Module 4_Week 4_Carbohydrates, Fats, and Protein_Performance Nutrition for the Physically Active

Module 4

Carbohydrates, Fats, and Protein

A. Carbohydrates in Food

Carbohydrates are compounds that contain carbon, hydrogen, and oxygen. The

presence of these three atoms also gives the word carbohydrate its common

abbreviation—CHO. Carbohydrates are found in food as sugars, starches, and

cellulose. Carbohydrates are found in the body predominantly in the form of glucose

(mostly in the blood) and in the storage form of glycogen. Carbohydrates are the

primary energy source for moderate to intense exercise and provide approximately 4

kcal/g. The largest amount of carbohydrate in the body is stored in the form of muscle

glycogen. Smaller amounts are stored as liver glycogen, which helps to maintain

normal concentrations of glucose (a sugar) in the blood. Carbohydrates found in food

replenish the carbohydrates used, although the body has a limited ability to make

glucose from other substances.

The sugars and starches found in food provide energy because the body can

digest and absorb these kinds of carbohydrates. The cellulose found in starchy foods

does not provide energy because humans do not possess the enzymes necessary to

digest it. However, cellulose and other fibers are important forms of carbohydrates

since fiber is needed for good health. In addition to energy, starchy foods also contain

vitamins, minerals, and other nutrients. Sugars provide only energy and do not contain

vitamins, minerals, or fiber.

To understand the differences in the various forms of carbohydrates in food,

one must look more closely at their chemical composition. Carbohydrates are

generally classified as monosaccharides, disaccharides, or polysaccharides. It helps to

know that saccharide means sugar, and mono means one, di means two, and poly

means many. Therefore, a monosaccharide consists of one sugar molecule, a

disaccharide two sugar molecules, and a polysaccharide many sugar molecules.

In which foods are these monosaccharides found? Although glucose can be

found by itself in foods, most of the time it is a component of food disaccharides and

polysaccharides. Fructose is naturally found in fruits and vegetables, but the largest

amount of fructose in American diets is added to foods when they are processed, such

as the addition of high-fructose corn syrup as a sweetener. Galactose is a

monosaccharide but it is found naturally in food only as part of the disaccharide,

lactose. There are three disaccharides found in food— sucrose, lactose and maltose.

Their chemical structures and characteristics are shown in Figure 4.2 and Table 4.2.

Sucrose is made of one molecule of glucose and one molecule of fructose. Lactose is

a combination of one molecule each of glucose and galactose. Maltose is made up of

two glucose molecules.

Sucrose is found naturally in fruits, vegetables, honey, and maple syrup. It is

also found in sugar beets and sugar cane, which are processed into white and brown

sugar. Sucrose is added to many processed foods. Lactose is naturally found in milk.

It is often referred to as milk sugar and is sometimes added to processed foods.

Maltose is produced during the fermentation process that is used to make maltose

syrup (commonly used in Asian cooking), beer, and other alcoholic beverages.

Polysaccharides are chains of glucose. These glucose chains are known as

starch, fiber, and glycogen. Starches in food may be straight chains (amylose) or

branched chains (amylopectin). Enzymes in the digestive tract help to break down

these chains into their basic component, glucose. Fiber is a tightly packed

polysaccharide that is the structural component of plants. Humans lack the enzymes

needed to break down fiber. Glycogen is a highly branched glucose chain and is the

form in which humans and animals store carbohydrates in their bodies (Figure 4.3).

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.

Starch is found in many foods including grains, legumes (beans), and tubers.

Grains are grasses that Table 4.2 Characteristics of Disaccharides bear seeds and

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. Beans are also known by their color—white, pink, red, or black.

Tubers, such as white or sweet potatoes and yams, have underground stems and are

often referred to as starchy vegetables.

Dietary fiber is found naturally in grains. Whole grains, which contain the

endosperm, the germ, and the bran, have more fiber than grains that are highly

refined. When whole grains are processed, the germ and the bran are removed. Since

these two parts contain most of the fiber, the processing results in a substantial loss of

fiber. Other sources of dietary fiber include legumes, seeds, fruits, and vegetables,

including the starchy vegetables.

There is no single, perfect way to classify the various carbohydrates found in

foods, so a number of terms have been used to distinguish them. Often carbohydrates

are divided into two categories: sugars and starches. Sugars are also known as simple

sugars or simple carbohydrates, and starches are referred to as complex

carbohydrates. The widespread processing of carbohydrate-containing foods has given

rise to new terms. 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). In contrast, whole grains and

foods made from them are referred to as minimally processed, fiber-containing, or

quality carbohydrates. All of these terms are an attempt to distinguish carbohydrate

foods based on their nutrient content.

Unfortunately, carbohydrates have also been referred to as good and bad. Bad

has been used to describe highly processed, fiber-deficient, and/or highly sweetened

carbohydrate foods and beverages. Although the original intention was to distinguish

more nutritious foods from less nutritious ones, this terminology is unfortunate since

the words good and bad carry powerful language and cultural connotations. Rather

than simply labeling carbohydrate foods as good or bad, one should consider the

context and frequency of their consumption. There are circumstances that make it

appropriate to consume carbohydrates typically referred to as bad. For example, the

consumption of large amounts of simple sugars is generally discouraged because they

are considered “empty calories.” Empty refers to a lack of nutrients, not a lack of

calories, and excess consumption of such foods and beverages may contribute to

obesity and malnutrition. But, there are circumstances when sugars (for instance,

sucrose or table sugar) may be a “good” choice. For example, the most appropriate,

rapid source of energy for athletes, particularly during or immediately after endurance

exercise, may be sucrose.

B. Digestion, Absorption, and Transportation of Carbohydrates

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. The digestive tract starts at the mouth and includes the stomach, small

intestine (that is, duodenum, jejunum, ileum), large intestine (that is, colon

[ascending, transverse, and descending], cecum, appendix, rectum), and anus (Figure

4.4). 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. The walls of the small intestine contain numerous

villi and microvilli to increase surface area for absorption of nutrients (Figure 4.5).

These small projections into the intestinal lumen contain a dense capillary and

lymphatic network to further aid the uptake of nutrients into the body.

The three disaccharides—sucrose, lactose, and maltose—are digested in the

small intestine (not the mouth or stomach). Enzymes specific to each disaccharide are

found in the brush border of epithelial cells lining the interior border of the small

intestine. The enzyme sucrase breaks down sucrose to yield glucose and fructose.

Sucrase can also break down maltose to two molecules of glucose, but dietary maltose

intake is usually small. The enzyme lactase breaks down lactose into glucose and

galactose. People who lack a sufficient amount of lactase are unable to break down

the lactose or milk sugar, resulting in a condition known as lactose intolerance.

The absorption of glucose is a two-step process, with each step involving a

carrier protein embedded in the membrane of the epithelial cells lining the small

intestine. Glucose is transported by secondary active transport from the intestinal

lumen into the epithelial cell by a sodium and glucose cotransporter (SGLT). Because

sodium is typically found in higher concentration outside the cell, it moves down its

concentration gradient into the cell using the SGLT transporter. 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. The sodium-potassium pump

then uses energy to pump sodium back out of the cell to maintain the concentration

gradient. As glucose enters the epithelial cell, its concentration increases relative to

the concentration in the capillaries on the basal side of the cell. Glucose then moves

from the cell into the blood by facilitated diffusion through another membrane-bound

glucose transporter, GLUT-2 (Figure 4.6). Galactose uses the same carrier and process

for absorption. However, once transported to the liver, the galactose is immediately

trapped by the liver cells and converted to glucose.

Once absorbed, glucose and fructose are carried directly to the liver via the

portal vein. The liver is extremely efficient in capturing fructose, which will

eventually be converted to glucose by the liver cells. The liver’s ability to capture all

of the fructose and galactose that is absorbed prevents either of these

monosaccharides from leaving the liver and circulating in the blood. Ultimately, all of

the sugars and starches found in food are broken down to monosaccharides and

absorbed. Glucose is the end point, the eventual form of sugar that the body uses,

regardless of whether the original compound was a polysaccharide, a disaccharide, or

a monosaccharide.

C. Glucose in the Body Metabolism

Metabolism refers to all of the physical and chemical changes that take place

within the cells of the body. Glucose is needed for cellular energy, and the metabolism

of glucose is regulated by a number of hormones. The most predominant of the

glucose-regulating (glucoregulatory) hormones are insulin and glucagon. 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, and (5) the production of glucose from lactate, amino

acids, or glycerol.

The normal concentration of blood glucose is approximately 70 to 110 mg/dl

(3.89-6.06 mmol/L), and sensitive hormonal mechanisms are used to maintain

equilibrium (homeostasis) within this fairly narrow range. For example, when blood

glucose concentration is elevated after a carbohydrate-containing meal, the hormone

insulin is secreted from the beta (B) cells of the pancreas to stimulate the transport of

glucose from the blood into the cells of various tissues. When blood glucose

concentration is too low, the hormone glucagon is secreted from the alpha (a) cells of

the pancreas to stimulate the release of glucose stored as liver glycogen into the

blood. Blood glucose concentration is always in flux, but hormonal mechanisms are in

place to bring blood glucose concentration back within the normal range

(equilibrium).

