Exploring life chapter 23

features of all vitamins and minerals

-They don't yield usable energy. -They need to be consumed in very small amounts. -Sufficient quantities are consumed in a healthy diet and we don't need to take supplements.

we convert foods into nutrients in four steps

1. ingestion: food is taken into the body 2. digestion: large pieces of food are dismantled by physically and chemically breaking them into absolute molecules 3. absorption: energy rich food molecules are taken into the cells of the body. where they can be used for energy and building materials 4. elimination: the remaining parts of t he consumed foods- mostly indigestible materials are discarded as waste products, after much. water reabsorption occurs The human digestive system is like an assembly line running backward. Imagine starting with an assembled car and dismantling it into its many parts: the tires, doors, windshield, steering wheel. The food entering the digestive system is like the intact car. It enters the assembly line and then passes through four distinct phases, during which the food is progressively chewed up and broken down, the nutrients absorbed by the body, and the non-usable portion of the raw materials discarded as waste products. In the next several sections, we examine in detail the four stages in the processing of food: ingestion, digestion, absorption, and elimination. Throughout these sections, it will be useful to picture the digestive system as a long tube with an opening at each end, the mouth and the anus, and some glands along the way that produce the necessary chemicals to help the process along.

saturated versus unsaturated fats

An important distinction among dietary fats is between saturated and unsaturated fats. If each carbon within the fatty acid chain is bonded to two hydrogen atoms, the molecule carries the maximum number of hydrogen atoms and is said to be saturated. (For a refresher, see Figure 3-9.) Conversely, if some of the carbons are bound to only a single hydrogen, the fatty acid is unsaturated. When saturated, fatty acids are very straight and the fat molecules can be packed together tightly. This causes saturated fats to be solid at room temperature, like butter. When unsaturated, the fatty acids have kinks in the hydrocarbon tail and cannot be packed together as tightly. Consequently, unsaturated fats do not solidify so easily and tend to be liquid at room temperature (think: vegetable oil). Because unsaturated fatty acids can accept one or more hydrogen atoms, they are a bit less stable and more reactive than saturated fatty acids—that isn't a bad thing, it just means they will take part in a greater variety of chemical reactions, making them less likely to be stored as body fat. Dietary trans fats have been in the news because of their tendency to raise levels of low-density lipoprotein cholesterol (LDL), increasing the risk of coronary heart disease. Because trans fats are no longer recognized as safe, the U.S. Food and Drug Administration is taking steps to have them removed from processed foods.

carbohydrates vary in important ways

Carbohydrates vary in complexity. All carbohydrates are formed from carbon, hydrogen, and oxygen in the approximate proportions of CH2O. But they vary considerably in their complexity and break down in the body at different rates. Animals consume carbohydrates in the form of simple sugars (monosaccharides), disaccharides and digestible complex sugars (starch and glycogen), and indigestible complex sugars (fiber). Simple sugars. These are linear or ring structures with three to seven carbon atoms, and include glucose and fructose. Animals can break them down directly through the steps of glycolysis (see Section 5.13), rapidly releasing the stored energy from the bonds of the sugar. Disaccharides and digestible complex sugars. Multiple simple sugars can bond together to form complex but digestible molecules. Disaccharides, such as sucrose (table sugar), are just two simple sugars joined together. Complex sugars, such as starch and glycogen, are large molecules that may consist of hundreds or thousands of glucose molecules connected in dense, branching patterns. For an animal to have access to the energy stored in the bonds of the individual simple sugars, it must first break the bonds that link the sugars together. As these bonds are broken, simple sugars become available for the energy-releasing reactions of glycolysis. Fiber. This is a complex carbohydrate, such as cellulose, that forms the structural parts of plants. Fiber differs from starch and other digestible complex sugars in the type of bond connecting the simple sugars together. Because this bond cannot be broken by any digestive enzyme in humans, fiber is indigestible. As we'll see later, it still plays an important role in digestion and is necessary in the diet to maintain health

Some animals have alternative means for processing their food.

Cellulose could feed the world. It is everywhere—it's the major carbohydrate making up the cell wall around plant cells—and a tremendous amount of energy is stored in its chemical bonds. But most animals don't produce (or acquire in any other way) the enzymes that break down cellulose. Those with the appropriate enzymes, however, gain access to one of the most plentiful sources of chemical energy on the planet. Let's investigate how some animals do it. Ruminant animals, including cows, bison, deer, goats, and sheep, have complex four-part stomachs in which they can digest plant matter that humans cannot. First, the grazing animals chew on the plant material for a while, grinding the tough cell walls. Then they swallow it, and it passes into the first part of their stomach, which contains a huge pool of enzymes and cellulose-digesting bacteria. There, the plant material gets broken down, and much of the cellulose is digested. To increase their energy-extraction efficiency, the animals regurgitate the food back into the mouth, chew it some more, and swallow it again. Called "chewing the cud," the additional chewing further breaks up the plant cell walls so that the bacteria have easier access to the cellulose, digesting more of it. From the first part of the stomach, the food then passes through the remaining chambers, where some additional digestion takes place, before moving into the small intestine and continuing the usual path of digestion. Although cellulose is the primary component of the ruminant diet, ruminants also get a significant amount of protein every day by digesting many of the bacteria working in their gut

