decrease by 40 to 70 percent.

Tetrahydrofolate (THF)

Once folate is absorbed by enterocytes, it combines with four hydrogen atoms, converting to its active form, tetrahydrofolate (THF). In its active form, folate facilitates single-carbon transfers, which are needed to synthesize many important organic substances such as amino acids. In a single-carbon transfer, a carbon atom in the form of a methyl group (CH3 is pulled from a molecule and bound to THF, producing 5-methyltetrahydrofolate (5-methyl THF). 5-methyl THF migrates to another molecule and allows it to bind to the methyl group, effectively transferring a single carbon from one molecule to another. Folate-requiring single-carbon transfers play a large role in amino acid metabolism. For example, to convert the amino acid homocysteine to the amino acid methionine, a methyl group must be transferred to homocysteine. This transfer results in the production of both THF (because 5-methyl THF loses its methyl group) and the essential amino acid methionine (see Figure 7.9). Because the production of methionine from homocysteine requires both folate and vitamin B12.

parathyroid hormone (PTH)

One of vitamin D's many important functions is the regulation of blood calcium levels. Low blood calcium stimulates the release of parathyroid hormone (PTH) from the parathyroid gland. PTH stimulates the conversion of 25-(OH) D3 to calcitriol in the kidneys. Together, calcitriol and PTH increase calcium absorption in the small intestine, decrease calcium excretion in the urine, and facilitate the release of calcium from bones (see Figure 7.20).

Phosphorus in body

Phosphorus is essential to the structures of cell membranes, bones, teeth, DNA, RNA, and ATP. Its functional roles include lipid transport, enzyme activation, and energy metabolism. Phosphorus is readily absorbed in the small intestine, and its blood concentration is regulated by the hormones calcitriol, parathyroid hormone (PTH), and calcitonin. When blood phosphorus levels are low, calcitriol and PTH increase both phosphorus absorption in the small intestine and phosphorus release from bone tissue. These actions help return blood phosphorus levels to normal. When blood phosphorus levels are high, calcitonin stimulates osteoblasts to take up phosphorus from the blood to build new bone, thus lowering blood phosphorus levels back to normal. The primary role of phosphorus in the body is as a component of cell membranes. Recall from Chapter 6 that cell membranes are made from phospholipids, which contain phosphorus. Because phospholipids also surround lipoproteins, phosphorus is critical to the transport of lipids in blood and lymph.

Starch

Recall that plants generate glucose through the process of photosynthesis. To store this important source of energy, plants convert the glucose to starch. Thus, starch is made entirely of glucose molecules bonded together. The glucose molecules in starch are arranged in either an orderly unbranched linear chain (amylose) or a highly branched configuration called amylopectin. (see Figure 4.6). Plants typically contain a mixture of these two types of starch. Examples of starchy foods include grains (corn, rice, and wheat), products made from them (pasta and bread), and legumes (lentils and split peas). Potatoes and hard winter squashes are also good sources of starch.

riboflavin

Riboflavin (vitamin B2) is an essential water-soluble vitamin involved in energy metabolism, the synthesis of a variety of vitamins, nerve function, and protection of biological membranes. Riboflavin is named for its ribose structural component and its yellow color—flavus means "yellow" in Latin. Riboflavin is found in a variety of foods, such as liver, meat, dairy products, enriched cereals, and other fortified foods (see Figure 7.2). Fruits and vegetables contain only marginal amounts of riboflavin, while whole-grain foods contain slightly more. Riboflavin is relatively stable during cooking but is quickly destroyed when exposed to excessive light. This is why milk is generally packaged in cardboard, opaque, or translucent (cloudy) containers to protect it from light and preserve its riboflavin content. Your body uses riboflavin to produce two coenzymes needed to transfer oxygen or electrons (negatively charged particles) from one molecule to another. This type of reaction, called a reduction-oxidation (redox) reaction, is critical for ATP production. Recall from Chapter 3 that an atom or molecule with a net positive or negative charge is called an ion. Ions are formed by the loss or gain of one or more electrons. Charged molecules, and certain ions, play an important role in redox reactions. In general, negatively charged ions (or molecules) donate oxygen or electrons to positively charged ions (or molecules). When this happens, the positively charged particle is reduced, and the negatively charged particle is oxidized, resulting in a redox reaction. Redox reactions assist in the synthesis of many bodily substances, such as collagen (a component of your skin, muscle tissue, and hair), carnitine (needed for fatty acid metabolism), and several neurotransmitters and hormones. Beyond its role in redox reactions, riboflavin is also necessary to convert vitamin A and folate (a B vitamin) to their active forms, to synthesize niacin (a B vitamin) from tryptophan (an amino acid), and to form vitamins B6 and K. Finally, riboflavin is critical to the metabolism of several neurotransmitters, and it is involved in reactions that protect biological membranes from oxidative damage.

intrinsic factor (IF)

Secondary vitamin B12 deficiency sometimes occurs when stomach cells fail to produce sufficient amounts of hydrochloric acid or intrinsic factor, a protein needed for vitamin B12 absorption. The production of either of these substances may slow or cease as a person ages, resulting in poor vitamin B12 absorption. Another cause of secondary vitamin B12 deficiency is pernicious anemia, an autoimmune disease whereby the immune system produces antibodies that destroy the intrinsic factor-producing cells of the stomach lining.