Since the 1980s, scientists have been studying the effect that different

carbohydrate foods have on blood glucose (known as glycemic response) and insulin

secretion. Hundreds of carbohydratecontaining foods have been tested and their

glycemic responses have been quantified (Atkinson, FosterPowell, and Brand-Miller,

2008). This has led to the classification of carbohydrate foods based on their glycemic

index (GI). Under normal circumstances, the result of carbohydrate consumption,

digestion, and absorption is a relatively rapid increase in blood glucose, which reaches

a peak and is then followed by a decline due to the secretion of insulin and the

subsequent increase in glucose uptake by tissues. The time course and magnitude of

this glycemic response are highly variable with different foods and do not fall neatly

into categories based on chemical structure (that is, mono-, di-, or polysaccharides) or

other descriptions, such as simple or complex carbohydrates. For example, the

consumption of the same amount of the two monosaccharides (simple sugars) glucose

and fructose results in very different blood glucose responses. Glucose ingestion

results in a rapid and large increase in blood glucose, which, in turn, rapidly returns to

baseline levels. Fructose consumption, on the other hand, results in a much slower and

lower glycemic response—it rises more slowly, does not reach as high a level, and

returns more slowly to baseline.

The variability in glucose and insulin response with different foods has

important implications, and the GI may be a useful tool, particularly for those who

must carefully control their blood glucose concentration, such as people with diabetes

(Doyle and Papadopoulos, 2000). Nondiabetic athletes may also benefit from

considering the GI of the carbohydrates they consume. There may be specific

situations in which an athlete would want to consume foods with a high glycemic

index and provoke a large blood glucose and insulin response, such as when

attempting to synthesize muscle glycogen quickly:after glycogendepleting exercise.

In glycolysis, glucose is broken down in a series of chemical steps to form

pyruvate, often referred to as a glycolytic intermediate. From pyruvate, the

completion of the metabolism of glucose follows one of two pathways: conversion to

lactate (anaerobic glycolysis) or oxidation of pyruvate in the mitochondria (oxidative

phosphorylation) (Figure 4.9 and Appendix J). How glucose is metabolized is

dependent upon a variety of factors: the type of cell, the enzymatic capability of the

cell, energy state, hormonal status, training history, and intensity of exercise. 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. For

example, erythrocytes (red blood cells) have no mitochondria and are therefore

incapable of metabolizing glucose aerobically; they derive their energy from

anaerobic glycolysis. Fasttwitch muscle fibers do contain mitochondria and the

inherent oxidative enzymes, yet they are biased to use glucose anaerobically. They

contain a highly active form of lactate dehydrogenase (LDH), the enzyme that

catalyzes the conversion of pyruvate to lactate, and are thus likely to use anaerobic

glycolysis even when oxygen delivery to the cell is sufficient. Fast-twitch muscle

fibers are also typically not recruited for use until exercise intensity is relatively high,

indicating that the energy need of the muscle cells is high, further favoring the use of

a more rapid energy-producing system such as anaerobic glycolysis.

The pathway to glycogen formation is favored when conditions exist to

activate the primary enzyme that controls this process, glycogen synthase. Glycogen

storage is favored when the energy need of the cell is low and insulin is elevated, as

occurs when a person is resting after a meal, particularly a meal that contains

carbohydrate. Glycogen synthesis is further enhanced in muscle when glycogen stores

have been reduced through exercise. Thus athletes who exercise regularly and

consume sufficient carbohydrate typically have higher muscle glycogen levels at rest

than sedentary people. For the athlete or physically active person, the majority of

carbohydrate that is consumed is either stored as glycogen or metabolized. However,

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.

Although most of the body’s glucose needs are supplied by dietary

carbohydrate, the body does have a limited ability to produce glucose from other

sources. The process of producing glucose from other sources is called

gluconeogenesis. The major sources for the production of glucose are lactate, amino

acids, and glycerol. During periods of fasting or starvation, proteins in the body can

be broken down into amino acids, which can then be metabolized (see Chapters 3 and

6). Amino acids that can be converted to certain intermediates in glycolysis or the

Krebs cycle can also be used in gluconeogenesis to form glucose in the liver. Alanine

is an example of an amino acid that can be used to form glucose. In fact, 18 of the 20

amino acids are biochemically capable of being converted to glucose.

D. Carbohydrates as a Source of Energy for Exercise

The fuel source utilized depends on a variety of factors, with exercise intensity

playing a major role. Very high-intensity, very short-duration anaerobic exercise

typically uses creatine phosphate as the energy source to replenish ATP. 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. Carbohydrate used by exercising

muscle can come from stored muscle glycogen or from glucose that is brought into

the muscle from the blood. Glucose is made available in the blood from the liver as a

result of at least three processes: the breakdown of liver glycogen, the production of

glucose from other sources (gluconeogenesis), or the ingestion of carbohydrates as

food or fluids, which are absorbed and passed through the liver.

Exercising muscle preferentially uses carbohydrate from stored glycogen. A

study conducted by Bergstré6m and Hultman (1967) has become the classic

description of muscle glycogen utilization during exercise. Subjects rode on a cycle

ergometer at a moderately hard aerobic intensity and their muscle glycogen utilization

was determined every 15 minutes by muscle biopsy. Results of this study clearly show

the decline in muscle glycogen as exercise time progressed (Figure 4.10). After 60

minutes of exercise, muscle glycogen fell to very low levels that corresponded with

fatigue, an inability of the subjects to complete the next 15-minute exercise period.

Although not shown in Figure 4.10, the study also demonstrated that the rate of

muscle glycogen usage was related to exercise intensity; muscle glycogen was used at

a higher rate at higher exercise intensities.

Exercise stimulates the uptake of glucose from the blood because it has a very

strong insulin-like effect. As glucose is being taken out of the blood by exercising

muscle, a fall in blood glucose is prevented by two metabolic adjustments, both

stimulated by the release of the hormone glucagon by the pancreas. Liver glycogen is

broken down and released into the blood as glucose, a process called glycogenolysis.

Glucagon also stimulates the process of gluconeogenesis by the liver to make glucose

available to maintain blood glucose.

The increase in ability to metabolize carbohydrate is due mostly to the

increased oxidative or aerobic capacity of the muscle. The activity of the enzymes

catalyzing the anaerobic pathway, glycolysis, do not change much, probably because

they are already at a high level of intrinsic activity in the muscle before training.

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. As

demonstrated elegantly in a onelegged cycling study by Bergstrém and Hultman

(1966), muscles that exercised and reduced muscle glycogen synthesized and stored

significantly more glycogen afterward than the leg muscles that were not exercised.

Resting muscle glycogen levels in average, sedentary adults are approximately 20-30

percent lower than those of trained athletes.

A training strategy that has become popular in the attempt to enhance fat

metabolism is to purposely reduce the amount of carbohydrate in the diet for a period

of time or withhold it prior to exercise in order to train with low carbohydrate stores.

Research studies do indicate that there may be some metabolic training enhancements,

such as an increase in mitochondrial function, as a result of these “depletion

workouts” or by “training low” (Hawley and Burke, 2010). However, training with

low carbohydrate availability may be difficult, and athletes commonly choose lower

exercise intensities during these workouts. In addition, immune system function may

be compromised, potentially exposing the athlete to increased risk of illness or injury.

Additional research is needed to determine if the enhanced metabolic adaptations that

occur with this type of training lead to improvements in performance without

detrimental effects on the athlete.

Glucose metabolism during exercise is regulated by several overlapping or

redundant hormonal mechanisms. Glucagon is a hormone secreted by the alpha (x)

cells of the pancreas that is a counter-regulatory hormone to insulin. It stimulates

essentially the opposite effect of insulin (that is, glycogen breakdown instead of

glycogen synthesis). Glucagon is secreted during periods of fasting or starvation, and

exercise is metabolically similar to these conditions. During exercise, insulin secretion

is suppressed and glucagon secretion is stimulated.

Glucagon stimulates glycogen breakdown in the liver and the release of

glucose into the blood, thus acting to maintain or increase blood glucose. It also

stimulates the process of gluconeogenesis by the liver, in which the liver can take

precursors such as lactate or amino acids and make new glucose that can subsequently

be released into the blood. This process begins early in exercise and can be thought of

as a preemptive response*to prevent a fall in blood glucose, rather than as a reactive

response to blood glucose declining (one of the rare feed-forward mechanisms in the

body, which usually operate in a feedback fashion).

Exercise intensity and duration have a substantial effect on metabolism, both

on the metabolic rate and on the source of fuel utilized. Carbohydrates, fats, and

proteins can be utilized as sources of energy for metabolism and are used in different

proportions under different conditions..As exercise intensity increases, the proportion

of energy that is derived from fat decreases and the proportion from carbohydrate

increases. Therefore, carbohydrate is the main source of energy for moderate- to high-

intensity exercise.

E. Translating Daily Carbohydrate Recommendations to Food Choices

Athletes must consume enough carbohydrates daily to replenish muscle

glycogen used during training. Prolonged moderate-intensity exercise, such as

distance running and distance cycling, depletes muscle glycogen, and studies have

shown that at least 5 g/kg/d is needed to replenish it to a level that allows for training

on consecutive days (Sherman et al., 1993). However, an intake of 5 to 7 g/kg/d does

not restore muscle glycogen levels of endurance athletes to pre-exercise or near

maximum capacity. Endurance athletes likely need to consume 8 to 10 g/kg/d to

maintain high levels of muscle glycogen over weeks and months of rigorous training.

The majority of studies conducted on muscle glycogen depletion and

replenishment have used endurance athletes as subjects (Jacobs and Sherman, 1999).

The body of scientific literature clearly shows that prolonged endurance exercise

depletes muscle glycogen and daily carbohydrate intake is needed to restore it. What

about athletes in other sports? Limited studies have shown that the recommendations

made for endurance athletes also apply to athletes in intermittent, high-intensity sports

such as soccer and ice hockey (Bangsbo, Norregaard, and Thorsoe, 1992; Akermark et

al., 1996; Balsom et al., 1999). The depletion of muscle glycogen when performing

intermittent, high-intensity exercise during practice and games is similar to the

depletion seen after prolonged moderate-intensity exercise.