absorption moves nutrients from. your gut to your. cells

Chewing, tearing, churning, acidifying, dismantling. You put a lot of energy into reducing food to its simplest component molecules. Eventually, though, there is a payoff. That payoff is absorption, the process by which the energy-rich food particles are taken from the digestive tract into the bloodstream and then into cells throughout the body, where they can be used for energy and building materials. Absorption occurs mainly in the small intestine; however, a few molecules are absorbed in the stomach, including aspirin and alcohol, which explains why drinking alcohol on an empty stomach can lead to unexpectedly rapid inebriation. The key to effective absorption is surface area. For a molecule such as a sugar or an amino acid to be absorbed by the body, the molecule must be in direct contact with the cell membrane of the cell that is going to absorb it. The greater the number of cells that can come in contact with the chyme passing through the small intestine, the greater the amount of nutrients that can be absorbed. For this reason, the tremendous surface area of the small intestine, as well as other structural characteristics, allows efficient absorption of nutrients. 1.The small intestine is long—about 20 feet. 2.Rather than being a straight, smooth tube, it has many folds. 3.The interior lining of the small intestine is made up of thousands of small finger-like projections, called villi. These create lots of nooks and crannies where chyme can come in contact with absorptive cells. 4.Each of the cells along the villi has hundreds of its own tiny thread-like projections, called microvilli. These create micro-nooks and micro-crannies that allow more of the molecules in chyme to directly touch the membranes of absorptive cells.

Digestion dismantles food into usable parts

Digestion occurs primarily in the stomach and small intestines. Food is fuel. But first, it must be completely broken down into its fundamental chemical constituents. Only then can it be absorbed into the bloodstream and used constructively. The process of dismantling large pieces of food, physically and chemically breaking them down into absorbable molecules, is digestion. It occurs primarily in the stomach and small intestine. The stomach As a ball of food moves through the digestive system, it empties from the esophagus directly into the stomach, a muscular J-shaped organ with thick, elastic walls that can expand to accommodate a large meal—as much as 4 liters of material! Where the esophagus connects to the stomach, there is a ring of muscle, called a sphincter. It seals off the stomach once food has entered, preventing regurgitation of the stomach's acidic contents into the esophagus.

small intestine

Digestion only begins in the mouth and stomach. Most chemical digestion actually occurs in the small intestine, a long thin tube connected to the stomach. It is about 20 feet long and winds all around your abdominal cavity. The small intestine begins its job of chemical digestion once the sphincter at the end of the stomach relaxes, creating an opening through which small amounts of chyme are squirted. And as the creamy substance is pushed through the small intestine by the rhythmic contractions of peristalsis, the macromolecules are gradually digested, with help from the pancreas and the liver. The pancreas, nestled at the point where the stomach connects to the small intestine, plays a central role in digestion by secreting pancreatic juice through a duct into the beginning portion of the intestine. This juice is a mixture of chemicals that neutralize the acidic chyme and enzymes that digest carbohydrates, proteins, and fats. Another organ that assists the small intestine with digestion is the liver, which produces bile, a juice that aids in the breakdown of fats. The liver sends the bile to the gallbladder, and from there it passes through a small duct into the small intestine, where the bile initiates the first step in fat digestion. Lipids are not water-soluble, so bile initiates the first step in fat digestion by separating globules of ingested fat into tiny, more easily digested and absorbed droplets that pancreatic enzymes can successfully break down. Enzymes generated by cells within the walls of the small intestine further digest fats, carbohydrates, and proteins. Digestion is completed in the small intestine when the consumed carbohydrates, proteins, and fats have been broken down into their component parts: simple sugars, amino acids, and fatty acids. These simple molecules can then be absorbed into the bloodstream.

vitamins and minerals are necessary for good health

It would be nice if we could take a pill that would make us more healthy or fit. Vitamin supplements are not such miracle pills. They're a bit like security guards at a museum: above a certain number, they don't make the museum any better, but in their absence, the museum is likely to become a whole lot worse. And while vitamin supplements are not a health panacea, vitamins and minerals do have important roles in nutrition. Vitamins are organic compounds required by the body in small amounts for normal growth and health. Minerals are chemical elements, other than those commonly found in organic molecules (carbon, hydrogen, oxygen, and nitrogen), some of which are required in the diet in small amounts. There are three features common to all vitamins and minerals. 1.They don't yield any usable energy. Rather, they serve as collaborators with enzymes to enable the processing of the proteins, carbohydrates, and fats we eat, and they catalyze a wide range of other chemical reactions around the body. 2.They need to be consumed in much smaller amounts than proteins, carbohydrates, and fats. This is because they tend to serve as reaction catalysts and so can be recycled and reused again and again. 3.If we have a healthy diet, we tend to consume sufficient quantities of vitamins and minerals in our food. Although their biological roles are frequently similar, vitamins and minerals have a fundamental chemical difference. Vitamins are organic molecules—they contain carbon—and are more fragile, easily destroyed by heat and other chemical or physical extremes. Minerals, on the other hand, are elements, so they can't be broken down further or they lose their chemical identity. They stay in your body until they are excreted. Seventeen minerals have been identified that are essential to the human diet.