Selenium in body

Selenium (Se) is an essential trace mineral that is critical to reduction-oxidation (redox) reactions, thyroid function, and the activation of vitamin C. The essentiality of selenium was only discovered in the 1950s. Since that time, much has been learned about how the body uses this trace mineral. Optimal selenium intake is believed to decrease the risk of cancer, protect the body from toxins and free radicals, activate vitamin C, and enhance immunity. Once absorbed, selenium circulates in the blood to the body's cells. Some selenium is incorporated into amino acids, which are subsequently used to make selenoprotein molecules. There are at least 14 selenoproteins in the body. One group of enzymatic selenoproteins helps protect against oxidative damage and may protect against certain types of cancer, but more research is required to understand the role that selenium plays in this process. The kidneys maintain blood selenium concentrations, and excess amounts are simply excreted in the urine. When consumption is high, selenium can also be expelled in the breath, causing a garlicky odor.

Chiral resolution

Separating right-handed from left-handed molecules. Chiral resolution in stereochemistry is a process for the separation of racemic compounds into their enantiomers.[1] It is an important tool in the production of optically active drugs. Other terms with the same meaning are optical resolution and mechanical resolution. 5-10% of all racemates are known to crystallize as mixtures of enantiopure crystals, so-called conglomerates.[2] Louis Pasteur was the first to conduct chiral resolution when he discovered the concept of optical activity by the manual separation of left-handed and right-handed sodium ammonium tartrate crystals in 1849. In 1882 he went on to demonstrate that by seeding a supersaturated solution of sodium ammonium tartrate with a d-crystal on one side of the reactor and a l-crystal on the opposite side, crystals of opposite handedness will form on the opposite sides of the reactor. This type of resolution, called spontaneous resolution, has also been demonstrated with racemic methadone.[3] In a typical setup 50 grams dl-methadone is dissolved in petroleum ether and concentrated. Two millimeter-sized d- and l-crystals are added and after stirring for 125 hours at 40 °C two large d- and l-crystals are recovered in 50% yield. Another form of direct crystallization is preferential crystallization also called resolution by entrainment of one of the enantiomers. For example, an added seed of (−)-hydrobenzoin to an ethanol solution of (±)-hydrobenzoin will have the (−)-enantiomer crystallizing out and after 15 cycles 97% optical purity can be obtained.

hydroxyapatite

Skeletal calcium is a component of a large crystal-like molecule called hydroxyapatite, which combines with other minerals such as fluoride and magnesium to form the structural matrix of your bones and teeth. Hydroxyapatite also functions as a storage depot for calcium.

kyphosis

Some people with osteoporosis lose significant height during old age and can develop a curvature in the upper spine, a condition called kyphosis (or dowager's hump).

iodized salt

Table salt that has been enriched with iodine as a nutritional supplement. Most salt is fortified with iodine. Iodised salt (also spelled iodized salt) is table salt mixed with a minute amount of various salts of the element iodine. The ingestion of iodine prevents iodine deficiency. Worldwide, iodine deficiency affects about two billion people and is the leading preventable cause of intellectual and developmental disabilities.[1][2] Deficiency also causes thyroid gland problems, including "endemic goitre". In many countries, iodine deficiency is a major public health problem that can be cheaply addressed by purposely adding small amounts of iodine to the sodium chloride salt.

Disaccharide Digestion

The digestion of disaccharides (maltose, sucrose, and lactose) takes place entirely in the small intestine. The small intestine is the lone source of enzymes needed for disaccharide digestion. Each disaccharide has its own specific digestive enzyme: • Maltase (which you just learned about in the digestion of starch) digests maltose, releasing two glucose molecules. • Sucrase digests sucrose, releasing glucose and fructose molecules. • Lactase digests lactose, releasing glucose and galactose molecules. Once disaccharides have been digested into their component monosaccharides, they can be absorbed across the cells (enterocytes) of the small intestine and circulated in the blood (see Figure 4.12). Once disaccharide and starch digestion is complete, the resulting monosaccharides (glucose, galactose, and fructose) are taken up by the cells (enterocytes) lining the small intestine and subsequently released into the blood. The blood then carries the monosaccharides directly to the liver. Because monosaccharides enter the bloodstream relatively quickly after consumption, a rise in blood glucose levels can be detected shortly after you eat most carbohydrate-rich foods. However, not all carbohydrates have the same effect on blood glucose levels. Some foods cause blood glucose levels to rise quickly and remain elevated, while others elicit a more subdued or gradual increase.t

epiglottis function

The epiglottis is a leaf-shaped flap of cartilage located behind the tongue, at the top of the larynx, or voice box. The main function of the epiglottis is to seal off the windpipe during eating, so that food is not accidentally inhaled.

quaternary structure

The fourth level of protein structure is called Ruaternary structure. Quaternary structure occurs when two or more polypeptide chains join together, as shown in Figure 5.5. This level of complexity is somewhat like putting two or three crumpled paper fans together. Not all proteins have a quaternary structure—only those made from more than one polypeptide chain. In addition to the quaternary structure, a nonprotein component called a prosthetic group must sometimes be positioned precisely within a protein for it to function. Prosthetic groups often contain minerals that are needed for the protein to carry out its purpose. Hemoglobin is an example of a protein with quaternary structure and prosthetic groups, because it is made from four separate polypeptide chains, each of which contains an iron-containing prosthetic group called heme. Heme is the portion of hemoglobin that actually transports the oxygen and carbon dioxide gases in the blood.

gastroesophageal reflux disease (GERD)

The gastroesophageal sphincter is sometimes unable to prevent the contents of the stomach from re-entering the esophagus, which is referred to as gastric reflux (see Figure 3.11). Over time, chronic reflux can result in a condition called gastroesophageal reflux disease (GERD), whereby the lining of the esophagus becomes irritated, causing a burning sensation in the chest (commonly referred to as heartburn). Fortunately, most people are able to manage heartburn by changing their lifestyles. Avoiding large meals and certain types of foods (e.g., caffeinated beverages, mint, and fried foods) are often effective prevention and management strategies. However if left untreated, GERD can lead to serious conditions such as inflammation of the esophagus. For this reason, it is important for anyone experiencing GERD to seek medical attention and treatment.