The needs of ultraendurance athletes may, at times, exceed the general range

for daily carbohydrate intake for other athletes. Examples of ultraendurance sports

include Ironman® triathlons and multipleday cycling events (stage races) such as the

Tour de France. These are grueling events that require huge amounts of energy, much

of which comes from muscle glycogen. Ultraendurance athletes may need more than

10 g/kg daily because of the higher carbohydrate needs associated with heavy training

and ultraendurance competitions (Burke, Kiens, and Ivy, 2004). Although many elite

ultraendurance athletes successfully consume the recommended amount of

carbohydrate (Coleman, in press b), it can be difficult for ultraendurance athletes to

meet these recommendations. They must eat frequently while awake and may need to

consume carbohydrate during the night (for example, after waking to go to the

bathroom or setting the alarm during the night to wake and eat). In addition to

carbohydrate-containing meals, ultraendurance athletes may include sports bars,

beverages, and gels before, during, and after training to try and reach their daily

carbohydrate goals.

Recommendations given as a percentage, relative to the amount of energy

consumed, can be misinterpreted. For example, a female endurance athlete states that

she consumes a high-carbohydrate diet consisting of 70 percent of her total calories as

carbohydrate. Is this an adequate amount? It depends on whether she consumes

enough energy, as shown in Table 4.5. This example shows how an athlete could meet

the percentage of carbohydrate recommended, but fall short of the minimum amount

of carbohydrate recommended. Also notice in this example that both athletes state that

they consume a “high” carbohydrate diet. High and low are relative terms, and unless

such terms are defined, using these words to describe dietary carbohydrate intake can

be misleading. To avoid misinterpretations, it is best to express recommendations on

an absolute basis: grams of carbohydrate per kilogram body weight (g/kg). However,

it is not uncommon to see carbohydrate recommendations made to athletes as a

percentage of total kilocalories. The usual recommendation for most athletes is 50-65

percent of total caloric intake, increasing to 70 percent for those athletes with higher

carbohydrate needs (for example, athletes in endurance sports). These percentages

assume that total daily energy intake is adequate.

Once the daily carbohydrate need is established, the focus turns to dividing the

total carbohydrate intake appropriately over the course of the day. The athlete’s

training and conditioning program and the unique demands of the competitive

environment before, during, and after exercise will dictate the amount and timing of

carbohydrate intake. 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). With the exception of preventing

hypohydration, all of these goals involve carbohydrate. Athletes should determine

both the amount and timing of carbohydrate consumption (as well as proteins, fats,

and fluids) needed to support exercise. For most athletes, dietary intake before

training becomes the basis for fine-tuning the amount and timing of carbohydrate and

other nutrients consumed prior to competition. Precompetition intake can be tricky

because start times for some events may not be known, familiar foods may not be

available, environmental conditions may be different from usual training conditions,

and the stress of competition may result in increased gastrointestinal distress.

The amount of carbohydrate in the pre-exercise meal depends upon how clése

the meal is consumed to the start of exercise. Gastrointestinal distress can be caused

by exercise, especially at the intensities at which athletes train and compete, because

blood flow to the gastrointestinal tract is reduced. It is recommended that

approximately 1 g of carbohydrate per kilogram body weight (1 g/kg) be consumed 1

hour prior to exercise. As the time before exercise increases, the amount of

carbohydrate can be increased; for example, 2 hours prior to exercise, 2 g of

carbohydrate per kilogram body weight can typically be tolerated (Coleman, in press

a). Larger amounts of carbohydrate (for example, 3—-4.5g/kg eaten 3-4 hours prior to

exercise) may be appropriate for athletes under certain circumstances and will depend

on the athlete’s tolerance. The adjustment of carbohydrate amount based on time prior

to exercise helps athletes prevent gastrointestinal distress and avoid hunger.

An active area of research regarding endurance athletes is whether low-

glycemic index foods are preferred for a precompetition meal. Most of the studies

have been conducted on trained distance cyclists who consumed 1 g/kg low-GI

carbohydrate approximately 1 hour before exercise. Some studies showed low-GI

carbohydrates to be beneficial because blood glucose concentrations were maintained

during 1 to 2 hours of exercise due to the slow absorption of glucose from the low-GI

carbohydrate. Other studies did not find a benefit, but no studies found that low-GI

foods were detrimental (Donaldson et al., 2010; Siu and Wong, 2004). Although the

research is not conclusive, some endurance athletes include low-GI carbohydrate-

containing foods as part of their pre-exercise meal. However, the benefit of a low-GI

carbohydrate prior to exercise may be small if carbohydrate is consumed during

exercise.

However, a small number of athletes may be prone to reactive (rebound)

hypoglycemia, a low blood glucose concentration that follows food intake. When

these athletes consume a food with a high glycemic index 1 hour or less before

prolonged exercise, blood glucose and insulin concentrations rise rapidly, which then

shortly results in low blood glucose (hypoglycemia). The reestablishment of a blood

glucose concentration within the normal range takes longer and performance may be

affected. Athletes who respond in this way will need to experiment with the amount

and timing of high-glycemic index foods prior to exercise.

Pre-exercise meals may contain protein and fat in addition to carbohydrate.

Protein and fat may be components of foods that are favored by the athlete but should

not be large in amount in order to allow for adequate gastric emptying and digestion.

The inclusion of foods with protein and/or fat may aid in satiety and prevent feelings

of hunger that may be felt soon after carbohydrate-only meals. 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 (see further

discussion in Chapter 5). Particularly for the endurance athlete, carbohydrate should

make up the majority of the pre-exercise meal.

The pre-exercise meal is important but it cannot completely offset the lack of

muscle and liver glycogen that results from repeated days of insufficient carbohydrate

intake. For athletes who restrict carbohydrates and energy to compete in a specific

weight category (for example, wrestlers, lightweight rowers, kick boxers), the

precompetition meal does provide an opportunity to replenish some fluid and

glycogen stores and to increase blood glucose concentrations. However, the time

between weight certification and the start of the competition is probably too short to

adequately replenish depleted glycogen stores. Nonetheless, these athletes try to

consume as much carbohydrate prior to competition as they can tolerate.

Athletes who perform endurance exercise or intermittent high-intensity

exercise for more than 1-2 hours are at risk for glycogen depletion, low blood glucose,

and fatigue during training and competition (Coyle, 2004). Ultradistance racers,

triathletes, marathon runners, distance cyclists, and other long-duration athletes must

consume both carbohydrates and fluids during heavy training and competition or they

may fail to finish. 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. Carbohydrate intake during training and

competition helps these athletes to spare muscle glycogen, maintain blood glucose

concentrations, delay fatigue, and reduce the athlete’s perception of fatigue.

There has been speculation that moderate- to highglycemic index foods are

beneficial during prolonged exercise because such foods are rapidly digested and

absorbed (Burke, Collier, and Hargreaves, 1998). However, there is a lack of scientific

studies to confirm or dispute the benefits of moderate- or high-GI foods during

exercise greater than 1 hour. A wide variety of commercial products have been

developed and marketed as vehicles to increase carbohydrate intake during exercise

(see Spotlight on... Sports Drinks, Bars, and Gels) although athletes can take in

carbohydrates through food (for example, bananas) or many types of fluids.

Carbohydrate beverages, or sports drinks, intended to be consumed during exercise

are typically formulated to contain less than 10 percent carbohydrate. One example is

the original Gatorade, rebranded in 2010 as Gatorade G Series 02 Perform, which

contains 6 percent carbohydrate. Another example is Powerade, rebranded as

Powerade ION4. Previously 8 percent carbohydrate, Powerade was reformulated in

2010 to contain 14 grams of carbohydrate in 8 fluid ounces, or 6 percent carbohydrate

(6 g carbohydrate per 100 ml), the same concentration of carbohydrate as the original

Gatorade.

Another recent trend in sports drink formulation is the lowering of

carbohydrate content for those participants whose exercise is not as demanding in

intensity and/or duration as that of other athletes, and therefore whose carbohydrate

needs are not as great. For example, Gatorade G Series G2 Low Cal contains

approximately one-third of the carbohydrate found in original Gatorade (5 grams in

240 ml or 2 percent carbohydrate). Powerade Zero contains no carbohydrate and no

calories. These “lite” versions of sports drinks may be useful for fluid and electrolyte

replacement but will obviously not provide sufficient carbohydrate for athletes under

more rigorous training and competition situations.

As contrasting examples, consider a triathlete and a bicycle racer. A triathlete

has virtually no opportunity to consume carbohydrate during the swim portion of the

race and must therefore develop a plan for consumption during the bike and run

portions. Although food and fluid can be carried on the bike, an Ironmanlength

triathlon is too long for the athlete to carry all the necessary food and fluid, so a

triathlete must depend upon products supplied by the race organizers. During a long

bicycle race, however, a cyclist can consume carbohydrate in the preferred form

throughout the race typically from sports drinks carried in water bottles on the bike

and from food supplied by team support vehicles.

Long, intense training sessions or competitive events may leave athletes with

substantially reduced or depleted liver and muscle glycogen stores. Athletes need to

consider a variety of factors to optimally replenish those stores. As discussed earlier,

one of the conditions that stimulate the synthesis of glycogen is its depletion. Some

muscle glycogen may be resynthesized after hard exercise even if the athlete does not

eat, although the amount is minimal. The glucose used for glycogen synthesis in this

case comes from the liver through gluconeogenesis, particularly from lactate. To

optimize glycogen replacement, however, two things are needed: carbohydrate and

insulin.

Glucose molecules are needed to re-form the glycogen chains and are typically

obtained by consuming carbohydrate-rich foods. Insulin plays an important role by

facilitating uptake of glucose into muscle cells and by activating the enzyme

principally responsible for glycogen resynthesis, glycogen synthase. Consumption of

foods or beverages containing carbohydrate provides the source of glucose and will

also stimulate the release of insulin from the pancreas. o 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 (Ivy, Katz et al., 1988). Athletes should therefore begin consuming

carbohydrate as soon as is practical after the exercise session or competition is over.

