Ch.4. Carbohydrates in Food

What are polysaccaharides?

Polysaccharides are chains of glucose. These glucose chains are known as starch, fiber, and glycogen.

Glucose metabolism is intricate and involves many metabolic pathways. These pathways include:

(1) the regulation of blood glucose concentration (2) the immediate use of glucose for energy (3) the storage of glucose as glycogen (4) the use of excess glucose for fatty acid synthesis (5) the production of glucose from lactate, amino acids, or glycerol

Why is a small amount of fructose usually added to sports drinks?

(1) to improve flavor, which encourages fluid consumption (2) to improve GI comfort, a practical issue for many distance runners (3) to enhance the ability to take up and utilize carbohydrate during exercise. - When too much glucose is consumed, the glucose transporters may become saturated, limiting glucose absorption. - The addition of fructose takes advantage of the different absorption pathway and may provide for increased carbohydrate uptake during exercise that may in turn aid performance.

Intake during Exercise Training and Competition

- Athletes who perform endurance exercise or intermittent high-intensity exercise for more than 1 to 2 hours are at risk for glycogen depletion, low blood glucose, and fatigue during training and competition -- Ultradistance racers, triathletes, marathon runners, distance cyclists, and other long-duration athletes must consume both carbohydrates and fluids during heavy training and competition - For exercise lasting approximately 1 to 2.5 hours, it is typically recommended that 30 to 60 g of carbohydrate be consumed each hour during prolonged exercise as either fluid or food -- Although the need is not as great, intermittent high-intensity athletes such as soccer and basketball players also benefit from carbohydrate (and fluid) intake during practices and games.

The current recommendation for total carbohydrate intake for athletes in training is

3 to 12 g/kg of body weight daily - This recommendation assumes that energy (caloric) intake is adequate.

Depending upon the amount and type of carbohydrate consumed, the rise of blood glucose and its return to its resting level may take from...

30 minutes to 1-2 hours

The normal concentration of blood glucose when measured under fasting conditions is approximately...

70 to 100 mg/dL (3.89-5.55 mmol/L)

The majority of research on pre-exercise carbohydrate intake has been conducted on...

Endurance athletes while training.

What do brush border enzymes do?

Enzymes specific to each disaccharide are found in the brush border of epithelial cells lining the interior border of the small intestine. - Brush border enzymes break down disaccharides into monosaccharides ex: sucrase, lactase, and maltase - Sucrase can also break down maltose to two molecules of glucose, but dietary maltose intake is usually small.

Fiber

Fiber is a tightly packed polysaccharide that is the structural component of plants. Humans lack the enzymes needed to break down fiber.

How does fructose absorption differ from glucose absorption?

Fructose must also attach to a specific carrier to cross the wall of the small intestine. However, the carrier that transports fructose from the intestinal lumen into the epithelial cell, GLUT-5, is different from the glucose carrier. - Fructose is absorbed into the intestinal epithelial cell by facilitated diffusion by the GLUT-5 transporter and into the blood by the same process and transporter (GLUT-2) as glucose. - Fructose is also absorbed into the epithelial cell by passive facilitated diffusion. - Once in the epithelial cell, fructose is transported into the blood by the same process and membrane-bound transporter, GLUT-2, as glucose

What monosaccharide is typically present in the largest amount after digestion?

Glucose is typically present in the largest amount because it is the only monosaccharide found in starch and maltose, and it is half of each molecule of sucrose and lactose.

What are good sources of dietary fiber?

Sources of dietary fiber are legumes, seeds, fruits, and vegetables, including the starchy vegetables and whole grains

Starch

Starches in food may be straight chains (amylose) or branched chains (amylopectin). - Starches in food may be straight chains (amylose) or branched chains (amylopectin). Enzymes in the digestive tract help break down these chains into their basic component, glucose - Starch is found in many foods, including grains, legumes (beans), and tubers. (Tubers, such as white or sweet potatoes and yams, have underground stems and are often referred to as starchy vegetables.)

Sugar Alcohols

Sugar alcohols can be formed from monosaccharides and dissacharides. - They are less sweet and typically have fewer kilocalories than sucrose (table sugar). - Common sugar alcohols such as glycerol (Glycerol is part of the structure of triglycerides, the most common type of fat found in food and in the body.) sorbitol, mannitol, and xylitol are often added to foods as sweeteners

Fructose

Sweetness (100 sweetness of table sugar)= 170 Glycemic index= 19 Location= In the body, found temporarily in the liver before being converted to glucose. In food, found naturally in fruits and vegetables and added to processed foods, often as high-fructose corn syrup.