ingestion is the first step in the. breakdown of food

The intake of an item of food into your body, called ingestion, is quick. Typically lasting less than a minute, ingestion involves your mouth, teeth, tongue, and esophagus, as well as your salivary glands. (In fact, just thinking about food can stimulate saliva secretion!) Salivary glands secrete mucus through tiny ducts throughout your mouth. Mucus lubricates the food to help it pass into your stomach. The glands also secrete an enzyme called alpha-amylase that initiates the process of digestion. Alpha-amylase breaks the bonds holding together the starch molecule and releases a bit of glucose that can be used for energy. About 20% of the ingested starch is broken down in the mouth. Little or no breakdown of protein and fat molecules occurs in the mouth. Interestingly, across cultures, humans have developed common ways of preparing their food, including using heat and marinating food in acidic solutions, such as vinegar or lemon juice. Because harsh conditions such as heat and acid disrupt the tissue of food items, they increase the efficiency with which digestive enzymes can make contact with the food and break it down. You start chewing food in your mouth to tear and grind it into little bits. This is a first important step toward harvesting the energy stored in the chemical bonds of the food. Several different types of teeth have evolved in mammals and other animal species that allow different types of food items to be processed. Incisor and canine teeth, in the front, are used for biting and tearing food. Behind them are premolars and molars, used for grinding and crushing food. Just by looking at the type of teeth an animal has, we can learn a lot about its diet. Birds have no teeth. This explains why they can sometimes be seen eating gravel. The gravel has no nutritional value, but it collects in the gizzard, one of the two chambers of a bird's stomach, where it helps to grind up food. This is an important digestive step for birds because, lacking teeth, they must swallow their food whole. Consequently, when it first arrives in the stomach, the food hasn't been ground up at all, which can reduce the efficiency of digestive enzymes.

absorption moves nutrients into the bloodstream

The molecules that can be absorbed in the small intestine include simple sugars, individual amino acids (and, occasionally, short chains of two or three amino acids), fatty acids, vitamins, and minerals. When a nutrient molecule is small enough to be absorbed, it gravitates toward the villi lining the interior of the small intestine. The tiny particles become trapped in the microvilli and are drawn across the cell membranes into the cells. The nutrients then diffuse out of the cells and into the interstitial fluid bathing the cells. From here, the nutrients are picked up by capillaries and move into the bloodstream, where they can be delivered to the organs and tissues that need them. There is a food myth that if certain foods are combined, digestion and absorption are impaired. This is not true. The myth ignores a couple of facts: the body is able to produce its digestive enzymes for fats, proteins, and carbohydrates simultaneously, and it can absorb nutrients regardless of which other nutrients are also in the digestive tract. In fact, the contrary is often true. Many foods enhance the absorption of nutrient molecules in other foods. Vitamin C in citrus fruits, for example, increases the efficiency of iron absorption from a meal of beef or beans.

essential vitamins

Thirteen vitamins essential to humans, also described in Figure 23-12, have been discovered. They fall into two groups, based on whether they are soluble in fat or water. The water-soluble vitamins include eight vitamins that are part of the vitamin B complex, plus vitamin C. There is some variation among species in the ability to manufacture (rather than needing to ingest) vitamins. Cats and dogs, for instance, can make vitamin C and so don't need to consume it. There are four types of fat-soluble vitamins: A, D, E, and K. Because they are stored in fatty tissue and the liver until they are needed, they don't need to be consumed as regularly as water-soluble vitamins. Because excess water-soluble vitamins can be removed by the kidney and excreted in urine, they are less likely to reach toxic levels than fat-soluble vitamins.

food passes from the mouth through the. esophagus to the stomach.

While you are chewing your food, your tongue forms the food into a ball shape that can be swallowed. The saliva-coated food is pushed to the back of your mouth by your tongue. There, your throat opens to two passageways, the trachea and the esophagus. The trachea, also called the windpipe, connects to your lungs. The esophagus connects to your stomach. Food destined for your stomach is kept from entering your trachea by a fast but critical maneuver in which your voice box moves up (due to muscle contractions), causing a flap of tissue (called the epiglottis) to be pushed over the entrance to the trachea just as you begin to swallow. If the ball of food you swallow is too big, it can get stuck at the beginning of the esophagus, wedging against the flap of tissue in front of the trachea—and this can cause you to choke by blocking air from getting into your lungs. Once the ball of chewed food makes its way into the esophagus, waves of smooth muscle contractions, called peristalsis, propel the food down the esophagus and into the stomach, where digestion continues. Because of peristalsis, it's not necessary for you to be sitting upright or standing when you eat. Even if you are standing on your head, the food is pushed down the esophagus to the stomach

Dietary supplements are usually only needed under special circumstances.