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

(Doyle, Sherman, and Strauss, 1993). With large single meals, blood glucose and

insulin rise rapidly and then return to baseline relatively quickly. Elevations in blood

glucose and insulin can be sustained for a longer period of time with smaller, more

frequent feedings, which maintains the appropriate environment for muscle glycogen

synthesis. It is also likely to be more palatable for the athlete to consume smaller

amounts of food and/or beverages over several hours than trying to consume a large

meal soon after fatiguing exercise.

Carbohydrate beverages that are consumed after exercise to replace glycogen

should contain mostly glucose and/or sucrose as the carbohydrate source. Studies

clearly show that beverages containing mostly fructose do not result in glycogen

synthesis rates that are as high as those beverages with glucose and sucrose (Blom et

al., 1987). Athletes may consume fructose because it is found naturally in fruit juices

and because it is often added to beverages to enhance flavor and sweetness. However,

fructosecontaining beverages should not be the primary recovery beverage because of

the reduced effect on muscle glycogen resynthesis and the potential for

gastrointestinal upset.

Along with commercial products formulated and marketed for consumption

before and during exercise, recovery products are available as well. Examples include

Gatorade G Series 03 Recover, G Series Pro 03, and Endurox R4 Recovery Drink.

These recovery drinks typically provide carbohydrate to stimulate the resynthesis of

muscle glycogen, as well as protein to aid in reducing protein breakdown and

stimulating protein synthesis during recovery from exercise (see discussion in Chapter

5). These products provide convenience and a known amount of carbohydrate and

protein, but sufficient carbohydrate and protein can easily be provided through other

foods as well.

The highest rates of muscle glycogen synthesis have been observed in the

hours after fatiguing exercise when approximately 1.5g/kg of carbohydrate were

consumed in the first hour immediately after exercise (Doyle, Sherman, and Strauss,

1993; Ivy, Lee et al., 1988). In these studies, subjects consumed approximately 120 g

of carbohydrate in the first hour postexercise. This might be considered the “priming”

dose of carbohydrate to initiate the glycogen synthesis process, with more

carbohydrate being consumed over the next few hours depending upon the need for

rapid resynthesis. For athletes needing maximal rates of muscle glycogen synthesis,

0.75 to 1.5g/ kg of carbohydrate should be consumed each subsequent hour until

approximately 4 hours after exercise. It is important to recognize that the higher end

of this range is a large amount of carbohydrate and may cause gastrointestinal upset.

Carbohydrate loading (also known as carbohydrate supercompensation) is a

technique that some athletes use to attain maximum glycogen stores prior to an

important competition. This technique is appropriate for endurance and

ultraendurance athletes who perform 90 minutes or more of continuous exercise, and

it may be used by some bodybuilders as part of their precontest preparations. In the

case of endurance and ultraendurance athletes, without maximum levels of glycogen

when the race begins these athletes could run out of stored carbohydrate as a fuel

source and be forced to reduce the intensity of their exercise or drop out of the race. In

the case of bodybuilders, maximum glycogen storage is a strategy used to promote

muscle definition, one feature on which contestants are judged. In these

circumstances, performance could be enhanced by carbohydrate loading.

Researchers continue to look at ways to modify the carbohydrate-loading

protocol, such as by manipulating the time period required. Since 2002, some studies

have shown that high levels of muscle glycogen can be attained within 1 day if large

amounts of carbohydrate are consumed. These levels can be attained in 1 to 3 days if

the athlete refrains from exercise (Bussau et al., 2002) or if the athlete performs 3

minutes of high-intensity exercise, then rests, and consumes a large amount of

carbohydrate (Fairchild et al., 2002). It is important to note that these studies do not

show higher muscle glycogen supercompensation than Sherman’s modified method,

and may involve strategies incompatible with the athlete’s precompetition preparation

(for example, complete cessation from exercise or performing very high intensity

exercise the day before). Carbohydrate-loading techniques remain an area of active

research.

The amount and timing of carbohydrates before, during, and after exercise

must be considered in the context of total dietary intake over days, weeks, and months

of training. Daily dietary intake considers not only the 24-hour consumption of

carbohydrates but also total caloric intake and the relative contribution of

carbohydrates, proteins, fats, and alcohol. Daily intake will vary, but over time both

energy and carbohydrate intake must be adequate. If not, training and performance

will be negatively affected.

For general health, it is recommended that carbohydrate intake for adults

should be 45-65 percent of total energy intake, assuming that energy intake is

adequate (Institute of Medicine, 2002). This recommendation is based on scientific

studies conducted in the general population. These studies found associations between

dietary intake and the prevention of nutrient inadequacies as well as a reduced risk for

chronic diseases, such as cardiovascular disease. Recommendations for the general

population are made as a percentage of total energy intake, so some calculations are

necessary before comparisons can be made to athletes.

Limited surveys of collegiate (Hinton et al., 2004) and elite (Ziegler et al.,

2002) athletes suggest that the fiber intake of athletes does not differ from that of the

general population. Female athletes report a fiber in- take of ~15 to 19 g/d whereas

male athletes consume about 18 to 19 g daily. Fiber intake is associated with the

consumption of carbohydrate-containing foods such as fruits, vegetables, whole

grains, legumes, beans, and nuts. Sugar is devoid of fiber, and grains that are highly

refined (for example., whole wheat that is processed to make white bread) lose most

of the fiber originally present before processing. Depending on the kinds of foods and

beverages consumed, it is possible for athletes to meet carbohydrate recommendations

but fall short of meeting fiber recommendations.

Carbohydrate recommendations for athletes are scientifically based and reflect

the need to maintain glucose homeostasis, maintain adequate muscle glycogen stores,

and to fuel exercise. It is not difficult to understand the physiological need for

carbohydrates. The challenge is to translate those recommendations into food choices

that will support training, performance, the immune system, and long-term good

health. Many athletes fail to consume an adequate amount of carbohydrate. In some

cases this may be due to a lack of knowledge about the amount of carbohydrate

needed. Table 4.10 lists the estimated daily total carbohydrate intake needed by

athletes based on their weight. For example, the 165 lb (75 kg) cyclist who needs 8 to

10 g/kg will need 600 to 750g of carbohydrate daily. Each gram of carbohydrate

contains approximately 4 kcal. Therefore, this athlete will need to consume 2,400 to

3,000 kcal from carbohydrates alone. This is not an easy task!

Vegetarians do not eat meat, fish, or poultry, but some consume animal

products such as milk or yogurt. Vegans avoid any foods of animal origin. Obtaining

an adequate amount of carbohydrates is not difficult for vegetarian athletes since so

many carbohydrate-containing foods are of plant origin. Those who avoid milk

products would still have ample food groups from which to choose—starches, starchy

vegetables, beans and legumes, nuts, fruits, vegetables, and sugar. Vegans could also

choose foods from these groups but would want to avoid any prepared products that

contain an animal-derived ingredient. The American Dietetic Association supports the

position that well-planned vegetarian diets are healthy, nutritionally adequate, and

able to meet sports nutrition recommendations.

The effect that a high sugar intake may have on chronic disease, such as

obesity, has been controversial. The consumption of sugar-sweetened beverages has

been scrutinized, in part, because soft drinks and other beverages sweetened with

sugar are one of the main sources of sugar in the diets of Americans of all ages

(Johnson et al., 2009). Some observational studies suggest an association between a

high intake of sugar-sweetened beverages and weight gain. However, there are also

studies that do not show an association (van Baak and Astrup, 2009; Gibson, 2008).

Not every person who gains weight does so because of excessive sugar intake, but for

many people, including some athletes, too much sugar is one major factor in their

weight gain.

Artificial sweeteners (technically known as nonnutritive sweeteners) are not

found naturally in foods; rather they are laboratory-manufactured compounds that

provide a sweet taste but few or no calories. Saccharin (Sweet ’n Low®), aspartame

(Nutrasweet®, Equal®), acesulfame potassium (Acesulfame K or Sunett®), sucralose

(Splenda®), and neotame are examples of nonnutritive sweeteners. Table 4.15

explains the various artificial sweeteners and their similarities and differences.

Questions about safety have been raised ever since the Food and Drug Administration

approved the first artificial sweeteners. The position of the American Dietetic

Association (2004) is that “nonnutritive sweeteners are safe for use within the

approved regulations.” However, concerns about artificial sweeteners are frequently

raised, most often via the media, and individuals who use artificial sweeteners should

evaluate any safety concerns raised.

Theoretically, the use of artificial sweeteners could improve diet quality

(American Dietetic Association, 2004). For example, suppose a person normally

consumed 12 oz (~360 ml) of a sugared soft drink, which has about 150 kcal. If an

artificially sweetened soft drink was consumed and food with the same amount of

kilocalories but more nutrients was eaten, an argument could be made that the quality

of the diet was improved. To date there have been no studies that have examined if

people usually make such substitutions, but athletes may choose to do so as part of

their overall diet plan.

The digestion of lactose (milk sugar) requires the enzyme lactase, known

scientifically as B-galactosidase. In humans and other mammals, lactase activity is

high during infancy. In most humans, lactase activity begins to decline at about age 2,

but in some Caucasians the decline does not begin until adolescence. Although the

decline is extensive, most adults do have a low level of lactase activity. Lactose

maldigestion occurs when there is insufficient lactase relative to the amount of lactose

consumed in the diet. In people of Northern European descent, lactase activity

remains at infant levels throughout adulthood and their tolerance of lactose remains

high.

Those with lactose intolerance typically can digest fermented milk products,

such as yogurt or Acidophilus milk better than other milks or unfermented milk

products, even though the same amount of lactose is consumed. One reason is that

fermented products contain bacteria that produce B-galactosidase (lactase). A second

reason is that solids, such as yogurt, tend to move more slowly through the

gastrointestinal tract than fluids, which allows for more time to digest the lactose (de

Vrese et al., 2001). This same effect is found when milk is consumed with a meal.