Galactose

Sweetness (100 sweetness of table sugar)= 30 Glycemic index= unknown Info: found naturally in food only as part of the disaccharide, lactose.

Glucose

Sweetness (100 sweetness of table sugar)= 75 Glycemic index= 100 Location: In the body, found circulating in the blood and stored as glycogen. In food, generally found as part of disaccharides and polysaccharides (starches). When added to food, glucose is referred to as dextrose.

Cells in the body that are considered to be highly aerobic (for example, heart muscle cells, slow-twitch skeletal muscle fibers) will predominantly metabolize glucose...

aerobically via oxidative phosphorylation.

The major sources for glucose formation by gluconeogenesis

are lactate and amino acids.

Carbohydrate is used as the predominant source of energy via anaerobic glycolysis during

high-intensity, short-duration anaerobic exercise or through oxidative phosphorylation during moderate- to high-intensity aerobic exercise. - muscle glycogen is a primary source of energy during moderate to intense aerobic exercise and during higher intensity, repeated intervals

During exercise...

insulin secretion is suppressed, and glucagon secretion is stimulated.

How glucose is metabolized depends on a variety of factors:

type of cell, enzymatic capability of the cell, energy state, hormonal status, training history, and intensity of exercise. - As glucose is taken up into a cell, it can be either metabolized or stored for later use, depending on the current energy state of the cell.

Digestion vs. Absorption

- Digestion is the breakdown of foods into smaller parts so the body can absorb them. - Absorption involves taking these smaller parts into the cells of the intestine, where they will then be transferred into the blood for transportation to other parts of the body.

How is glycerol converted to sugar?

- Glycerol is a sugar alcohol. When triglycerides are metabolized, the fatty acids are broken down and oxidized for energy. - The majority of glycerol produced from the breakdown of triglycerides will be used to resynthesize other triglycerides, but a small amount may be converted to glucose - Glycerol contains approximately 4.3 kcal/g, so its caloric content is approximately the same as sucrose (~4 kcal/g).

What are grains? What are legumes?

- Grains are grasses that bear seeds; they include wheat, corn, rice, rye, oats, barley, and millet, and foods that are made from them, such as breads, cereals, and pasta. - Legumes are plants that have a double-seamed pod containing a single row of beans. Examples of legumes are lentils, split peas, black-eyed peas, and many kinds of beans, such as soy, kidney, lima, and northern beans.

Functional Fibers

- In addition to dietary fiber, some processed foods may contain added fibers that have been extracted from plants. Ingredients such as cellulose, guar gum, or pectin are often added to foods in small amounts. These are known as functional fibers and are generally added to affect the food's texture by making it more solid. - Fiber supplements may also contain these fibers or another functional fiber, psyllium.

What happens when you consume sugar alcohols?

- Only a small amount of glycerin (glycerol) is typically used in a product, and its absorption from the gastrointestinal tract is slow, so glucose and insulin concentrations are not rapidly elevated. - When these sugar alcohols have been added to foods, the label must carry a warning stating that excess consumption may have a laxative effect because the unabsorbed portion remains in the digestive tract and may cause diarrhea. - Excess consumption is defined as 20 g/d of mannitol or 50 g/d of sorbitol, although some people have symptoms at lower doses.

Exercise Training Increases the Capacity for Carbohydrate Metabolism

- Regular exercise training, specifically aerobic training, stimulates an increase in the oxidative (aerobic) capacity of muscle, primarily through an increase in the number and size of mitochondria and an increase in oxidative enzyme activity --> increase in oxidative capacity increases the muscle's maximal capacity to utilize carbohydrates and to oxidize lactate. *the utilization of carbohydrate as a percentage of the total energy expenditure is approximately the same. - Another adaptation that increases the muscle's total capacity to utilize carbohydrate is the increase in stored muscle glycogen as a result of regular exercise training - Study: muscles that exercised and reduced muscle glycogen synthesized and stored significantly more glycogen afterward than the leg muscles that were not exercised

What is absorption of sugars dependent on?

- The number of glucose and fructose carriers: The number of glucose and fructose carriers is limited; therefore, if the amount of these carbohydrates in the small intestine is greater than the number of carriers present, some of these sugars will not be absorbed. **The small intestine can adapt somewhat to increased amounts of glucose and fructose in the diet by upregulating (increasing) the number of transporters in order to increase absorption ability. - Concentration gradient (Absorption of fructose also depends on a concentration gradient, but when fructose is present in the small intestine, it is greater than the concentration in the blood because there is no fructose circulating in the blood.)