Salivary amylase

The long, arduous process of breaking starch into individual glucose molecules begins in the mouth. Chemical digestion starts when the salivary glands release saliva, which contains the enzyme salivary amylase. Salivary amylase breaks some of the chemical bonds that join glucose molecules. This results in the formation of multiple, shorter chains of glucose molecules. However, because food stays in the mouth only a short time, very little starch digestion actually takes place there.

Iron functions in body

The majority of iron in the body is incorporated into the oxygen-carrying molecules hemoglobin and myoglobin. Hemoglobin, found in red blood cells, is a complex protein that transports oxygen from the lungs to cells and carbon dioxide, a waste product of cellular metabolism, from cells to the lungs. Myoglobin serves as an oxygen-storage molecule within muscles. When circulating hemoglobin is not able to deliver enough oxygen to muscles (as is the case during strenuous physical exertion), the oxygen stored in myoglobin is released. This allows the muscle cells to continue to produce ATP, even when blood oxygen availability is low. Beyond its roles in hemoglobin and myoglobin, iron is a component of several metalloenzymes, is necessary for DNA synthesis, and plays a role in immunity

lipogenesis

The metabolic process by which fatty acids combine with glycerol to form triglycerides.

primary structure

The most basic level of protein structure; determined by the number and sequence of amino acids in a single peptide chain. Although understanding the concept of primary structure is relatively simple, take a moment to contemplate the enormous number of primary structures that can be made from just 20 amino acids. Consider, as an analogy, the English alphabet, which has a similar number of letters. As you know, the number and variety of words that can be constructed from just 26 letters is astounding. Some words are short; some are long. Some words contain just a few different letters; others contain many different letters. The same holds true for proteins: some are short; some are long; some contain a handful of different amino acids, whereas others contain all 20 amino acids. The possibilities are seemingly endless.

Cretinism

The most severe form of iodine deficiency, cretinism, affects babies born to iodine-deficient mothers. During fetal growth, the baby relies on the mother's thyroid hormones for its own growth and development. If a mother is iodine-deficient, she does not produce sufficient thyroid hormones, which affects the baby's growth and development. Cretinism causes severe mental impairment, poor growth, infertility, and increased risk of death. Cretinism is preventable by consuming iodine-rich food or iodine supplements early in pregnancy

tertiary structure

The next level of protein complexity, tertiary structure, involves additional folding caused by interactions between the amino acids' R-groups. This folding transforms the entire protein into an even more complex, three-dimensional structure. It is this exact, precise process of folding and twisting that gives each protein its unique shape. And, it is this shape that allow proteins to recognize and interact with other proteins.

kwashiorkor

The other extreme type of PEM, called kwashiorkor, is distinguished from marasmus by the presence of severe edema (swelling) in the extremities. Edema is sometimes present in children with marasmus, but those with kwashiorkor usually have more extensive edema, which typically starts in the legs but often occurs throughout the entire body. Remember that one of the bodily functions of protein is regulation of fluid balance. Children with kwashiorkor sometimes have large, distended abdomens due to fluid accumulation in the abdominal cavity. This condition is referred to as ascites. Because malnourished children often have intestinal parasites, worms sometimes contribute to this abdominal distension as well. Children with kwashiorkor are often apathetic and sometimes have cracked and peeling skin, enlarged fatty livers, and sparse unnaturally blond or red hair. Although many characteristics of kwashiorkor were once thought simply to be caused by protein deficiency, this does not appear to be the case. Researchers now believe that many of the signs and symptoms of kwashiorkor are the result of micronutrient deficiencies, such as vitamin A deficiency, in combination with infection or other environmental stressors. In addition, emerging research suggests that alterations in a child's gastrointestinal microbes might play a role.

Pancreas roll in digestion

The pancreas plays many important roles in digestion. It protects the small intestine from the acidity of chyme and it also supplies various enzymes necessary for digestion (see Figure 3.14). The arrival of chyme in the small intestine stimulates the pancreas to release pancreatic juice, a mixture of water, bicarbonate, and digestive enzymes. Pancreatic juice is released into a duct that empties directly into the upper region (duodenum) of the small intestine. Being a particularly strong base, the bicarbonate quickly neutralizes the acidic chyme as it enters the duodenum. Another fluid that plays an important role in digestion—especially when fatty foods are consumed is bile. Although bile is produced in the liver, it is stored in the gallbladder for quick release into the small intestine. When high-fat foods are consumed, the gallbladder releases bile, which acts like a detergent. Bile causes large globules of fat to disperse into smaller fat droplets that are easier to digest. This process, called emulsification, enables fats and water to form a smooth, uniform mixture.

LDL cholesterol

The removal of fatty acids from VLDLs causes them to become smaller and denser. The resulting lipoprotein, which is smaller and less dense than a VLDL, is referred to as an intermediate-density lipoprotein (IDL). Some IDL's are taken up by the liver, whereas others remain in circulation where they continue to give up additional triglycerides. Eventually, an IDL becomes a cholesterol-rich low-density lipoprotein (LDL), which is smaller and denser still. An LDL receptor is a specialized type of protein located on cell membranes. LDL receptors bind to the apoproteins (called apolipoprotein B) embedded in the surface of the LDLs, allowing the LDLs to be taken up and broken down by the cell. In this way, much of the cholesterol is removed from the blood and used by cells to maintain their cell membranes and synthesize other compounds such as steroid hormones and vitamin D. Sometimes, however, cholesterol-containing LDL particles penetrate walls of arteries, such as those supplying the heart with blood. When this happens, the lipids within the LDLs become oxidized, and the apolipoprotein B molecules are modified in such a way that they attract immune cells to the area. In an effort to repair any damage that has been done to the blood vessel, these immune cells take up and dismantle the LDL particles. This, in turn, initiates a cascade of inflammatory responses that can further damage the blood vessel and can result in the buildup of a fatty substance called plaque, which is comprised of cholesterol, fatty substances, cellular waste products, calcium, and fibrin. Eventually, the accumulation of plaque can slow or even block blood flow.11 Epidemiologic studies suggest that high levels of LDL cholesterol in the blood, or at least cholesterol contained in some types of LDL, are related to increased risk for cardiovascular disease.12 Thus LDL cholesterol is often referred to as "bad cholesterol."