Those who maldigest lactose typically use trial and error to determine the

lactose-containing foods to include in their diets. They experiment with the amount

consumed, the form (that is, solid, semisolid, liquid), and the presence of fermentation

bacteria. Other tactics include adding lactase tablets to food or drinking milk that has

lactase added. These strategies allow people to consume dairy products, a

concentrated source of calcium in the diet. In several studies of Caucasians with

lactose intolerance, subjects limited their dairy intake, and therefore, their calcium

intake. Lactose maldigestion is thought to be one factor that contributes to low

calcium intake and low bone mineral density, both of which are associated with

osteoporosis.

The hormonal response to carbohydrate intake described earlier in the chapter

helps the body to keep blood glucose in homeostasis. About 90 percent of Americans

have a normal response, but at least 23 million Americans have diabetes. In those with

type 1 diabetes, insulin secretion is absent. These individuals will need to match their

food intake with the proper amount of insulin, which is injected or released from an

insulin pump. In those with type 2 diabetes (about 21 million of the 23 million people

with diabetes), insulin secretion is diminished or the cells are resistant to the influence

of insulin. These individuals may take medications to stimulate the release of more

insulin. They should reduce the intake of high-glycemic index foods that rapidly

increase blood glucose concentrations. If insulin resistance is present, reducing excess

body fat and engaging in at least 30 minutes of moderately intense exercise 5 days a

week will help reduce the body’s resistance to the action of insulin.

F. Protein Basics

Protein functions in the body in many ways. A primary function of protein is

to build and maintain tissues. To increase muscle mass, an athlete must engage in

resistance exercise, consume a sufficient amount of energy (kcal), and be in positive

nitrogen (protein) balance. Because so much emphasis is put on protein and muscle

growth, it can easily be forgotten that protein is also the basis of enzymes and many

hormones, and of structural, transport, and immune system proteins. Although it is not

the primary function of protein, the amino acids that make up protein can be used to

provide energy. In prolonged endurance exercise, amino acids are an important energy

source even though carbohydrates and fats supply the majority of the energy.

The basic component of all proteins is the amino acid, a nitrogen-containing

compound. Proteins found in food and protein supplements are broken down into

amino acids through the processes of digestion and absorption. Once absorbed, the

amino acids are transported to the liver, which plays a major role in amino acid

metabolism. Body proteins are constantly being manufactured and broken down.

Dietary protein and degraded body proteins provide a steady stream of amino acids

for protein synthesis. Sufficient energy intake is needed to support the growth and

maintenance of skeletal muscle; insufficient caloric intake does not support growth

and undermines the ability to build and maintain skeletal muscle. In a prolonged

starvation state, the body's ability to maintain nitrogen balance is compromised and

skeletal muscle tissue will be sacrificed to ensure survival.

Proteins are made up of amino acids, which contain carbon, hydrogen,

oxygen, and nitrogen. It is the nitrogen that distinguishes them from the composition

of carbohydrates, fats, and alcohol, which are made up of only carbon, hydrogen, and

oxygen. To understand their functions one must understand the structures of proteins.

The basic structural component is an amino acid. An amino acid is a chemical

compound that contains an NH, (that is, an amino) group and a COOH (that is,

carboxyl) group of atoms. The basic structure of an amino acid is shown in Figure 5.1.

The nitrogen content of an amino acid is approximately 16 percent. There are a total

of 20 different amino acids that will be used by the body to make various proteins.

Amino acids may have side chains (e.g., glycine, leucine), acid groups (e.g.,

glutamine), basic groups (e.g., lysine), or rings (e.g., tryptophan). Some contain sulfur

(e.g., cysteine, methionine).

Of the 20 amino acids needed by healthy adults, 9 are considered

indispensable because the body cannot manufacture them. The remaining 11 amino

acids are termed dispensable because they can be manufactured in the liver. Six of

these 11 amino acids are referred to as conditionally indispensable, because during

periods of stress the body cannot manufacture a sufficient amount. Illness, injury, and

prolonged endurance exercise are examples of physiologically stressful conditions. In

the past, the terms essential and nonessential were used to describe indispensable and

dispensable amino acids, respectively. Nonessential is a misleading term because it

implies that such amino acids are not needed. In fact, they are needed but the body has

the ability to manufacture them if they are not consumed directly from food.

Indispensable and dispensable are now the preferred terms when describing amino

acids.

Protein quality is determined based on the amounts and types of amino acids

and the extent to which the amino acids are absorbed. Protein quality is a critical issue

in human growth and development. In countries where protein foods are abundant,

sufficient protein quality is a near certainty, an assumption that should not be made in

countries where protein foods are limited. Humans must obtain through diet all of the

indispensable amino acids, which are found in lower concentrations in plant proteins

than in animal proteins. Animal proteins are termed complete proteins because they

contain all the indispensable amino acids in the proper amounts and proportions to

each other to prevent amino acid deficiencies and to support growth. In contrast, plant

proteins may lack one or more of the indispensable amino acids or the proper

concentrations and are termed incomplete proteins. The indispensable amino acids

that are of greatest concern are lysine, threonine, and the sulfur-containing amino

acids, cysteine and methionine. If the intake of these specific amino acids is limited,

then protein deficiencies could occur.

Peptide refers to two or more amino acids that are combined. Specifically,

dipeptide refers to two amino acids, tripeptide to three amino acids, and polypeptide

to four or more amino acids. Most proteins are polypeptides and are made up of many

amino acids, often numbering in the hundreds or thousands. Protein and polypeptide

are terms that are used interchangeably. Dipeptide and tripeptide are terms that are

typically used when discussing digestion and absorption.

The secondary structure of the polypeptide is a result of bonding of amino

acids that are located close to each other. These bonds give more rigidity and stability

to the protein, an important characteristic for structural proteins such as collagen. The

tertiary (third) level of structure is a result of interactions of amino acids that are

located far away from each other. These interactions, if present, cause the polypeptide

to form a loop. The loop results in a clustering of certain amino acids, which then

function in a particular way. For example, a cluster of amino acids may have a

positive or negative charge and accept or repel other compounds, such as water.

Quaternary (fourth) level structure involves more than one polypeptide, typically two

or four. Because of their quaternary structure, these proteins can interact with other

molecules. Insulin, which interacts with glucose, is an example of a compound made

up of two polypeptides.

The immune system is a protein-based system that protects the body from the

invasion of foreign particles, including viruses and bacteria. One immune system

response is the activation of lymphocytes, cells that produce antibodies. All antibodies

are compounds that are made of polypeptide chains (usually four) in the shape of a Y.

The antibody fits the virus or bacteria like a key ina lock, aiding in their destruction.

The shape of the “key” is due to disulfide bonds and the sequence of the amino acids.

Some amino acids help to regulate the immune system (see Spotlight on...Amino

Acids as Regulators).

G. Proteins and Amino Acids Processes of Metabolism

Digestion begins as soon as proteins found in food arrive in the stomach.

Absorption takes place primarily in the middle and lower small intestine by several

mechanisms. Once absorbed, the amino acids will be transported to the liver, which

acts as a clearinghouse. After a meal the majority of the amino acids absorbed will

remain in the liver for metabolism. The remainder will circulate in the blood and be

transported to other parts of the body.

Protein digestion begins when a food protein comes in contact with the gastric

juice of the stomach. The hydrochloric acid (HCl) in the gastric juice begins to

denature (change the structure of) the protein. At the same time, the HCl activates

pepsin, an enzyme that will break down the polypeptides into smaller units. Pepsin

prefers to break the bonds of certain amino acids, such as leucine and tryptophan. This

initial stage of digestion generally breaks down very large polypeptides into smaller

units, but these smaller units are still very large amino acid chains.

As the denatured and partially digested polypeptides move from the stomach

to the small intestine, a number of digestive enzymes are activated. Some of these are

found in pancreatic juice, which is secreted from the pancreas into the small intestine.

Other digestive enzymes are released from the cells of the brush border that line the

gastrointestinal tract. Similar to pepsin, these enzymes prefer to break the bonds of

specific amino acids. For example, some enzymes break down amino acids with rings

whereas other enzymes break down amino acids with side chains.

The liver serves as a clearinghouse for the amino acids by monitoring the

supply arid dictating which amino acids will be transported to which tissues.

Exceptions to this are the branched chain amino acids (BCAA)— leucine, isoleucine,

and valine. The liver has very low levels of the enzyme BCAA transferase, which is

needed to transfer these amino acids to tissues. Therefore, the branched chain amino

acids leave the liver, circulate in the plasma, and are taken up by skeletal muscle cells,

which have high levels of the enzyme that the liver lacks. BCAA transferase is also

found in the heart, kidneys, and adipose tissue.

Protein from food provides about two-thirds of the amino acids absorbed from

the small intestine. These amino acids are exogenous, meaning they originate from

outside of the body. The other one-third of the amino acids is endogenous, originating

from inside the body). Endogenous proteins include mucosal cells shed into the

gastrointestinal tract and gastrointestinal secretions that contain enzymes and other

protein-based compounds. These endogenous proteins are broken down and absorbed

in a manner similar to proteins originally derived from food, although they are often

absorbed lower in the gastrointestinal tract, including the colon. This is the body’s

way of recycling amino acids, but not all of them can be reclaimed. Those that are not

absorbed are excreted in feces, which represents one way that nitrogen is lost from the

body and is one reason why dietary protein must be consumed daily. Once amino

acids are absorbed, the body does not distinguish between the amino acids originally

obtained from food and those from endogenous sources.