Why is it better to use g/kg for carbohydrates recommendation instead of percentage of total calories consumed?

- The percentage of total calories assumes that the person will consume adequate calories for the day and sometimes it may fall short --> room for misinterpretation ex: total calories someone consumed may be 1,400 and not the recommended 2,000 --> 70% of 1,400 is less than 2,000 so if they followed that recommendation, they would fall short - g/kg is more straightforward

What structural features of the small intestine aid in absorption?

- The walls of the small intestine contain numerous villi and microvilli to increase surface area for absorption of nutrients - These small projections into the intestinal lumen contain a dense capillary and lymphatic network to further aid the uptake of nutrients into the body.

What happens if there's too much sugar and it can't all be absorbed?

- not absorbed from the small intestine passes through to the colon, ferments, and causes gastrointestinal distress such as bloating and gas.

General Carbohydrate Recommendations Based on Exercise Intensity and Duration

-Low-intensity or short-duration skill-based activities: 3-5 g/kg - Low-intensity, long-duration (more than 1 hour): 5-7 g/kg - an intake of 5 to 7 g/kg/d does not restore muscle glycogen levels of endurance athletes to pre-exercise levels when training at or near maximum capacity. - Endurance athletes likely need to consume 6 to 10 g/kg/d to maintain high levels of muscle glycogen during rigorous training. - Ultraendurance athletes may need more than 10 g/kg daily, perhaps as much as 12 g/kg, because of the higher carbohydrate needs associated with heavy training and ultraendurance competitions

For exercise, how do cells get the energy they need for exercise?

1) Liver glycogen is broken down and released into the blood as glucose, a process called glycogenolysis. 2) If exercise continues for a prolonged duration, reductions in blood glucose may occur as liver glycogen is reduced and gluconeogenesis fails to produce glucose at the rate that it is being utilized 3) Although the exercising muscles begin to take up and use glucose, they rely most heavily on muscle glycogen if sufficient stores are available. **Cells prefer glycogen over glucose

What happens when you have excess carbohydrates?

Carbohydrate consumed in amounts in excess of what can be stored as glycogen can be converted to other stored forms of energy, namely fat. The term for the synthesis of fatty acids is lipogenesis. - They are synthesized by attaching carbons two at a time in the form of acetyl CoA until a chain of 16 or 18 carbons is formed --> fatty acid chains can be attached in groups of three to a glycerol molecule to form a triglyceride, which is the major storage form of fat in the body - Lipogenesis takes place in the liver or in adipocytes (fat cells).

Glycemic Index (GI)

A method of categorizing carbohydrate-containing foods based on the body's blood glucose response after their ingestion, digestion, and absorption. - Highly refined starchy foods and starchy vegetables (for example, white bread, corn flakes, and baked potatoes) have a high GI, which means that blood glucose levels rise quickly after their consumption. -- Foods with a GI >85 are considered high-GI foods - Legumes, beans, fruits, and nonstarchy vegetables have a low GI. Blood glucose rises slowly because the digestion and subsequent absorption of glucose is slower when compared to highly refined starchy foods. -- Foods with a GI <60 are considered low-GI foods

FODMAP is an acronym for fermentable oligo-, di-, and mono-saccharides and polyols.

All of these are short-chain carbohydrates that can cause gastrointestinal upset due to poor absorption, the drawing of water into the gastrointestinal tract, and rapid fermentation by bacteria in the gut

Why are energy bars not preferred for a mid-exercise snack for runners?

Although an energy bar marketed to athletes may have about the same amount of carbohydrates as two packets of a gel, its fiber and fat content is typically too high to be tolerated during higher-intensity exercise such as distance running. - However, adventure athletes, who may engage in many hours of mountain biking, trail running, and orienteering, often snack on sports bars during or between events, and the higher carbohydrate and caloric content is beneficial.

How do certain cells respond to glucose?

Certain types of cells are more likely to use glucose anaerobically and as a result produce lactate. Some cells lack the organelles or enzymatic capability to oxidize glucose. - Erythrocytes (red blood cells) have no mitochondria and are therefore incapable of metabolizing glucose aerobically; they derive their energy from anaerobic glycolysis - Fast-twitch skeletal muscle fibers do contain mitochondria and the inherent oxidative enzymes, yet they are biased to use glucose anaerobically.