Who needs to take supplements? •Women with iron deficiencies •Pregnant and breastfeeding women •Post-menopausal women •Those on extremely low-calorie diets •People with limited consumption of milk or very low sun exposure •Those with intestinal absorption problems

Animals have a variety of diets

carnivorous: animals that consume only other animals herbivorous: animals consume only plants omnivorous: animals consume both plants and animals While all organisms need food, there are some very different ways to get it. Plants and other photosynthetic organisms (including many bacteria and protists) make their own food, through photosynthesis. But animals do not have the cellular machinery to do this, and so they must eat other organisms. Most animals eat plants, while some animals eat other animals. But in every case, though some organisms are able to capture solar energy to make food, all animals must acquire that energy indirectly by consuming other organisms. In the animal world, species fall into three groups based on their diets: Carnivores Predatory animals that consume only other animals are carnivores. They include spiders and snakes; mammalian species such as wolves, seals, bats, and cats; and hawks, owls, and other birds of prey. Some carnivores don't actually kill their prey but instead just suck nutrient-rich fluids from them. Mosquitoes and ticks are carnivorous fluid-feeders. Herbivores Food that is easy to get, but hard to digest— that sums up life as an herbivore, an animal that consumes only plants. Because plants are plentiful in most habitats and can't run away, they are easy targets for predation. To protect themselves, however, plants tend to carry many toxic compounds that are difficult or impossible for animals to break down chemically. Nonetheless, many herbivores have developed digestive adaptations to overcome these difficulties. We'll explore a couple of them in detail in Section 23.13. Examples of herbivores include beavers, tortoises, and caterpillars, as well as many species of birds, squirrels, and large grazing mammals. And just as there are fluid-feeding carnivores, there are fluid-feeding herbivores. Aphids, for example, pierce the surface of plants and suck their sugary sap. Omnivores Most humans, as omnivores, eat plants and animals and can digest both efficiently. We share this diet with numerous other species, including cockroaches. Omnivores come in all sizes, from bears to raccoons, from chickens to flies and wasps.

animal versus plant proteins complete, incomplete proteins

complete proteins: contain all nine essential amino acids almost all animal proteins are complete incomplete proteins: do not contain. all nine essential amino acids almost all plant proteins are incomplete, but a mix of proteins from two or. more plants is often complete such as. rice. and beans All proteins are not created equal. Animal products vary in their amino acid composition. All nine essential amino acids are found in milk, eggs, meat, poultry, cheese, and fish; these animal products have "complete" proteins. Most proteins that we get from eating plants are not complete. Corn proteins, for example, have only six of the essential amino acids, as do beans (but not the same six). In fact, with only a few exceptions (including soybeans and quinoa), plant proteins do not have all nine. A healthy diet is one containing not just a sufficient amount of protein but a sufficient amount of each of the nine essential amino acids that we need.

essential minerals

minerals that animal cells need in small amounts

carbohydrates and lipids provide bodies with energy and more: carbohydrates

function: once carbohydrates are broken down, glucose provides energy to fuel mocement, growthm and all cellular activities in the body source: fruits, vegetbales, and grains storage: carbohydrates are stored in the liver and muscle cells as glycogen for about a day before being broken down to provide energy can be converted to fat and stored in fat cells Carbohydrates: Fuel for living machines Carbohydrates are the primary fuel on which animal bodies run. In humans, nearly all of the energy used by our brain every day comes from the simple carbohydrate glucose. As we saw in Chapter 3, all carbohydrates are made primarily from carbon, hydrogen, and oxygen. When the bonds between these atoms are broken, energy is released that can be captured by the body and used to fuel movement, growth, and all the other cellular activities that require energy. We get the majority of our dietary carbohydrates from fruits, vegetables, and grains (Figure 23-8). And, as with proteins, the breakdown of carbohydrates for energy generates 4 kilocalories per gram.

Energy rich fats.

function: provide a dense source of energy that can efficiently stored in the body, and fats aid in keeping the body warm source: butter, cheese, oils, eggs, and meat storage: fats are stored in fat cells throughout the body Fats are long-term energy-storage experts. Fats in the animal diet function primarily as a dense source of energy that can be efficiently stored in the body. The average person has about four or five weeks' worth of stored energy in the form of fat. Compared with carbohydrates or proteins, a given amount of fat contains more than twice as much stored energy: 1 gram of fat produces 9 kilocalories. Another feature that makes fats particularly efficient as energy-storage molecules is that, because they are hydrophobic, they are stored without binding to water. (Because fats are poor conductors of heat, fats stored in a layer just beneath the skin can also help to keep the body warm.) Fats vary in the nutrition they provide. In our diet, fats usually come in the form of fatty acids, long chains of carbon atoms with hydrogens attached. With proteins, we saw that there are essential and nonessential amino acids. Similarly, with dietary fats, there are essential and non-essential fatty acids. The essential fatty acids—including linoleic acid, in the omega-6 family of fatty acids—are those that humans cannot produce and must be consumed. Linoleic acid is essential as a building block for signaling molecules, such as some hormones; a deficiency can lead to infertility and difficulty lactating. Another essential fatty acid is linolenic acid, essential for normal growth and development, especially in the eyes and brain.