The liver is a major site for amino acid metabolism. The liver monitors the

body’s amino acid needs and responds accordingly with anabolic or catabolic

processes. Anabolic is defined as building complex molecules from simple molecules.

An example is the synthesis of a protein. Catabolic is the breakdown of complex

molecules into simple ones. The use of protein for energy is a catabolic process, as the

protein must be broken down into its amino acid components, some of which are then

metabolized for energy. The liver plays the primary role in amino acid metabolism but

it functions in concert with other tissues, such as skeletal muscle and kidneys.

The liver uses approximately 20 percent of these amino acids to make proteins

and other nitrogen-containing compounds. The liver catabolizes (breaks down) the

majority of the amino acids delivered from the gastrointestinal tract. Two important

metabolic processes are deamination and transamination. Deamination refers to the

removal of the amino group from the amino acid (Figure 5.4). When the amino group

is removed, the remaining compound is an alpha-keto acid (a-keto acid), frequently

referred to as the carbon skeleton. Transamination involves the transfer of an amino

group to another carbon skeleton, whereby an amino acid is formed. Transamination

allows the liver to manufacture dispensable amino acids from indispensable amino

acids. Deamination and transamination are regulated by enzymes and are part of an

intricate system that the liver uses to monitor and respond to the body’s amino acid

and protein needs.

One of the major functions of the liver is protein anabolism. Some amino acids

will be incorporated into liver enzymes. Others will be used to make plasma proteins.

For example, the liver manufactures albumin, a protein that circulates in the blood and

helps to transport nutrients to tissues. Many of the proteins made in the liver are

synthesized and released in response to infection or injury. As mentioned earlier, the

liver continually monitors the body’s amino acid and protein needs and responds to

changing conditions.

Those adults who want to be in a growth state must achieve positive nitrogen

balance and positive net protein balance. Adult growth states include pregnancy and

substantial increases in skeletal muscle mass. Athletes who are trying to increase

skeletal muscle size (hypertrophy) must be in positive nitrogen balance. To achieve

positive nitrogen balance, both energy and protein intake must be sufficient. As

athletes train hard and work toward their goal of increasing muscle size, there will be

periods of protein imbalance. Resistance training results in muscle protein breakdown

at the time of the exercise, but during recovery, rest and food intake stimulates muscle

protein synthesis and results in positive net protein balance.

There are many factors that influence muscle protein synthesis and

degradation, including one’s genetic potential for cellular protein synthesis. The

synthesis of skeletal muscle protein is strongly influenced by exercise, specifically

strength training. The mechanical force that is developed by muscle during strength

training stimulates both protein synthesis and protein breakdown. Protein synthesis is

stimulated to a greater degree because one of the adaptations of skeletal muscle in

response to strength training over time is hypertrophy, an increase in the amount of

muscle tissue. The greatest increase is in myofibrillar proteins, the proteins that make

up the force-producing elements of the muscle.

Prolonged strenuous exercise also results in some positive effects on the

immune system, such as a reduction of systemic inflammation. Systemic

inflammation is related to an increase in certain cy- tokines, signaling proteins that

can affect how a cell functions. Some cytokines induce inflammation, which is

believed to be a major factor associated with many chronic diseases, such as

cardiovascular disease. Strenuous exercise results in an increase in some of the anti-

inflammatory cytokines. In this respect, prolonged strenuous exercise is a positive

factor for good health.

H. Protein Recommendations for Athletes

The Dietary Reference Intake (DRI) for adults is 0.8 g of protein/kg body

weight daily (Institute of Medicine, 2002), although this amount may be too low to

protect against the loss of skeletal muscle mass associated with aging (Elango et al.,

2010). Athletes in training need more protein than nonathletes. There is a general

consensus on the amount of protein needed daily by strength, endurance, and

ultraendurance athletes, although controversies still exist and more research on highly

trained athletes is needed. In addition to daily protein intake, athletes in training

should also plan the timing of their protein intake, particularly immediately after

exercise. An emerging area of research is the type of the protein consumed and its

potential role in optimal protein synthesis.

The precise amount of protein needed each day by athletes in training is not

known. Relatively few studies have been conducted in athletic populations, and one of

the traditional measures used in research, nitrogen balance, has known limitations for

predicting requirements in well-trained athletes. Therefore, protein recommendations

for athletes, especially those who wish to substantially increase skeletal muscle mass,

are somewhat controversial.

Vegetarians do not consume the flesh of animals, such as meat, fish or poultry,

but may consume animal products, such as eggs, milk, or cheese. Vegans do not

consume any product derived from animals. Due to these differences in food intake, it

is advisable to distinguish vegans from vegetarians and nonvegetarians. There have

been no studies examining the amount of protein required by vegetarian or vegan

athletes. A common recommendation for physically active vegetarians or vegans is to

consume 10 percent more protein than the amount recommended for nonvegetarian

athletes. This 10 percent figure is an adjustment for the lower digestibility of plant

proteins when compared to animal proteins. Consuming this amount of protein is not

difficult if energy intake is sufficient.

The acute effect of exercise is to put the body into a catabolic state, breaking

down certain tissues to provide the energy to sustain the exercise. For example,

muscle glycogen is broken down to provide glucose, stored fats are broken down to

mobilize fatty acids, and skeletal muscle proteins are broken down to provide amino

acids that can be used as energy (for example, leucine). However, the catabolic state is

followed by an anabolic state, an environment that allows for recovery from the acute

effects of exercise and for skeletal muscle growth.

There is strong scientific evidence that postexercise protein intake is effective

for support of skeletal muscle growth after resistance exercise (Tipton et al., 2004).

However, protein intake immediately after exercise does not appear to be effective for

increasing strength or power or changing body composition (Hoffman et al., 2009).

Foods or supplements containing intact proteins, such as whey, casein, or soy,

stimulate an increase in skeletal muscle mass. Particularly effective is the combination

of whey and casein, which naturally occurs in milk but is also found in some protein

supplements.

As a practical matter, many athletes choose a postexercise food or drink that

contains both carbohydrate and protein. As described in Chapter 4, carbohydrates,

especially high-glycemic carbohydrates, consumed immediately after exercise are

beneficial because they help restore muscle glycogen. The carbohydrates also

stimulate the release of insulin. Although its primary role is cellular glucose uptake,

insulin also increases amino acid uptake into muscle and inhibits the process of

muscle degradation. The important point is to provide the body with the nutrients it

needs immediately after exercise to begin resynthesis of tissue that has been

catabolized during exercise. In this respect, consumption of carbohydrate and protein

(both of which also provide energy) shortly after exercise ends is advantageous. Some

popular choices for athletes to consume after exercise include low-fat chocolate milk,

fruit-in-the-bottom yogurt, and sports beverages that have been formulated for

consumption during recovery.

I. Effect of Energy Intake on Protein Intake

The amount of protein required is related to energy intake. Under normal

conditions, an adequate energy intake from either carbohydrate or fat spares amino

acids from being used for energy and helps maintain nitrogen balance. Athletes need

to be in nitrogen balance to maintain muscle mass and need to be in positive nitrogen

balance to increase muscle mass. Adjustments to protein intake will need to be made

when energy intake is deficient. The amount of protein needed depends on the

magnitude of the energy deficit and whether it is acute or chronic. The most serious

situations are those athletes who self-impose starvation over several months or years.

Some athletes maintain small chronic energy deficits, and they may need to slightly

increase their protein intake as a compensatory measure.

When a chronic energy deficit is present, more protein is needed than when

energy intake is sufficient. Nitrogen balance cannot be maintained if energy and

protein intakes are too low. Studies of athletes with anorexia nervosa report low

intakes of both energy and protein. In one study, the mean daily protein intake of

athletes with anorexia nervosa was 0.7 g/kg body weight (range 0.5 to 1.0 g/kg/d)

(Sundgot-Borgen, 1993). Athletes with eating disorders are struggling with

psychological issues that interfere with the consumption of food and will need intense

counseling from well-trained practitioners. Among the nutritional goals will be

appropriate energy and protein intakes.

Low-calorie diets can lead to a significant loss of lean body mass. There is

much evidence that high-protein, low-energy diets can achieve positive nitrogen

balance in rats. The body of literature in humans is smaller and not as well defined

(Millward, 2001, 2004). Nevertheless, it is also recommended that when humans are

energy deficient they should increase protein intake in an effort to maintain nitrogen

balance.

Many athletes periodically want to lose body fat quickly and will substantially

reduce caloric intake for a short time (often a few weeks). When energy deficits are

large and short term, such as when an athlete is “making weight,” the goals are to lose

body weight through fat and water loss but maintain as much skeletal muscle protein

as possible. This is difficult to accomplish. Under semistarvation conditions, the loss

of weight comes from several components, including water, glycogen, protein, and

fat. In the first 10 days of fasting/ starvation, only about one-third of the body weight

lost is typically lost as body fat (Brownell, Steen, and Wilmore, 1987), whereas 6-16

percent is lost from protein stores. This type of severe energy restriction raises serious

concerns about hydration status, the potential for heat illness, maintenance of skeletal

muscle mass, ability to exercise due to depleted glycogen stores, hypoglycemia, and

declines in resting metabolic rate.

Studies have shown that athletes who consume an inadequate amount of

protein also usually consume an inadequate amount of energy (kcal), so the problems

they face are numerous. The lack of protein is an especially important problem

because of its critical role in building and maintaining muscle mass and supporting

the immune system. Skeletal muscle hypertrophy (an increase in muscle size) cannot

take place in the absence of food. Resistance exercise is a powerful stimulus for

skeletal muscle protein synthesis, but it also results in skeletal muscle protein

breakdown. Thus protein and carbohydrate ingestion, particularly in the two to three

hours after exercise, is necessary for a net gain in skeletal muscle protein. To achieve

positive net protein balance, a postexercise feeding is needed.