Fiber

The term fiber (or dietary fiber) refers to a diverse group of polysaccharides found in a variety of plant-based foods such as whole grains, legumes, vegetables, and fruits. Different foods contain different types of dietary fiber. The type of chemical bond found in fiber differs from that found in starch. Because humans lack the enzymes needed to break these chemical bonds, fiber is virtually indigestible in the human small intestine (see Figure 4.8). As a result, undigested fiber passes from the small to the large intestine relatively intact. Once in the large intestine, fiber is broken down by intestinal bacteria. There is evidence that dietary fiber helps promote the growth of beneficial bacteria in the large intestine, which in turn helps inhibit the growth of disease-causing (pathogenic) bacteria. Dietary fiber is commonly classified on the basis of its solubility in water. Soluble fiber attracts water and turns to gel as it passes through the GI tract, slowing the process of digestion. Soluble fiber is found in oat bran, barley, nuts, seeds, beans, lentils, peas, and some fruits and vegetables. It is also found in psyllium, a common fiber supplement. Some types of soluble fiber may help lower risk of heart disease. *nsoluble fiber is found in foods such as wheat bran, vegetables, and whole grains. It adds bulk to the stool, which aids in the process of elimination.

Vitamin A

The term vitamin A refers to a series of three compounds referred to as retinoids (or preformed vitamin A): retinol, retinoic acid, and retinal. Although all three forms are important, retinol is the most biologically active and is synthesized in the body from retinal (see Figure 7.13). Retinoic acid can also be synthesized from retinal, but retinoic acid itself cannot be converted to any other retinoid. Beyond the retinoids, the vitamin A family also includes several carotenoid compounds, which have structures similar to those of the retinoids. Some carotenoids can be converted to vitamin A; one such carotenoid is called a provitamin A carotenoid. One of the most common provitamin A carotenoids is beta-carotene (beta-carotene), which the body converts to two retinal molecules. A carotenoid that cannot be converted to vitamin A is called a nonprovitamin A carotenoid. Lycopene, astaxanthin, zeaxanthin, and lutein are examples of nonprovitamin A carotenoids and often referred to as phytochemicals. Whereas animal-based foods tend to contain preformed vitamin A, plant-based foods tend to contain provitamin A carotenoids. The yellow and red hues of many carotenoids make carotenoid-rich plant fibers and animal tissues brightly colored. As such, yellow, orange, and red fruits and vegetables, such as cantaloupe, carrots, and peppers, are particularly good sources of the carotenoids, as are brightly colored animalbased foods such as egg yolks, lobsters, crabs, and shrimp. Anecdotal evidence suggests that ancient Egyptian physicians prescribed vitamin A-rich liver to treat poor vision. It was not until much later, however, that scientists began to understand the mechanisms by which vitamin A-rich foods improved eyesight. As illustrated in Figure 7.15, when light enters your eyes, it passes to an inner back lining called the retina. The retina consists of a layer of nerve tissue and millions of cells called cones and rods. Cones enable you to see color, whereas rods distinguish black from white (a critical aspect of night vision). Rods contain thousands of rhodopsin molecules, each of which is composed of cis-retinal (a form of vitamin A) and opsin (a protein). When light strikes the rhodopsin, the cis-retinal is converted to trans-retinal and separates from the opsin. This reaction causes a neural signal to be sent to the brain. The light signal is then interpreted by the brain as a recognizable image. Because cis-retinal is a form of vitamin A, adequate consumption of vitamin A is needed for this cascade of events to occur. Vitamin A is also important for the health of the eye's outermost tissue layer, the cornea, and especially important to vision in low-light environments. This is why adequate vitamin A intake is needed to prevent night blindness, a condition whereby vision is impaired in dim light.

There are literally hundreds of fatty acids found in foods, and as you know, they differ from one another in terms of their chain length and, for unsaturated varieties, the number, placement, and configuration of their double bonds. Similarly, trans fatty acids can differ as well. This is important because various forms of trans fatty acids influence health in different ways. Those produced during the partial hydrogenation of vegetable oils can have negative effects on heart health, but other naturally occurring forms demonstrate potent anticarcinogenic effects. The most well-studied naturally occurring trans fatty acid is commonly referred to as conjugated linoleic acid because, like linoleic acid, it has 18 carbons and 2 double bonds. However, unlike linoleic acid, which has two single bonds between its double bonds, the double bonds in CLA are separated by only one single bond. Furthermore, one of CLA's double bonds has a trans configuration. The most common dietary CLA is primarily found in dairy and beef fat, and its common name is rumenic acid. Because naturally occurring trans fatty acids, such as rumenic acid, are thought to promote health (or at least not pose health risks), the FDA does not regulate their presence in foods.