proteins in food are broken down to build proteins in the body

protein: raw material for growth function: once proteins are broken down, the amino acids are used as. the raw materials. to build new complex proteins such as hemoglobin and muscles source: animals: egg whites, shrimp, tuna, poultry, and meat plants: grains, vegetables, nuts, seeds, and legumes storage: amino acids are usually stored for less than half a day. before being reassembled into proteins throughout the body proteins can be converted to fat and stored in fat cells •Proteins are building materials, catalysts (enzymes), and a source of energy. For optimum health, organisms must consume many types of nutrients. In this section and the next, we investigate the nutritional features of the three types of calorie sources: proteins, carbohydrates, and fats. Proteins provide the raw materials for growth. Protein in our diet is principally a building material. As described in detail in Sections 3.10-3.12, protein molecules are made of long chains of amino acids linked together, like beads on a string. Once eaten, protein molecules are broken down into the individual amino acids, much like removing the beads from the string one at a time. It's only in the form of individual amino acids (and, occasionally, chains of two or three amino acids) that proteins can be absorbed by the digestive system. Our bodies can then reorder the beads, making proteins that are different from those we ate. Besides serving as building materials, the proteins we build from amino acids function as enzymes, catalyzing chemical reactions (see Sections 3.13 and 3.14). Proteins, whether from our diet or from our own body tissues, can also be broken down to release energy or to be converted to and stored as fat. Protein breakdown happens, for example, when we are consuming too few (or too many) calories to sustain necessary growth and activities. It also happens during a long exercise session. Our bodies can use protein as fuel because we have a variety of methods of converting one type of chemical to another to release or store energy (see Figure 5-36). The breakdown of proteins for energy generates 4 kilocalories per gram. However, when a protein is broken down for energy, the nitrogen it carries cannot be used to build new proteins; instead, it is excreted in our urine. In essence, burning protein for fuel wastes valuable nitrogen (which is not found in carbohydrates and lipids) that could be used for building new protein.

amino acids are the building blocks of proteins

•Animals require 20 different amino acids to make proteins. •Humans can only make 11 of the 20. -Essential amino acids are the 9 amino acids we must get from the foods we eat. -The remaining 11 amino acids are non-essential amino acids. Animals, including humans, require 20 different amino acids to make proteins. Most animals, however, can produce only about half of these amino acids themselves—for example, under most normal conditions humans can make only 11 of the 20 amino acids. The 9 amino acids we cannot produce we must get from the food we eat; we say that these 9 amino acids are essential amino acids, and the remaining 11 are non-essential amino acids.

how many calories do you need?

•BMR is relative to the size of an organism. •It also varies with activity level. To estimate how many kilocalories an individual expends each day, we first establish an energy baseline. This is called the basal metabolic rate, or BMR, and refers to the amount of energy the individual expends with no food in the digestive tract, in a neutral-temperature environment, doing nothing more than the equivalent of sitting on a couch all day. For humans, the BMR is approximately 1 calorie per hour per gram of body weight, or about 1,400 kcal/day for a woman (weighing 120 pounds) and about 1,700 kcal/day for a man (weighing 160 pounds). The difference between the sexes is mostly a function of the difference in body size. The BMR allows easy comparisons across species because it is so clearly defined, but to accurately evaluate organisms' energy needs, we must account for how active they are. Individuals need about 50% to 100% more kilocalories per day than their BMR. A 120-pound woman requires 1,800-2,400 kcal/day, and a 160-pound man requires about 2,400-3,200 kcal/day. From one animal species to another, basal metabolic rates differ tremendously. The BMR of a tiny shrew, for example, is about 35 times higher than that of a human. The shrew's heart beats more than 500 times per minute at rest! Let's estimate what a shrew needs to eat each day. If it weighs 5 grams and has a BMR of 35 calories per hour per gram, we can calculate as follows: Body weight x Energy needed each hour x hours per day = Animal's caloric need Or, 5g x 35 cal/g/hr x 24 hr/day = 4,200 cal/day (= 4.2 kcal/day) Thus, the shrew would need 4.2 kcal/day if it were at rest. But if it needed another 4 kcal/day for its normal activities, it would have to eat about 8 kcal/day. A pure source of carbohydrate or protein carries about 4 kcal/gram. Thus, every day, the shrew must find and consume at least 2 grams of food. This is a challenge when you weigh only 5 grams. The task is equivalent to a 200-pound man finding and eating 80 pounds of food every day. Animals that fast or hibernate for long periods of time must also prepare by consuming many kilocalories. During the breeding season, male elephant seals eat little or nothing; they spend much of their time battling with other males for dominance and (if they are among the most dominant) mating with females. Over these 100 days, a male that begins the season at 6,600 pounds (3,000 kg) may lose one-third of his body weight. Consequently, the male seal's caloric requirements are much higher in the months leading up to the breeding season.

carbohydrate storage

•Carbohydrates in our body are stored mostly as glycogen in liver and muscle cells. -Breakdown of stored glycogen releases glucose into the bloodstream. •Large amounts of water are bound to stored glycogen. When dieting, there is dramatic initial loss of weight due to the loss of water that was bound to glycogen Carbohydrates are stored in the body as glycogen. We store carbohydrates mostly as glycogen (see Section 3.3) in liver and muscle cells. At any given time, we can store only about one day's worth of energy. When our bodies need energy for an activity, a signal is sent that causes the release of enzymes that break the bonds holding together the highly branched glycogen. This glycogen breakdown produces a flood of glucose into the bloodstream and in the muscles where the energy is needed. Large amounts of water are bound to stored glycogen: 4 pounds of water for every pound of glycogen. Consequently, as glycogen in your liver and muscles is used, the water bound to it is released from the tissue and lost as urine. If stores of glycogen are depleted in the initial stages of a weight-loss diet, there is a dramatic initial loss of weight due to the loss of water that was bound to the glycogen. As your body starts utilizing stored fat, the rate of weight loss slows considerably.