J. Supplementation with Individual Amino Acids

Proteins are made up of amino acids, which have specific biochemical

functions. For example, some amino acids can stimulate growth hormone release or

muscle protein synthesis in the presence of resistance training, whereas others are

integral to immune system function. A logical question is whether supplementation

with individual amino acids can enhance specific biochemical functions and be an

effective way to increase skeletal muscle size or strength or enhance immune system

function, which in turn could lead to positive changes in body composition and

performance.

Beta-alanine is a dispensable amino acid. It is not the same as the amino acid

alanine, which has a different structure. Beta-alanine is found in protein-containing

foods such as meat, poultry, and fish as part of the dipeptide, carnosine. Carnosine is

broken down in the gastrointestinal tract to beta-alanine and histidine. The liver can

also manufacture beta-alanine.

HMB is a metabolite of leucine, one of the indispensable amino acids. Leucine

is one of the branched chain amino acids and has some anticatabolic properties. In

theory, supplementation with HMB could minimize the protein breakdown that

follows resistance exercise. Research has focused on whether HMB supplements

could reduce muscle damage, increase muscle strength, and change body

composition.

Leucine, isoleucine, and valine are the branched chain amino acids, so named

because of their chemical structure. During prolonged endurance exercise when

glycogen stores are low, skeletal muscle can metabolize these amino acids for energy.

In addition to being used as an energy source, BCAAs compete with tryptophan, an

amino acid associated with mental fatigue. BCAAs are also involved in the immune

system (Newsholme and Blomstrand, 2006; Gleeson, 2005). Resistancetrained

athletes use BCAA supplements in an effort to reduce skeletal muscle damage and

muscular fatigue.

Glucosamine and chondroitin, which are usually sold together, is a dietary

supplement marketed to relieve joint pain in those with osteoarthritis and prevent

cartilage breakdown in athletes. Glucosamine is manufactured by the body from

glucose and the amino acid glutamine, and is not related to dietary intake. It is part of

glycosaminoglycan (an unbranched polysaccharide), which is found in the

extracellular matrix of the joints. Because of its ability to attract water, it is referred to

as a “joint lubricant.” Chondroitin is also synthesized by the body and is part of a

protein that aids in elasticity of cartilage.

Exercise is a potent stimulator of growth hormone. Certain amino acids, such

as arginine, ornithine, and lysine, can also stimulate growth hormone release

(Kanaley, 2008; Collier, Casey, and Kanaley, 2005; Collier, Collins, and Kanaley,

2006). Some athletes consume these amino acids before strength training to enhance

theexercise-inducedrelease of growth hormone and to promote greater gains in muscle

mass and strength. These amino acids may be consumed individually or together and

are often advertised as growth hormone releasers.

K. Fatty Acids, Sterols, and Phospholipids

The word fat is used in many different ways. In nutrition, fats are energy-

containing nutrients found in food. In medicine, fats are known as lipids, large fat-

containing components in the blood. In physiology, a fat is a long chain of carbon

molecules. In all of these disciplines, fat is also used to describe the body’s long-term

storage site for fats, although the precise term is adipose tissue. In everyday language

fat is often used as an adjective, describing a body weight that is greater than

desirable.

Fats vary in their chemical composition. The predominant fats in food and in

the body are triglycerides, which are made up of three fatty acids attached to a

glycerol molecule. Sterols, such as cholesterol, and phospholipids, phosphate-

containing fats, are also found in food and in the body. These three classes of fat

compose the category known as lipids. To understand the differences in the various

fats, one must look closely at their chemical composition. This discussion begins with

fatty acids, which are chains of carbon and hydrogen ending with a carboxyl group (a

carbon with a double bond to oxygen and a single bond to an oxygen/hydrogen,

written as COOH). The length of the fatty acid chain can range from 4 to 24 carbons.

The number of carbons will be an even number because fatty acid chains are

manufactured by adding two car-' bons at a time. An example of a fatty acid is shown

in Figure 6.1. The fatty acid in this example, oleic acid, has 18 carbons. The fatty

acids used most commonly in human metabolism have 16 or 18 carbons.

Unsaturated fatty acids contain one or more double bonds between carbons,

reducing the number of hydrogen atoms that can be bound to the structure. When only

one double bond between carbons is present, it is referred to as a monounsaturated

fatty acid (mono means “one”). When two or more double bonds are present, these

fatty acids are referred to as polyunsaturated fatty acids (poly means “many”). When

double bonds between carbons are present, as in the case of mono- and

polyunsaturated fatty acids, the fatty acid can be in the cis or trans formation. Cis

refers to groups that are on the same side of the double bond between carbons. Trans,

which means. “across” or “on the other side,” refers to groups that are on opposite

sides of the double bond between carbons. The vast majority of unsaturated fatty acids

occur naturally in the cis form. The cis form allows fatty acids to “bend,” which is an

important feature when these fatty acids are incorporated into cell membranes.

Although some trans fatty acids are found in nature, most are produced synthetically

through the addition of hydrogen atoms to an unsaturated fatty acid. This results in the

fatty acid chain being “straight.” This hydrogenation process is used in commercial

food processing to make liquid oils more solid (for example, soybean oil made into

margarine) and to increase the shelf life of the product.

Two 18-carbon fatty acids are essential fatty acids— linoleic and alpha-

linolenic (a-linolenic). The body cannot manufacture these essential fatty acids, so

they must be consumed in the diet. Fortunately, these two essential fatty acids are

widely found in food. Linoleic, an omega-6 fatty acid, is in many vegetable oils such

as corn, soy, safflower, and sunflower oils. Alpha-linolenic, a member of the omega-3

family, is found in soy, canola, and flaxseed oils. It is also found in leafy green

vegetables, fatty fish, and fish oils.

The omega series fatty acids include omega-3, -6, and -9 fatty acids, which are

briefly summarized in Table 6.1. The best-known omega-3 fatty acids are alpha-

linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid

(DHA). As noted on page 202, alpha-linolenic acid is an essential fatty acid and is

widely distributed in food. The body can convert ALA into EPA and DHA, but the

amount that can be converted is very limited. In the past 20 years, researchers have

focused on the beneficial effects of EPA and DHA, which include their potential for

reducing cardiovascular disease risk by having a positive effect on blood lipids (see

Chapter 13); their impact on the immune system, and their roles in acute

inflammation. The latter two are of particular interest for athletes who engage in

prolonged and intense exercise, which results in an acute increase in inflammatory

compounds and is a chronic physiologic stress on the immune system (see Spotlight

on... Omega-3 Fatty Acids and Athletes).

Almost 95 percent of the fat found in foods is in the form of triglycerides; the

remaining fats in food are either sterols or phospholipids. Fatty acids are chains of

carbon, but sterols have a different chemical composition. Sterols belong to a group of

fats whose core structure is made up of four rings. This four-ringed nucleus is known

as a steroid. Various side chains can be added to the steroid nucleus and many

different compounds can be made, including cholesterol, vitamin D, and the steroid

hormones, including the sex hormones (Figure 6.7). If the steroidbased compound has

one or more hydroxyl (OH) groups attached and no carbonyl (=C=O) or carboxyl

(COOH) groups, then the compound is known as a sterol.

L. Metabolism of Fats

Fats are large and complex molecules that do not mix easily with water.

Therefore, fats must be exposed to bile salts and digestive enzymes before they can

cross the membranes of the intestinal cells (see figure 6.8). An important digestive

enzyme is pancreatic lipase. As the name implies, this enzyme is secreted by the

pancreas into the small intestine and helps to break down the large fatty acids into

smaller components. Recall that all but 5 percent of the fat found in food is

triglyceride (that is, triacylglycerol), which is composed of three fatty acids attached

to a glycerol molecule. These three-unit fats are broken down by enzymes to two-unit

fats known as diglycerides (that is, diacylglycerols), one-unit fats, monoglycerides

(that is, monoacylglycerols), and free fatty acids. Phospholipids are involved in a

similar digestive process, although the enzymes are different. Cholesterol is not

broken down at this point. The process of digestion reduces the size of the fat particles

and readies them for absorption by the mucosal cells of the intestine.

To understand how fats are transported, one must understand the body’s main

transport fluids: blood and lymph. Blood consists of water, red and white blood cells,

and many other constituents, including oxygen and nutrients. Blood enters tissues

through arteries, leaves tissues through veins, and circulates within the tissues via the

capillaries. Some components of blood are filtered out of the capillaries into the

spaces of the tissue. This fluid is known as interstitial fluid. Most of the interstitial

fluid is returned to the capillaries but some is not. That which is not returned is

referred to as lymph. Lymph consists of white blood cells (which play an important

role in immune function), proteins, fats, and other compounds. Lymph moves through

its own set of vessels (the lymphatic system) that are separate from capillaries.

Eventually, the lymph and blood vessels are joined near the heart.

Once fats are digested, absorbed, and circulated, they are stored in the body

largely in the form of triglycerides. The main sites of fat storage in the body are

adipose tissue (in adipocytes [fat cells]), liver, muscle (as intramuscular triglycerides),

and to a small degree in the blood. As has been previously discussed in Chapter 3, fats

are metabolized for energy through oxidative phosphorylation, the aerobic energy

system. In order to be metabolized, fats must be removed from storage, transported to

cells, and taken up into mitochondria. There they are oxidized via the Krebs cycle,

and ATP is produced via the electron transport chain.

The process of triglyceride formation is called esterification. The enzyme

lipoprotein lipase exists in the walls of the capillaries that perfuse fat cells. When this

enzyme is activated, it results in the breakdown of circulating triglycerides from

lipoproteins, freeing fatty acids for uptake into the fat cells. Once taken up into

adipocytes, the fatty acids are re-formed into triglycerides for storage. The activity of

LPL and the process of triglyceride formation are primarily stimulated by the

hormone insulin. The pancreas secretes insulin in response to food consumption,

particularly a meal containing carbohydrate. Therefore, in the hours after a meal

(particularly a meal containing fat and carbohydrate), the body has the hormonal

environment and the substrates that favor triglyceride formation aud fat storage. An

abundance of adipocytes can be found just beneath the skin (subcutaneous fat) and

deep within the body surrounding the internal organs (visceral fat).

