Role of Nutrition in Athletic Performance
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Exercise training and genetic makeup are major determinants of athletic performance.
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A healthy diet will not substitute for either factor, but making wise food choices will allow athletes to maximize their athletic potential by contributing to endurance, speed, and recovery of muscle tissue.
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Athletes usually lack nutrition knowledge regarding diet and performance, which is often not a priority and leads to misinformation regarding diet and exercise performance.
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In order to educate and assess the dietary practices of athletes, sports medicine professionals require an understanding of sports nutrition to be able to recommend eating patterns that allow athletes to perform at their full potential.
Energy Sources for Muscles
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A basic understanding of how muscles use food as fuel is an important step in understanding an athlete’s diet.
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The body cannot directly use the energy released from food; it must first convert the chemical energy found in food to adenosine triphosphate (ATP) ( Fig. 5.1 ).
Figure 5.1
Sources of ATP for biologic work. ATP is harvested from macronutrients, carbohydrates, proteins, and fats to synthesize ATP for biologic work.
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A small amount of ATP can be found in resting muscle cells—just enough to keep the muscle working maximally for about 1–2 seconds.
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During extended periods of exercise, the body produces additional ATP by adding phosphocreatine to ADP, thereby producing more ATP for muscle contractions.
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Moreover, energy-yielding macronutrients such as carbohydrates, fats, and occasionally proteins are used as energy sources to produce more ATP ( Table 5.1 ).
TABLE 5.1
ENERGY SOURCES IN THE BODY
Energy Source
Storage Area
When Used
Activity
ATP
All tissues
All the time
Sprinting (0–3 seconds)
Phosphocreatine
All tissues
Short bursts
Shot put, high jump, bench press
Carbohydrate
(anaerobic)
Muscles
High intensity lasting 30 seconds to 2 minutes
200-meter sprint, 50-meter swim
Carbohydrate
(aerobic)
Muscles and liver
Exercise lasting 2 minutes to ≥3 hours
Jogging, soccer, basketball, swimming
Fat (aerobic)
Muscles and fat cells
Exercise lasting more than a few minutes; greater amounts are used at lower exercise intensities
Long-distance running, marathons, ultra-marathons, day-long hikes
Carbohydrate: Major Fuel for Short-Term, High-Intensity Exercise
Anaerobic vs. Aerobic Metabolism
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The primary pathway to provide energy for sporting events where exercise intensity is near maximal for 30–90 seconds is glycolysis or anaerobic metabolism.
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During anaerobic glycolysis, lactate is produced; it accumulates in the muscles and increases acidity. As lactate increases, pH in the muscle lowers and is associated with the onset of fatigue. The muscle lactate produced from glycolysis is eventually released into the bloodstream and taken up by the liver, which resynthesizes the lactate into glucose. This pathway is called the Cori cycle and allows glucose to enter the bloodstream and be absorbed by cells to be used as energy.
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During anaerobic exercise, carbohydrate is the only substrate that can be used to resupply ATP.
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If oxygen is available to muscle cells and exercise is performed at a moderate or low intensity, then most of the pyruvate produced during glycolysis is shuttled into the mitochondria for ATP production via aerobic metabolism. Approximately 95% of ATP produced from carbohydrate metabolism is aerobically formed in the mitochondria.
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Aerobic metabolism supplies ATP more slowly than anaerobic pathways, but it releases greater energy and can be sustained for hours.
Glycogen
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Carbohydrates are stored in the form of glycogen in the body; it is stored both in the liver and the muscles.
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Through glycogenolysis, glycogen is broken down to glucose and metabolized via the anaerobic and aerobic pathways. Liver glycogen is used to maintain blood glucose, whereas muscle glycogen supplies glucose to working muscles.
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In events lasting <30 minutes, muscles primarily rely on muscle glycogen. As exercise time increases, muscle glycogen stores decline, and they begin to absorb glucose from the blood as an energy source ( Fig. 5.2 ).
Figure 5.2
Rate of glycogen utilization in exercising muscle.
(Data from Grisham CM, Garrett RG. Biochemistry, 3 rd Ed. Brooks/Cole Publishing Company, Belmont CA; 2006.)
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Once glycogen stores are exhausted, exercise can continue, but the muscles can only work at 50% of maximal capacity; this state is often called “hitting the wall” or “bonking” because further exertion is hampered.
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In certain cases, before a competition, high-carbohydrate diets can be used to increase muscle glycogen stores to up to double the typical amount, thereby delaying the onset of fatigue and improving endurance.
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Maintenance of blood glucose becomes increasingly vital as exercise duration increases beyond 20–30 minutes. By maintaining blood glucose, the body saves muscle glycogen to use during sudden bursts of effort or for a second workout later in the day.
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A carbohydrate intake of 0.7 g/kg/hr during strenuous endurance exercise that lasts for approximately ≥1 hour can help maintain adequate blood glucose, which, in turn, results in delayed fatigue.
Fat: Main Fuel Source for Prolonged, Low-Intensity Exercise
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At rest and during prolonged exercise at low to moderate intensity, fat becomes the predominant fuel source for exercising muscles. In fact, during very long activities such as ultra-marathons, fat supplies approximately 50%–90% of the required energy.
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The rate of fatty acid oxidation in the muscle is affected by training level. The better trained a muscle is, the greater its ability to use fat as a fuel. The more unfit a muscle is, the greater its reliance on carbohydrate as opposed to fat. Training increases the size and number of mitochondria and the level of fatty oxidative enzymes, thus allowing athletes to use fat more readily as a fuel source, eventually conserving glycogen.
Protein: a Minor Fuel Source During Exercise
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Although protein can be used as fuel by muscles, its contribution is relatively small compared with that of carbohydrates and fat.