USDA recommendations

•Control weight. •Engage in regular physical activity. •Eat a variety of fruits, vegetables, and grains, and nonfat or low-fat milk/milk products. •Keep your diet low in saturated fat, cholesterol, and total fat. •Keep your diet low in sugar and sodium. •Limit alcohol consumption. The U.S. Department of Agriculture (USDA) has established dietary guidelines to help people plan a healthy diet. The USDA updates the guidelines every five years in order to incorporate advances in nutritional science. In addition to urging people to take steps to ensure that their food is safe to eat, the USDA also recommends the following: 1) Keep your weight within the recommended range. 2) Be physically active. 3) Choose a variety of fruits, vegetables, grains—particularly whole grains—and nonfat or low-fat milk and milk products. 4) Keep your diet low in saturated fat, cholesterol, and total fat. 5) Keep your diet low in sugars relative to complex carbohydrates and fiber. 6) Keep your diet low in sodium. 7) If you consume alcoholic beverages, do so in moderation. Food labels are a useful tool that can help in adhering to these guidelines and selecting a balanced diet. Ingredient lists, too, are very valuable, listing every ingredient in order of amount (by weight). Ultimately, following these guidelines can help reduce the incidence and severity of chronic diseases. Complicating the USDA's dietary guidelines is the fact that we need to reduce our caloric intake as we get older. Metabolic rate falls slowly but surely, beginning around age 30. Consequently, without an increase in activity, eating the same amount of food— even if it is a healthy diet—leads to weight gain. Most adults gain about half a pound every year throughout their thirties, forties, and fifties. It is important to note that people who do not consume meat, poultry, fish, and/or milk products can still have a balanced diet. Legumes, seeds, and nuts can provide many of the same nutrients found in meat, including protein. Dark leafy vegetables can provide iron, another nutrient plentiful in meat. And in fact, because such diets tend to be lower in fat content, they can be valuable in helping to maintain a healthy body weight.

Elimination removes unusable material from your body

•Fiber is indigestible, but necessary for proper colon function. •Water absorption is balanced by the colon. •Diarrhea •Constipation You are much better at conservation and recycling than you imagine, especially when it comes to the fluids and solids you consume. The last phase in the breakdown of food, elimination, takes place as what's left of the chyme—mostly indigestible materials—leaves the small intestine and enters the large intestine, also called the colon. Much larger in diameter than the small intestine (about 3 inches vs. 1 inch for the small intestine) but only about 3-6 feet long, the large intestine serves to absorb water, salts, and some vitamins (Figure 23-21). The last part of the large intestine, the rectum, serves as a storage compartment for the remaining parts of consumed food, the feces, which is later defecated. Some material in food cannot be digested or absorbed. This material is called fiber and includes plant gums and cellulose. For this reason, when fiber is consumed, it increases fecal mass. With additional mass, more water is attracted, speeding the movement of chyme through the colon, softening the feces, and making it easier to eliminate (that's why too much fiber can lead to diarrhea). Fiber also binds to bile, causing some of it to be eliminated, reducing the body's ability to absorb cholesterol from food. It is important to achieve just the right balance of water absorption in the large intestine. If too much is absorbed, the remaining indigestible material becomes too solid and can't move easily through the last part of the digestive system, causing constipation. Alternatively, if too little water absorption occurs, the body loses more water through diarrhea. In the colon, water is removed from feces through osmosis. First, salts are pumped out of the colon into surrounding cells, and then water moves out by simple diffusion (toward the area with more ions). This process can be manipulated as a treatment for constipation. Laxatives contain magnesium salts, which are so slowly absorbed that they are still largely intact when they reach the colon. Laxatives work by increasing the salt concentration in the colon, so that more water remains. This additional water makes it easier to excrete feces (but can sometimes cause diarrhea).

why do organisms need food?

•Food provides the raw materials for growth and the fuel to make it happen. Chili fries. Lime-marinated tofu kabobs. A big vanilla milkshake. Although we dress food up in an almost infinite variety of combinations, it really boils down to two simple biological needs: raw materials and fuel. Just as a carpenter needs wood and a car needs gas, living organisms need raw materials and fuel to function. Our need for fuel is one of the two reasons why we must eat: building a body, moving around, reproducing, and just staying alive all require energy. Food provides that energy. The other reason is that to grow and to build the myriad complex molecules required for life, we need raw materials: molecules containing carbon and nitrogen and phosphorus, to name just a few. Food provides these raw materials. The food we eat is physically and chemically broken down into its fundamental macromolecular components in the process of digestion. Our body quickly breaks down food and separates it into the usable and unusable. The usable materials are carbohydrates, lipids, proteins, vitamins, minerals, and water and are referred to as nutrients. These six crucial substances are used for energy, raw materials, and maintenance of the body's systems. The unusable materials pass through the digestive system and are eliminated. Ultimately, an organism's body weight reflects a balance between the energy carried within the molecular bonds of the food it consumes and the energy burned in the processes of living. Any surplus calories are stored, usually as fat or as glycogen (see Section 3.3), a form of carbohydrate stored primarily in muscle and liver tissues.