M. Fats Energy

Fat is an important fuel source for energy production at rest and during

exercise. As mentioned in previous chapters, carbohydrate and fat are the two major

fuel sources, with protein playing a much smaller role. The degree to which either of

these fuel sources may contribute to the body's energy needs is dependent upon a

variety of factors that will be discussed in this section, with an emphasis on the factors

that influence fat oxidation. Next, observe the metabolic response when he begins

running at a steady pace of 9 minutes and 30 seconds (9:30) per mile, a modest

exercise intensity for this athlete. As one would expect, heart rate increases above

resting (127 bpm) and total energy expenditure increases substantially to 11.5 kcal/

min. In other words, he burns 11.5 kcal every minute he runs at this pace. The RER

rises to 0.88, indicating the percentage of energy derived from fat has dropped to 40.8

percent whereas that provided by carbohydrate has increased to 59.2 percent. Even at

this fairly modest exercise intensity, fat is no longer the predominant source of energy

for running. However, even though the percentage of energy from fat has declined, the

absolute number of kcal from fat metabolism has increased dramatically over what

was seen at rest. This makes perfect sense—although the percentage is less (40.8

percent compared to 78.2 percent at rest), it is a smaller percentage of a much larger

number—the total energy expenditure has increased 8-fold, from 1.4 kcal/min at rest

to 11.5 kcal/min during exercise.

The subject in this case study had a goal to run a marathon, a prolonged

distance run of 26.2 miles. The data from the metabolic study show what happens to

energy expenditure and fuel utilization if the athlete runs at different intensities, but

what metabolic response occurs when he runs at a steady pace for several hours? This

runner did indeed complete a marathon in celebration of his 50th birthday. His final

time for the marathon was 3 h, 55 min, 48 sec, an average pace of 9:00 minutes per

mile. Reviewing the metabolic study data, at a pace of 9:00 minutes per mile this

runner was obtaining approximately 37.4 percent of his energy from fat metabolism

and 62.6 percent from carbohydrates. As a runner continues to exercise for a

prolonged period of time, however, carbohydrate stores are reduced significantly,

eventually leading to muscle glycogen depletion. As the available carbohydrate stores

are diminished, the body has no choice but to rely more on fat oxidation. Therefore, a

very common metabolic response to prolonged exercise is a slight, gradual decline in

the RER, indicating a reduced reliance on carbohydrate and an increased dependence

on fat oxidation.

Endurance exercise training results in an enhanced ability to oxidize fat. This

is potentially advantageous to endurance athletes—if they can rely more on fat

metabolism during an endurance event, they may be able to “spare” the body’s limited

carbohydrate stores (that is, muscle glycogen) for use later in the event and improve

their performance. The regular stimulus of chronic exercise training taxes the

oxidative phosphorylation energy system and the oxidative pathways of fat and

carbohydrate metabolism. Over time a number of physiological adaptations occur that

enhance the body’s fat oxidation capability. Fatty acids are mobilized from adipocytes

more easily and are taken up into muscle cells more readily. Cardiovascular

adaptations include an increase in the capillary network in muscle, which allows for

an enhanced delivery of fatty acids. One of the most important adaptations to

endurance training that aids fat oxidation is an increase in mitochondrial mass in the

muscle due to an increase in both the number and size of these important oxidative

organelles. Mitochondrial mass can double in response to endurance training and

results in an overall increase in activity of oxidative enzymes, once again enhancing

the body's ability to metabolize fat.

N. Fat-Related Dietary Supplements

Athletes in various sports use caffeine to improve performance and to delay

fatigue. The strongest scientific evidence shows that caffeine can enhance endurance

performance in distance runners, cyclists, and cross country skiers. There is also

evidence that caffeine can improve performance for those engaged in high-intensity

activities lasting 1-20 minutes, including runners, cyclists, swimmers, and rowers.

Athletes in other sports have been studied, but the evidence for improving

performance is not conclusive.

Caffeine improves endurance performance because it is a central nervous

system stimulant that results in a heightened sense of awareness and a decreased

perception of effort. Although caffeine may enhance free fatty acid mobilization

during endurance exercise, more fat is not oxidized and reliance on muscle glycogen

is not reduced. Many athletes engaged in resistance training use caffeine to delay

fatigue while training. Caffeine may enhance contractile force in skeletal muscle

during submaximal contractions. It may also increase the athlete’s threshold for pain

or perceived exertion, which could result in longer training sessions.

Carnitine is essential to transport fatty acids into the mitochondria where they

can be broken down for energy. Whenever a substance is known to have a direct role

in metabolism, an intriguing question is raised: Would a concentrated amount, such as

that found in a supplement, enhance the normal metabolic process? Carnitine is found

in food and can be synthesized in the body from the amino acid lysine. Deficiencies

have been reported in humans but they are rare. As would be expected, in carnitine-

deficient individuals supplementation normalizes long-chain fatty acid metabolism.

Healthy adults are not carnitine deficient and exercise does not result in the loss of

carnitine in the muscle.

As described earlier in the chapter, medium-chain triglycerides contain 6 to 10

carbon atoms. They are rapidly absorbed via the portal vein and are easily transported

into the mitochondria. For these reasons, MCT are sometimes advertised as being an

energy source that is as readily available as carbohydrate. It is important to know if

the use of MCT by endurance athletes could increase fat oxidation during moderate-

to high-intensity exercise, reduce reliance on muscle glycogen stores, or enhance

performance.

Omega-3 fatty acid supplements (fish oil supplements) are marketed to

athletes as a way to reduce inflammation, reduce the effects of oxidative stress, and

counteract im- mune dysfunction associated with strenuous exercise. Typically,

omega-3 fatty acid supplements contain eicosapentaenoic acid (EPA) and

docosahexaenoic acid (DHA), although some may also contain alphalinolenic acid

(ALA). EPA is usually found in the greatest proportion. The dosages used in research

studies are generally greater than 2.4 g/day of omega-3 fatty acids, with various

mixtures of EPA, DHA, and, sometimes, ALA.

Ads targeting athletes often suggest supplementing with 1-2 g of omega-3

fatty acids daily. Recommendations for higher doses for trained athletes, particularly

endurance athletes, have been made (Simopoulus, 2008); however, there is a lack of

evidence to support such recommendations and there are concerns about high-dose

supplementation with omega-3 fatty acids. No Dietary Reference Intake (DRI) has

been established to date; however, the Food and Drug Administration recommends

that consumers not exceed a total of 3 g/d EPA and DHA from all ‘sources, with no

more than 2 g/d from a dietary supplement.

O. Best Fat for Athletes

The appropriate amount of dietary fat for the athlete will depend on two

factors—overall energy (caloric) need and macronutrient balance. Recall that the four

energy-containing nutrients are carbohydrate, fat, protein, and alcohol. Though each

nutrient can be considered separately, the relationships among them are also

important. Typically only carbohydrate, fat, and protein are included in macronutrient

balance discussions. Alcohol is usually not included in general recommendations for

athletes because it contains no essential nutrients and many athletes cannot legally

consume alcohol due to age restrictions.

To determine the amount of dietary fat needed, one must also know how much

carbohydrate, protein, and total energy (kcal) the athlete needs. This discussion

assumes that the athlete is in energy balance. In other words, the athlete does not want

to change body weight or composition and energy intake is equal to energy output. In

such cases, how much dietary fat does such an athlete need to consume? In some

respects this is a mathematical problem. Because the athlete wishes to remain in

energy balance, the daily number of kcal needed to match energy expenditure must be

determined. The amount of carbohydrate necessary to support the demands of the

sport and the goals of the training cycle must be established. The daily protein goal is

also important information. Once those figures are obtained, the amount of fat can be

calculated.

Many athletes do not wish to be in energy balance; instead they wish to

maintain an energy deficit for a period of time. The reason for the deficit is to force

the body to use stored body fat for energy. The athlete’s goal is to attain a lower

percentage of body fat, which presumably will translate to a performance advantage.

A low percentage of body fat may be advantageous to performance depending on the

sport. For example, there is a potential performance advantage for a 10,000 m (6.2-

mile) runner to attain a low percentage of body fat because there is less body mass

(weight) to be moved. Excess body fat is “dead weight” as it does not produce any

force to help with the exercise task and is extra weight that must be carried. However,

performance and health can be negatively affected when body fat stores are too low or

when dietary fat intake is severely limited.

Chronic inadequate fat intake, and the energy restriction that usually

accompanies it, has the potential to negatively affect training, performance, and

health. These effects may include: (1) inadequate replenishment of intramuscular fat

stores, (2) inability to manufacture sex-related hormones, (3) alterations in the ratio of

high- and low-density lipoproteins (HDL:LDL), and (4) inadequate fat-soluble

vitamin intakes. Chronic fat restriction may negatively impact the manufacture of sex-

related hormones such as testosterone. Some studies of healthy men have shown that a

low-fat diet (between 18 and 25 percent of total calories) with a high ratio of

polyunsaturated to saturated fat lowered testosterone concentration (Dorgan et al.,

1996; Hamalainen et al., 1984). There have also been reports of lowered testosterone

concentration in wrestlers who consumed fat- and energy-restricted diets (Strauss,

Lanese, and Malarkey, 1985). These studies did not examine the effect that low

testosterone concentration may have on muscle mass, but they have been interpreted

to mean that chronic and severe fat restriction is not desirable for male athletes

because of the potential effect on testosterone production.

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