There are different types of trans fat.

essential fatty acids

There are potentially hundreds of different fatty acids, each with its own distinct structure, physiologic properties, and name. Although foods provide a variety of different fatty acids, many of which are needed for optimal health, only two are considered essential. Because cells are unable to make these fatty acids, it is important that you get them from food. The two essential fatty acids are linoleic acid and linolenic acid (also called α-linolenic acid). Linoleic acid has 18 carbons and two double bonds, and it is an ω-6 fatty acid. Linolenic acid has 18 carbons and three double bonds, and it is an ω-3 fatty acid. Linoleic acid and linolenic acid are essential nutrients because the body cannot insert double bonds in the ω-3 and ω-6 positions of fatty acids. As such, you need linolenic acid and linoleic acid to make all of the other ω-3 and ω-6 fatty acids, respectively, and to make other related substances. For example, as illustrated in Figure 6.6, linoleic acid is used to make arachidonic acid (a 20-carbon, ω-6 fatty acid). Similarly, linolenic acid can be converted to eicosapentaenoic acid (EPA), a 20-carbon, ω-3 fatty acid, which can subsequently be converted to docosahexaenoic acid (DHA), a 22-carbon, ω-3 fatty acid. This is accomplished by increasing the number of carbon atoms in the chain, a process called elongation, and by increasing the number of double bonds in the fatty acid chain, or through desaturation. These important long-chain PUFAs have many important functions in the body. For example, they are necessary for normal cell function, forming structural components of cell membranes, and the regulation of gene expression. They also serve as precursors to hormone-like compounds called eicosanoids.

Vitamin D

There are two forms of vitamin D in foods: ergocalciferol (vitamin D2) is found in plant-based foods, and cholecalciferol (vitamin D3) is found in animal-based foods. Cholecalciferol is also synthesized in the body. The body's synthesis of vitamin D actually involves two steps, as illustrated in Figure 7.18. First, a cholesterol derivative is converted by ultraviolet light to previtamin D3 (or precalciferol) in the skin. Tanning machines also emit the ultraviolet light that allows the first step to take place, but relying on tanning sessions to obtain enough vitamin D3 is not recommended. This is because tanning can damage the skin and increase the risk of skin cancer. After the cholesterol derivative is converted to previtamin D3, this substance is converted in the skin to vitamin D3 (or cholecalciferol), which then diffuses into the blood and circulates to the liver. People with darker skin may need up to three times more sunlight exposure than do people with lighter skin to produce enough vitamin D3. For all people, vitamin D3 production decreases with age. Vitamin D deficiency is also more common in obese individuals, as compared to those who are lean. This may be because, once vitamin D is synthesized, it can be taken up by adipose tissue, making it less available to other tissues of the body. Whether consumed through the diet or produced in the skin, vitamin D3 must be further metabolized before it can be used by the body. This two-step process, illustrated in Figure 7.19, occurs in the liver and kidneys. First, vitamin D3 is converted to 25-hydroxyvitamin D3 (25-[OH] D3) in the liver. Next, the 25-(OH) D3 circulates in the blood to the kidneys, where it is converted to 1,25-dihydroxyvitamin D3 (1,25-[OH]2 D3), the active form of vitamin D known more commonly as calcitriol.

Thiamin

Thiamin (vitamin B1 ) is an essential water-soluble vitamin involved in energy metabolism and the synthesis of the genetic material DNA and RNA. It is found in a wide variety of foods, such as pork, peas, fish, legumes, soymilk, enriched cereal products, and whole-grain foods (see Figure 7.1). As you will notice throughout this chapter, whole-grain foods tend to be good sources of many water-soluble vitamins. Thiamin is sensitive to heat and is easily destroyed during cooking. Shorter cooking times and lower temperatures can decrease this loss, although it is important to cook meat until it is done. Several factors influence thiamin bioavailability. In general, its absorption increases when the body needs more of it, and decreases when thiamin status is adequate. Compounds found in raw fish, coffee, tea, berries, Brussels sprouts, cabbage, and alcohol can interfere with thiamin's absorption, and sulfites, often added to processed foods as preservatives, can destroy thiamin.2 Conversely, vitamin C can increase thiamin bioavailability. Although thiamin is not an energy-yielding nutrient per se, it is intimately involved in ATP production. To aid in energy synthesis, thiamin acts as a coenzyme, meaning that an enzyme will not function unless thiamin is present. As a component of a coenzyme, thiamin plays a role in the synthesis of DNA, RNA, and triglycerides. Without thiamin acting as a coenzyme, the enzymes that make DNA and RNA cannot work, and protein synthesis is halted. As a result, cell division is impeded because the genetic material (DNA and RNA) cannot be replicated.

Iron absorption, transport, and storage

Transport proteins begin the process of absorption by escorting iron from the microvilli into the enterocytes. Once iron is absorbed into the intestinal cell, the first level of iron regulation begins. Iron-storing proteins acting as gatekeepers determine the initial fate of iron by either releasing it into the blood or retaining it in the intestinal cell for subsequent elimination in the feces. This dynamic process optimizes iron bioavailability in relation to the body's need for iron (see Figure 8.13). Dietary iron released from the intestinal cell into the blood binds to a protein called transferrin, which delivers the iron to special receptors on cell membranes. The number of iron-binding receptors on a cell membrane changes in response to the cell's need for iron: cells increase the number of receptors when they need more iron, and decrease the number of receptors when they need less. When more iron is absorbed than is needed, small amounts of excess iron are stored in the body. Surplus iron is incorporated into ferritin, a protein found primarily in liver, skeletal muscle, and bone marrow cells. Ferritin forms a protein complex that only releases iron when it is needed. In this way, ferritin serves as the body's iron reserve. In fact, measuring blood ferritin levels is one way to assess a person's iron status. Another important regulator of iron is called hepcidin, which is made by the liver. Hepcidin regulates the level of iron in blood and its distribution to various tissues of the body by reducing dietary iron absorption.