what constitutes a healthy diet

•Food quality and quantity are the main factors in choosing a healthy, balanced diet. Choosing a variety of foods from each of the basic food groups creates a healthy diet At the most basic level, just two requirements—quality and quantity—must be satisfied in the design of a healthy diet (Figure 23-24). First, a diet must contain sufficient amounts of each of the six categories of nutrients: water, proteins, carbohydrates, lipids, vitamins, and minerals. And second, a healthy diet must contain sufficient energy to support an individual's metabolic needs without containing a surplus of calories. But it is not easy to find the best strategy to satisfy these requirements. Balance in a diet is important, because no one food is completely adequate. Milk, for example, is a good source of protein and calcium but does not contain sufficient iron for most adults. Meats, on the other hand, are rich in iron (as well as protein) but generally are poor sources of calcium. For this reason, nutritionists recommend consuming a variety of foods from each of the basic food groups— (1) grains, (2) vegetables, (3) fruits, (4) milk and other dairy products, and (5) meats, poultry, and beans—and small quantities of unsaturated fats, such as olive oil and canola oil, and exercising daily to manage body weight.

Protein storage

•Proteins and amino acids cannot be stored for very long in the human body. -Usually less than a day •Essential amino acids need to be consumed daily. -45g protein/120-lb. woman per day -60g protein/160-lb. man per day •Health and activity levels affect protein needs. •Plants need proteins, too! Proteins cannot be stored in the body for long. Proteins and the amino acid pool they generate on digestion cannot be stored very long in our bodies, usually less than a day. By that point, they are broken down and the nitrogen we don't need is excreted. Because our bodies are making proteins constantly, we must take in all of the essential amino acids over the course of each day. The recommended daily intake of protein is approximately 0.8 grams per kilogram of body weight. This translates to about 45 grams of protein for a 120-pound woman (and about 25 additional grams per day for pregnant or lactating women) and 60 grams for a 160-pound man. Regular muscle-building exercise can double or even triple this requirement, but it is important to remember that protein on its own does nothing to stimulate muscle growth; it simply serves as the source for the raw materials. Every organism—even plants—must build enzymes and so needs protein. This is dramatically apparent in the Venus flytrap, a plant that captures and consumes insects in order to supply itself with protein. The plant releases digestive juices that break down the insect into its usable nutrients. Venus flytraps grow in places where the soil is of poor quality, particularly with respect to nitrogen; they can extract nitrogen from the animal prey they capture. The green leaves and stems of Venus flytraps, however, should serve as a reminder that these plants are also photosynthetic. The nitrogen they need (and resort to carnivory to obtain) helps them build the enzymes necessary to conduct photosynthesis.

the cecum

•Some insects make cellulose-digesting enzymes. •Some animals employ bacteria in their digestive tract. -Cecum -Human cecum •Coprophagy Some insects, including silverfish, produce cellulose-digesting enzymes. These enzymes make it possible for them to eat books and paper, which are made of plant products containing cell walls made from cellulose. Not all animals that have cellulose-digesting bacteria in their guts are ruminants. Horses, rodents, rabbits, and koalas have an outcropping of the digestive tract, called the cecum, in which bacteria live. Usually considered the beginning of the large intestine, the cecum is a separate chamber where the food goes for a while and the cellulose gets digested, before the food continues down the intestine. In animals that don't break down cellulose, the cecum is much smaller. The cecum is almost nonexistent in the meat-eating coyote, for example, while the similarly sized koala has a cecum that is 2 meters long. Within this lengthy chamber, bacteria convert shredded eucalyptus leaves into usable food. In humans, no cellulose-digesting bacteria live in the cecum. (Connected to the human cecum, and developing from it, is a small pouch-like structure called the appendix.) Rabbits and rodents have mastered another way to increase their ability to extract energy from their food: they pass it through their entire digestive system twice. Eating some of their feces—a process called coprophagy—allows them to significantly increase their nutritional intake from their cellulose-laden diet. Numerous other species—including pigs, elephants, cats, gorillas, and chimps—also resort to coprophagy from time to time, although the reasons are not always clear and may be related to vitamin and mineral absorption or the acquisition of bacteria to aid in digestion.

calories count: organisms need sufficient energy

•The energetic value of food is measured in calories. •1 kilocalorie = 1,000 calories Have you ever gone a whole day without eating? Or have you ever spent a few weeks trying to reduce your caloric intake? Living in a state of hunger can be torturous. In the early 1990s, eight human "guinea pigs" learned this the hard way. Living in the Biosphere 2 dome—a self-contained, 3.2-acre world filled with plants and animals that was designed to explore sustainable living with minimal environmental impact—they took part in an experiment on the effects of a low-calorie diet, among other things. The findings were not surprising: all the participants lost weight, but they also became very unhappy. They argued constantly and frequently squabbled over meal portions in what they dubbed "the hunger dome." Put simply, humans (and all other living organisms) need a steady and sufficient flow of energy to function well. The energetic value of food is measured in very small amounts of energy called calories, with a single calorie defined as the energy required to raise the temperature of 1 gram of water by 1° C. However, the term can be a bit confusing: in discussing human consumption, the term "calorie" actually refers to a kilocalorie (kcal), which is 1,000 calories