The stress of higher exercise intensity results in a substantial stimulation of the sympathetic nervous system

Epinephrine (also known as adrenaline) is secreted from the adrenal glands (adrenal medulla), whereas the majority of the norepinephrine originates from the nerve endings of sympathetic nervous system cells. - The catecholamines also stimulate glycogen breakdown and gluconeogenesis and thus provide an additional mechanism to regulate blood glucose. - Another stress hormone released from the adrenal cortex, cortisol, aids blood glucose regulation indirectly by stimulating the breakdown of proteins into amino acids that may be used in gluconeogenesis

How does galactose absorption differ from glucose?

Galactose uses the same carrier (GLUT-2) and process for absorption. However, once transported to the liver, the galactose is immediately trapped by the liver cells and converted to glucose.

How is glucose transported from the intestine lumen into an epithelial cell?

Glucose is transported by secondary active transport from the intestinal lumen into the epithelial cell by a sodium and glucose cotransporter, SGLT 1) Because sodium is typically found in higher concentration outside the cell, it moves down its concentration gradient into the cell using the SGLT transporter. 2) Sodium binding to the transporter increases the affinity of the SGLT for binding glucose molecules, so glucose is transported into the cell along with sodium 3) The sodium-potassium pump then uses energy to pump sodium back out of the cell to maintain the concentration gradient. 4) As glucose enters the epithelial cell, its concentration increases relative to the concentration in the capillaries on the basal side of the cell. 6) Glucose then moves from the cell into the blood by facilitated diffusion through another membrane-bound glucose transporter, GLUT-2 7) Once absorbed, glucose and fructose are carried directly to the liver via the portal vein.

Where is glycogen predominantly stored?

Glycogen can be stored in many types of cells, but two of the major storage sites for carbohydrate in the body are skeletal muscle and liver. - The pathway to glycogen formation is favored when conditions exist to activate the primary enzyme that controls this process, glycogen synthase. - Glycogen synthesis is further enhanced in skeletal muscle when glycogen stores have been reduced through exercise - Athletes who exercise regularly and consume sufficient carbohydrate typically have higher skeletal muscle glycogen levels at rest than sedentary people.

Glycogen

Glycogen is a highly branched glucose chain and is the form in which humans and animals store carbohydrates in their bodies - Although glycogen is found in muscle and liver tissue of live animals, it is not considered a food source of carbohydrates for humans because it degrades rapidly.

Exercise has a very strong insulin-like effect; that is, it stimulates glucose uptake.

If exercise is initiated at the time insulin is high and blood glucose is being lowered, the additional glucose uptake by exercising muscles could result in blood glucose being temporarily lowered too much.

Sucrose

Monosaccharide composition: glucose + fructose Sweetness (100= sweetness of table sugar)= 100 Glycemic index= 68 Info: Found in fruits, vegetables, honey, and maple syrup; sugar beets and sugar cane are processed into white and brown sugar.

Lactose

Monosaccharide composition: glucose + galactose Sweetness (100= sweetness of table sugar)= 15 Glycemic index= 46 Info: Most adults lose their ability to digest lactose (milk sugar).

Maltose

Monosaccharide composition: glucose + glucose Sweetness (100= sweetness of table sugar)= 40 Glycemic index= 105 Info: Minor disaccharide in most diets; Maltose is produced during the fermentation process that is used to make maltose syrup (commonly used in Asian cooking), beer, and other alcoholic beverages.

Do all cells in the body need insulin to take up glucose?

No, exceptions are brain and liver cells, which are not dependent on insulin for the transport of glucose - Exercising muscle cells can also take up glucose from the blood without insulin, a process known as non-insulin-dependent glucose transport.

Nutrients found in pre-exercise meals

Protein and fat may be components of foods that are favored by the athlete but should not be present in large amounts in order to allow for adequate gastric emptying and digestion. - Although suggested by some research studies, the inclusion of protein/amino acids in the pre-exercise meal is not likely to aid performance over the consumption of carbohydrate alone

What does salivary amylase (in mouth) and pancreatic amylase (digestive tract lumen) do with polysaccharides?

They break them down into disaccharides - The three disaccharides—sucrose, lactose, and maltose—are digested in the small intestine (not the mouth or stomach).