Type 1 diabetes

Type 1 diabetes occurs when the pancreas is no longer able to produce insulin. Without insulin, certain cells (e.g., skeletal muscle and adipose cells) cannot take up glucose, causing blood glucose levels to become dangerously high. Approximately 5 to 10 percent of all people with diabetes have type 1. Although type 1 diabetes can develop at any age, it typically develops during childhood and early adolescence. Type 1 diabetes develops when a person's immune system produces antibodies that mistakenly attack and destroy the insulin-producing cells of the pancreas (see Figure 4.17). This inappropriate immunologic response is thought to be triggered by an environmental factor such as exposure to a virus. For this reason, type 1 diabetes is classified as an autoimmune disease, an illness that occurs when an abnormal immunological response results in the destruction of body tissues. Although the majority of people who develop type 1 diabetes have no family history of the disease, most experts agree that a genetic tendency does increase a person's risk. Destruction of the insulin-producing pancreatic cells and the subsequent inability to produce insulin cause blood glucose levels to become dangerously elevated (severe hyperglycemia). Symptoms tend to develop rapidly, and because of their severity, are not easily ignored. Signs and symptoms associated with type 1 diabetes include rapid weight loss, extreme thirst, and frequent urination.

Type 2 diabetes

Type 2 diabetes is by far the most common form of diabetes - 90 to 95 percent of people with diabetes fall into this category. In the United States, type 2 diabetes has become so widespread that an estimated 29.1 million people or 9.3 percent of the population have diabetes; about 1 out of every 11 people.20 Although type 2 diabetes can occur at any age, it most frequently develops in adults, middle-aged, and older. However, the rising prevalence of obesity in children and teens throughout the United States has contributed to a new and alarming trend: an escalation in the number of children and teens diagnosed with type 2 diabetes.21 Given this trend, type 2 diabetes can no longer be thought of as a condition that only affects adults. Unlike people with type 1 diabetes, most people with type 2 diabetes have normal or even elevated levels of insulin in their blood. Type 2 diabetes is caused by insulin resistance, meaning that insulin receptors are less responsive to insulin. Because insulin-requiring cells do not respond appropriately to insulin's signal, the amount of glucose taken up from the bloodstream is notably diminished. When blood glucose levels rise beyond the normal range, a person begins to experience symptoms associated with type 2 diabetes. Symptoms tend to develop gradually and are often ignored—this is why type 2 diabetes can go undiagnosed for many years. Some of the early symptoms associated with type 2 diabetes include fatigue, frequent urination, and excessive thirst.

Cis and trans fatty acids

Unsaturated fatty acids differ in the ways that their hydrogen atoms are arranged around the carbon- carbon double bonds. In most naturally occurring fatty acids, the hydrogen atoms are positioned on the same side of the double bond. This is called a cis double bond (see Figure 6.4). When the hydrogen atoms are positioned on opposite sides of the double bond, it is called a trans double bond. Unlike cis double bonds, trans double bonds do not cause bending. A fatty acid containing at least one trans double bond is called a trans fatty acid. Trans fatty acids have fewer bends in their backbones than do their cis counterparts. For this reason, trans fatty acids are also more likely to be solid (fats) at room temperature.

Cobalamin (B12)

Vitamin B12 (cobalamin) is a water-soluble vitamin involved in energy metabolism and production of methionine (an amino acid). Cobalamin is so-named because it contains the trace element cobalt and several nitrogen (or amine) groups. Vitamin B12 is a unique vitamin because it can only be synthesized by microorganisms such as bacteria and fungi. That is, plants and higher animals (like humans) do not synthesize vitamin B12 themselves—they must obtain it from microorganisms living in either their environments or colonized in gastrointestinal tracts. When you eat these plants and animals, you absorb the vitamin B12 that they themselves have produced. Good dietary sources of vitamin B12 include shellfish, meat, fish, and dairy products; many ready-to-eat breakfast cereals are also fortified with vitamin B12.

Vitamin K

Vitamin K functions as a coenzyme in a variety of reactions that ultimately constitute the life-or-death process by which blood clots form. Without this process, called coagulation, you might bleed to death after even a minor scrape. For your blood to coagulate and form a clot, a cascade of chemical reactions must first take place. After a cut or scrape occurs, various clotting factors are activated by vitamin K, allowing the next reactions in the cascade to take place. These reactions ultimately result in the production of fibrin, a protein that forms a web-like clot that stops the bleeding. Beyond its coagulation functions, vitamin K is also essential for the synthesis of proteins needed for bone and tooth formation. The term vitamin K refers to three compounds that have similar structures and functions. Vitamin K is found naturally in plant-based foods in a form called phylloquinone. This form is also found in some vitamin K supplements. Another form of vitamin K called menaquinone is produced by bacteria in the large intestine. Because this bacterial production does not produce sufficient amounts of vitamin K to sustain health, vitamin K is considered an essential nutrient. A third form of vitamin K called menadione is neither found naturally in food nor synthesized by intestinal bacteria, but is produced commercially.

Glycogen

Whereas plants store extra glucose as starch, the human body stores small amounts of excess glucose in the form of glycogen. Glycogen is a polysaccharide found primarily in liver and skeletal muscle. Like amylopectin the glucose molecules in glycogen are arranged in a highly branched configuration (see Figure 4.7). The numerous branches of glucose molecules found in glycogen can be broken down quickly, providing cells with an immediate source of energy when needed. However, unlike glycogen stored in skeletal muscle, only liver glycogen can release its glucose into the blood.

protein complementation

You may be wondering how people who only eat plant-based foods (vegetarians) get all of their essential amino acids if their diets are limited to incomplete proteins. The answer to this question is that diverse foods with different incomplete proteins can be combined to provide adequate amounts of all the essential amino acids. This dietary practice, called protein complementation, is customary around the world, especially in regions that traditionally rely heavily on plant-based foods for protein. Examples of commonly consumed foods whose proteins complement each other are rice and beans, or corn and beans. Both rice and corn have several limiting amino acids (for example, lysine) but provide adequate amounts of others (e.g., methionine). By contrast, dried beans and other legumes tend to be limiting in methionine but provide adequate amounts of lysine. In general, protein complementation allows diets containing a variety of plant-based protein sources to provide all of the necessary essential amino acids.