Stomach function

•The stomach physically and chemically breaks down food into chyme. -Pepsin •Stomach acid kills most of the bacteria we consume. The stomach has three functions: 1.It physically breaks down and mixes food, through churning of the muscles surrounding the stomach. 2.It secretes acid to further break down food chemically and to kill bacteria. 3.It begins some chemical digestion of proteins. The presence of food in the stomach causes cells in crevices within the stomach lining, called gastric pits, to rapidly produce hydrochloric acid and pepsinogen, a precursor molecule quickly converted to the protein-dismantling enzyme pepsin. The acid corrodes proteins in the food, breaking them into smaller pieces, which can then be broken into their constituent amino acids by the pepsin. The burning sensation of indigestion occurs when the sphincter between the esophagus and the stomach leaks, and acidic contents of the stomach move back up the esophagus. It usually occurs when a person eats too much or too quickly. Antacids can neutralize acid from the stomach. Remember, though, that the high acidity in the stomach is important for digestion, so while reducing it may alleviate heartburn, antacids can reduce the digestive efficiency of the stomach. Besides helping to chemically break down food in your stomach, the strong acid also kills most of the bacteria that you might consume. Generally, bacteria can't survive when the pH gets so acidic (although some microbes occasionally sneak through in higher-pH pockets of food passing through the stomach). The end result of all the churning and digesting is that the food you have eaten is no longer recognizable. It becomes a creamy, acidic liquid called chyme. While some carbohydrates and proteins have been partly or completely digested in the stomach, fiber is undigested, as are the lipids, which are suspended as greasy droplets within the chyme. At the other end of the stomach is another ring of muscle. About every 20 seconds, it opens just a tiny bit and squirts a few tablespoons of chyme into the small intestine

water is an essential nutrient

•Water -transports nutrients and waste in the body. -takes part in chemical reactions. -serves as a solvent for many vitamins and minerals, amino acids, and sugars. -lubricates many joints, the spinal cord, and the eyes. -helps regulate body temperature. Water constitutes about 65% of the body weight of most mammals and is considered an essential nutrient. This water plays important roles in both the intracellular fluid and the extracellular fluids, including blood. Water in body fluids serves a variety of critical purposes, all of which can be impaired when the animal becomes dehydrated. Water has the following functions: •Transports nutrients and waste in the body •Takes part in chemical reactions •Serves as a solvent for many vitamins and minerals, amino acids, and sugars •Lubricates many joints, the spinal cord, and the eyes •Helps regulate body temperature A person expending about 2,000 kcal/day needs about 2-3 liters of water each day. Water intake can come from drinking water, milk, and other beverages and from water in foods, and water is released as a by-product of many chemical reactions. Water intake must offset the water lost in urine, feces, respiration, and sweating. It is important to remember, too, as we saw in the Chapter 4 Street Bio, that drinking too much water too quickly can lead to water intoxication—which sometimes happens to marathon runners during a race. When coupled with the loss of salt through sweating, over-consumption of water can lead to severe sodium imbalance. This imbalance causes dizziness, nausea, confusion, and swelling of the extremities. It can even lead to swelling of the brain, causing death. Water usage varies among animal species. Tremendous water efficiency has evolved in some desert mammals—the kangaroo rat, for example, gets all of the water it needs not from drinking but from food and metabolic processes. Among the marine birds and reptiles, most are able to drink salt water. They can do this with the aid of salt glands that remove and excrete the excess salt they consume.

huge colonies of bacteria live in the colon

•We acquire bacteria both during and after birth. -Vaginal and fecal bacteria -Food and environment •Usually, our own body cells are outnumbered by the bacterial cells living on and in us. •We obtain nutrients we need as metabolic by-products made by bacteria. •Antibiotics kill both harmful and beneficial bacteria. Huge colonies of bacteria live in the colon. Before we are born, our guts are free of bacteria. During birth, we acquire some of our mother's vaginal and fecal bacteria. By the age of two, all of the numerous bacterial species present in adults have been acquired from food and the environment. At any given time, we generally contain more bacterial cells than our own body cells (see Figure 15-4). The bacteria are usually harmless and live off the undigested materials that end up in the colon. Bacteria also release important metabolic by-products, such as vitamin K and one of the B vitamins, biotin. About half of the feces that we excrete each day is made up of dead bacterial cells; the rest is mostly indigestible materials such as cellulose and other types of fiber. Antibiotics frequently (and unintentionally) kill a large proportion of the colon bacteria, in addition to whatever illness-causing microbe they were prescribed to kill. This can have negative side effects. First, the transit of undigested materials through the colon may not be slowed down as much as usual, in which case less water is removed and diarrhea results. Also, a reduction in the production of vitamin K and biotin can lead to deficiencies.

minerals

•are chemical elements required for normal body functioning.

vitamins

•are organic compounds required in small amounts for normal growth and health.