Carbohydrate intake prior to training and competition

The dietary goals of the athlete prior to exercise include avoiding hunger, delaying fatigue, minimizing gastrointestinal distress, and preventing hypohydration (below a normal state of hydration). - Studies show that carbohydrate intake up to 3 or 4 hours prior to endurance exercise is beneficial - The effect is similar to "topping off" a car's gas tank before starting on a long trip. - It is recommended that approximately 1 to 4 g of carbohydrate per kilogram of body weight (1 to 4 g/kg) be consumed 1 to 4 hours prior to exercise. - Athletes must fine-tune this broad recommendation based on their body size, degree of hunger, food preferences, gastrointestinal tolerance, and the amount of time prior to exercise - The bottom line: for most athletes the GI of the pre-exercise meal should rarely be of concern, and using low-glycemic foods during this time is primarily based on the athlete's personal preference - The pre-exercise meal is important, but it cannot completely offset the lack of muscle and liver glycogen that can result from repeated days of insufficient carbohydrate intake.

The majority of digestion and absorption takes place where?

The majority of digestion and absorption takes place in the small intestine, although the mouth, stomach, and large intestine do account for some digestion and for absorption of some nutrients.

When would be a time where it would be preferred to consume "bad" carbohydrates or sugary carbohydrates?

The most appropriate, rapid source of energy for athletes, particularly during or immediately after endurance exercise, may be sucrose to replenish carbohydrate stores since you burn a lot during endurance exercise - The immediate energy needs of exercise will outweigh the longer-term "healthy diet" perspective in this case

What are the most predominant glucose-regulating hormones?

The most predominant of the glucose-regulating (glucoregulatory) hormones are insulin and glucagon. - Glucose is needed for cellular energy, and the metabolism of glucose is regulated by a number of hormones.

Why is xylitol used in chewing gum?

The sugar alcohol xylitol (2.4 kcal/g) is frequently used in chewing gum because it can help prevent dental caries.

What does "highly processed" usually refer to in terms of food

The term highly processed refers to foods that are primarily sugar (for example, sugared beverages) or products made from grains that have been highly refined and sweetened (for example, sugared cereals) b/c manufacturers alter them and in the process of making the product, they add sugars *Vegetables are considered quality Carbohydrates because they are minimally processed and contain fiber

What are the 3 main monosaccharides?

The three monosaccharides found in foods are glucose, fructose, and galactose.

What are the 3 disaccharides?

There are three disaccharides found in food—sucrose, lactose, and maltose. sucrose- glucose + fructose lactose- glucose and galactose maltose- glucose + glucose

Guidelines for optimal muscle glycogen resynthesis

Timing—To maximize the rate at which muscle glycogen is replaced, carbohydrate should be consumed as soon after the exercise bout as possible. Studies show that waiting as little as 2 hours after exercise to begin consuming carbohydrate will significantly slow the rate of muscle glycogen resynthesis Meal size—Consumption of carbohydrate in smaller, more frequent meals appears to further aid the rate at which muscle glycogen is replaced in the hours after exercise Type of carbohydrate—Carbohydrate beverages that are consumed after exercise to replace glycogen should contain mostly glucose and/or sucrose as the carbohydrate source; fructose-containing beverages should NOT be the primary recovery beverage due to the reduced effect on muscle glycogen resynthesis and the potential for gastrointestinal upset. *Consumption of high-GI foods results in higher blood glucose and insulin responses, conditions that favor the rapid synthesis of muscle glycogen. Amount of carbohydrate—The highest rates of muscle glycogen synthesis have been observed in the hours after fatiguing exercise when approximately 1.5 g/kg of carbohydrate were consumed in the first hour immediately after exercise Addition of protein/amino acids—A number of studies suggest that adding proteins to the meal after exercise may increase the rate at which muscle glycogen is replaced

The Digestive Tract starts...

at the mouth and includes the stomach, small intestine (duodenum, jejunum, ileum), large intestine (colon [ascending, transverse, and descending], cecum, appendix, rectum), and anus

Hypoglycemia is defined as...

blood glucose concentration below 50 mg/dL (2.76 mmol/L)

When blood glucose concentration is too low...

the hormone glucagon is secreted from the alpha (α) cells of the pancreas to stimulate the release of glucose stored as liver glycogen into the blood.

When blood glucose concentration is elevated after eating a carbohydrate-containing meal...

the hormone insulin is secreted from the beta (β) cells of the pancreas to stimulate the transport of glucose from the blood into the cells of various tissues. - Insulin mediates the transfer of glucose out of the blood and into most cells, such as red blood cells, resting muscle cells, and fat cells. - GLUT-4 is the most abundant glucose transporter in the body and is found in major tissues that respond to insulin—skeletal muscle and adipose tissue. - GLUT-4 transporters respond to insulin by increasing their numbers on the cell membrane to increase glucose uptake into these tissues in what is called insulin-dependent glucose transport.