Gastric juice

Your stomach produces more than 2 quarts (roughly 2 liters) of gastric juice daily. Although the hydrochloric acid and digestive enzymes in gastric juice would normally damage most tissues, the lining of the stomach is protected by a layer of mucus called the gastric mucosal barrier. This thick, tenacious gel-like substance explains why the stomach lining can withstand this hostile, corrosive environment. Damage to this protective layer of mucus can result in inflammation or gastritis, which can subsequently lead to the formation of a gastric ulcer. Contrary to popular belief, the majority of ulcers are not caused by stress or by eating spicy foods. Most ulcers are caused by a small, spiral-shaped bacterium called Helicobacter pylori (H. pylori). However, some medications (such as aspirin) and chronic, excessive consumption of alcohol can cause ulcers as well. The presence of food in the stomach stimulates the release of a hormone called gastrin, which is produced by endocrine cells in the stomach's lining. Gastrin stimulates the release of gastric juice and causes the muscular wall of the stomach to contract vigorously. These powerful muscular contractions, much like the action of kneading bread, force the bolus to mix with the acidic gastric juice. Within 3 to 5 hours after eating a meal, the partially digested food is mixed thoroughly with the gastric juice. By the time the food leaves the stomach, it has been transformed into a semi-liquid paste called chyme. The pyloric sphincter, located at the base of the stomach, regulates the flow of chyme into the small intestine. With each peristaltic wave, a few teaspoons of chyme squeeze through the sphincter as it briefly relaxes. The remaining chyme tumbles back and forth in the stomach, allowing for even more mixing. This slow release of chyme allows the small intestine to prepare for its role in the digestive process.

hydrolytic reaction

a chemical reaction in which a water molecule is added to break a covalent bond e.g., those required for carbohydrate and triglyceride digestion

Myoglobin

a red protein containing heme, which carries and stores oxygen in muscle cells. It is structurally similar to a subunit of hemoglobin. Myoglobin serves as an oxygen-storage molecule within muscles. When circulating hemoglobin is not able to deliver enough oxygen to muscles (as is the case during strenuous physical exertion), the oxygen stored in myoglobin is released. This allows the muscle cells to continue to produce ATP, even when blood oxygen availability is low. Beyond its roles in hemoglobin and myoglobin, iron is a component of several metalloenzymes, is necessary for DNA synthesis, and plays a role in immunity.

ascites

abnormal accumulation of fluid in the abdomen Abdominal swelling caused by accumulation of fluid, most often related to liver disease.

metalloenzyme

an enzyme that is activated when it combines with a mineral When a mineral such as copper, magnesium, or zinc combines with and activates an enzyme, the mineral is called a cofactor and the enzyme is called a metalloenzyme.

babushka

an old woman or grandmother.

deflationary

associated with or tending to cause decreases in consumer prices or increases in the purchasing power of money

osteomalacia

disease marked by softening of the bone caused by calcium and vitamin D deficiency

What are hormones?

eicosanoids steroids amino acid/protein derivatives (amines, peptides, and proteins) Hormones serve to communicate between organs and tissues for physiological regulation and behavioral activities such as digestion, metabolism, respiration, tissue function, sensory perception, sleep, excretion, lactation, stress induction, growth and development, movement, reproduction, and mood manipulation.

Basal Metabolism (BMR)

energy burned for life-sustaining activities An expenditure of energy to sustain vital, involuntary physiologic functions such as respiration, beating of the heart, nerve function, and muscle tone.

Enantiomer

is one of two stereoisomers that are mirror images of each other that are non-superposable (not identical), much as one's left and right hands are mirror images of each other that cannot appear identical simply by reorientation. another term for left and right handed molecules; chirality

Hyponatremia

low sodium in the blood

Human Growth Hormone (HGH)

stimulates secretion of hormones that stimulate body growth and metabolism Growth hormone (GH) or somatotropin, also known as human growth hormones (hGH or HGH) in its human form, is a peptide hormone that stimulates growth, cell reproduction, and cell regeneration in humans and other animals. It is thus important in human development. GH also stimulates production of IGF-1 and increases the concentration of glucose and free fatty acids.[1][2] It is a type of mitogen which is specific only to the receptors on certain types of cells. GH is a 191-amino acid, single-chain polypeptide that is synthesized, stored and secreted by somatotropic cells within the lateral wings of the anterior pituitary gland.

What drives osmosis?

the difference in water concentration across a selectively permeable membrane The driving force behind osmosis is the difference in the concentration of solutes in bodily fluids. As an example of osmosis in action, consider what happens when vegetables are pickled. When a cucumber is placed in a salty brine solution, water moves out of the cucumber in the direction of the greater solute (salt) concentration. As water leaves the cucumber, it shrivels. When the right spices are added to the brine, the cucumber takes on a unique, delicious flavor and is referred to as a pickle.

Coulomb barrier

the energy barrier due to electrostatic interaction that two nuclei need to overcome so they can get close enough to undergo a nuclear reaction.

metabolic pathway

A series of chemical reactions that either builds a complex molecule or breaks down a complex molecule into simpler compounds. A series of interrelated chemical reactions that require enzymes.

Mesonic molecule

A mesonic molecule is a set of two or more mesons bound together by the strong force. Unlike baryonic molecules, which form the nuclei of all elements in nature save hydrogen-1, a mesonic molecule has yet to be definitively observed. The X(3872) discovered in 2003 and the Z(4430) discovered in 2007 by the Belle experiment are the best candidates for such an observation. _____________ Charged pions are the lightest and longest-lived mesons. Mesonic atoms are formed when an orbital electron in an atom is replaced by a negatively charged meson. Laser spectroscopy of these atoms should permit the mass and other properties of the meson to be determined with high precision and could place upper limits on exotic forces involving mesons (as has been done in other experiments on antiprotons. Determining the mass of the π− meson in particular could help to place direct experimental constraints on the mass of the muon antineutrino. However, laser excitations of mesonic atoms have not been previously achieved because of the small number of atoms that can be synthesized and their typically short (less than one picosecond) lifetimes against absorption of the mesons into the nuclei1. Metastable pionic helium (π4He+) is a hypothetical14,15,16 three-body atom composed of a helium-4 nucleus, an electron and a π− occupying a Rydberg state of large principal (n ≈ 16) and orbital angular momentum (l ≈ n − 1) quantum numbers. The π4He+ atom is predicted to have an anomalously long nanosecond-scale lifetime, which could allow laser spectroscopy to be carried out17. Its atomic structure is unique owing to the absence of hyperfine interactions18,19 between the spin-0 π− and the 4He nucleus. Here we synthesize π4He+ in a superfluid-helium target and excite the transition (n, l) = (17, 16) → (17, 15) of the π−-occupied π4He+ orbital at a near-infrared resonance frequency of 183,760 gigahertz. The laser initiates electromagnetic cascade processes that end with the nucleus absorbing the π− and undergoing fission20,21. The detection of emerging neutron, proton and deuteron fragments signals the laser-induced resonance in the atom, thereby confirming the presence of π4He+. This work enables the use of the experimental techniques of quantum optics to study a meson.

saturated vs. monounsaturated vs. polyunsaturated

Aside from the number of carbon atoms they contain, fatty acids also differ in the types of chemical bonds between their carbon atoms (see Figure 6.3). A fatty acid's carbon-carbon bonds can be either single bonds or double bonds. If a fatty acid contains only carbon- carbon single bonds in its backbone, it is a saturated fatty acid (SFA); if it contains at least one carbon-carbon double bond, it is an unsaturated fatty acid. A fatty acid with just one double bond in its backbone is a monounsaturated fatty acid (MUFA), and one with more than one double bond is a polyunsaturated fatty acid (PUFA). As with chain length, the number of double bonds can influence the physical nature of a fatty acid. As you can see in Figure 6.3, each carbon atom in an SFA is surrounded (or saturated) by hydrogen atoms. Saturation prevents the fatty acid from bending. Because of this rigidity, SFAs form straight chains. Foods with a high proportion of saturated fatty acids tend to be solid at room temperature because the straight-chain SFAs are highly organized and stack together in a tight array. Foods containing large amounts of SFAs (such as butter) tend likewise to be solids (fats) at room temperature. Compared with SFAs, unsaturated fatty acids (those with double bonds) have fewer hydrogen atoms associated with the carbon backbones—this allows them to bend. In fact, whenever there is a carbon-carbon double bond, there is a kink or bend in the fatty acid backbone. These bends cause unsaturated fatty acids to become disorganized, preventing them from being densely packed. In general, organized molecules such as SFAs are solid at room temperature, while disorganized molecules like PUFAs are liquid. MUFAs have chemical characteristics that lie between those of SFAs and PUFAs, making them thick liquids or semi-solids at room temperature. You may have noticed that olive oil, which is high in MUFAs, is a thick oil at room temperature.

Carbohydrates

Carbohydrates are organic molecules made up of carbon, hydrogen, and oxygen atoms (Figure 4.1). The simplest type of carbohydrate is often referred to as a sugar—a carbohydrate molecule that cannot be broken down further to produce other sugars. Most people think of sugar as a substance used to sweeten their foods. Although this is true, the uses and functions of sugars extend far beyond sweetening. For example, cells use a special type of sugar called glucose as an important source of energy. A carbohydrate consisting of a single sugar molecule is called a monosaccharide, and a carbohydrate consisting of two sugar molecules bonded together is called a disaccharide. Because of the small sizes of these molecules, monosaccharides and disaccharides are referred to as simple carbohydrates (or simple sugars).

Sucrose

Composed of fructose joined with glucose, sucrose is found in many plants (and is especially abundant in sugar cane and sugar beets). Crushing these plants produces a juice that can be processed to make a thick brown liquid called molasses. Further treatment and purification of molasses forms pure crystallized sucrose, otherwise known as refined table sugar. Because most people enjoy the intense sweetness of sucrose, it is often added to foods.

hypoglycemia

low blood sugar (low glucose in blood) blood glucose = blood sugar

omega naming system

system for naming fatty acids; (# of C in fatty acid: # of C=C bonds, omega, position of C=C bond from the methyl end) Image preceding text: "If the first double bond is located between the third and fourth carbons from the omega end, the fatty..."

epigenetics

the study of environmental influences on gene expression that occur without a DNA change The term epigenetics refers to inheritable changes in gene expression that do not involve changes in the DNA sequence itself. A person's epigenome, therefore, refers to all of the chemical compounds that attach to the strands of DNA or proteins associated with DNA that collectively influencing gene expression by turning genes on or off. Even if two people have exactly the same genetic sequences in their DNA, as is the case with identical twins, they may differ in terms of how those genes are expressed due to epigenetic variation. Interestingly, like mutations in DNA, some epigenetic differences can be passed on to the next generation. Scientists now think that epigenetic modifications may play important roles in the development of many chronic degenerative diseases such as cancer, type 2 diabetes, and cardiovascular disease. Growing evidence that nutritional status may affect long-term epigenetic modifications is of great importance to the field of nutrition. For example, there is strong evidence that babies who are malnourished during fetal life, but then experience accelerated growth in childhood, may be at increased risk for cardiovascular disease and type 2 diabetes as adults, partly due to epigenetic modifications in gene expression.