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Only approximately 5% of the body’s energy needs is supplied by amino acid metabolism; however, proteins can contribute to energy needs during endurance exercise.
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If an athlete exhausts his or her muscle glycogen stores, then proteins can contribute as much as 15% of the energy expenditure for exercise.
Dietary Recommendations for Athletes
Energy Needs
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Athletes need varying amounts of energy depending on their body size, body composition, and type of training or competition.
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Monitoring body weight is an easy way to assess the adequacy of caloric intake; however, several sport dietitians are now using the concept of energy availability to monitor caloric requirements.
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Energy availability is the amount of energy left for bodily functions after the energy costs for training and competition have been calculated. Table 5.2 provides an example of how to calculate adequate energy availability for an athlete.
TABLE 5.2
SAMPLE CALCULATIONS TO DETERMINE ENERGY AVAILABILITY
Athlete’s weight (kg)
68 kg
Workout expenditure (kcal/d)
900 kcal/d
Energy intake (kcal/d)
3200 kcal/d
Percent body fat (%)
13
Lean body mass (kg)
68 kg × .13 = 8.84 kg; 68 kg − 8.84 kg = 59.16 kg
Energy availability (kcal/lean mass)
3200 kcal − 900 kcal = 1900 kcal/59 kg lean mass = 32.2 kcal/lean mass
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Typically, athletes with an energy availability of <30 kcal/kg of lean body mass have a greater risk of metabolic and hormonal disruptions, which could include reproductive functions as well.
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Regardless of the technique used to monitor caloric requirements, both an excess and inadequate intake of calories can result in fatigue, increased risk of injury, prolonged recovery period, and overall poor athletic performance.
Carbohydrates
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Carbohydrates are the primary source of energy for exercising muscle, particularly when exercise intensity reaches 65% or greater of the maximum oxygen consumption (VO 2 max).
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In the past, carbohydrate recommendations have often been expressed as a percentage of total calories; however, this percentage is poorly correlated to both the amount of carbohydrate actually consumed and the required fuel needed to support an athlete’s training and competition; thus, the concept of carbohydrate availability is now being used.
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Carbohydrate availability matches the athlete’s carbohydrate intake to his or her fuel needs for training and competition.
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If athletes fail to consume adequate carbohydrates and energy during daily training, then muscle glycogen levels decrease, and training and competition performance becomes impaired.
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If athletes overconsume carbohydrates, the excess glucose is stored as fat in the body.
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In general, a wide variety of whole grains such as pasta, breads, and rice along with fruits and vegetables provide carbohydrates as well as essential vitamins, trace minerals, and fiber that athletes need.
Carbohydrate Recommendations
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Low-intensity exercise: Carbohydrate intake ranges 3–5 g/kg of body weight
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Moderate-intensity exercise: Carbohydrate intake ranges 5–7 g/kg of body weight
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Endurance exercise for several hours: Carbohydrate intake ranges 6–10 g/kg of body weight
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Extreme exercise lasting 4–5 hours: Carbohydrate intake ranges 8–12 g/kg of body weight
Application
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Carbohydrate intake is particularly important when performing multiple training bouts in 1 day, such as swimming practice or track and field events or in tournament play when multiple games may be played in 1 day. Consumption of multiple meals or snacks that contain carbohydrates throughout the day will ensure a fuel source for exercising muscles.
Protein
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Protein consumption facilitates muscle synthesis and repair.
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Athletes need to consume more than double the protein amount required by a sedentary adult.
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Most athletes can easily meet their protein requirements through a well-planned diet that includes high-quality proteins that are spread throughout the day rather than consumed in large amounts in one single meal.
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Researchers have found that consuming >40 g of protein in a single meal has no additional benefits in stimulating more muscle protein synthesis (MPS), and any excess over this amount is simply oxidized by the body. Moreover, they have suggested that multiple meals containing 20–30 g of protein must be consumed throughout the day to stimulate muscle protein synthesis.
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While the amount of protein an athlete needs is important, so is the quality of the protein because it influences the body’s ability to synthesize MPS.
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Protein quality depends upon its essential amino acid profile, which includes both the specific amount and proportion of essential amino acids.
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Essential amino acids (EAAs) are ones that the body cannot make and must therefore be consumed in the diet.
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There are 9 EAAs, including 3 branched-chain amino acids (BCAAs).
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BCAAs are absorbed faster than small amino acids, and EAAs are absorbed faster than nonessential amino acids with the essential BCAAs leucine, isoleucine, and valine being absorbed the fastest. These physiologic properties give BCAAs the ability to stimulate the pathway that accelerates MPS.
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Research has shown that the amino acid leucine in combination with other BCAAs is critically important in stimulating MPS. Leucine is a key signaling protein of the mammalian target of rapamycin (mTOR) pathway, which is the pathway for stimulating MPS ( Fig. 5.3 ). An effective dose of leucine is about 2–3 g/day. Box 5.1 lists food items that have a high leucine content.
Figure 5.3
Regulators that influence the mTor pathway.
Box 5.1
Concentration of Leucine in Food and Dairy Products
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Meats, fish, chicken, turkey, eggs, and dairy products contain leucine and the other BCAAs
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Effective dose of leucine is approximately 2–3 g/day
12 ounces of milk = 2000 mg leucine
3 egg whites = 990 mg leucine
1 cup of 1% cottage cheese = 2880 mg leucine
3 oz tuna = 1740 mg leucine
3 oz chicken breast = 3690 mg leucine
3 oz beef = 3005 mg leucine
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Protein Recommendations
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Ideally, a mix of animal and plant protein sources should be included in the diet of an athlete.
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Protein consumption for athletes is more than double the requirements for sedentary individuals; however, several athletes exceed their protein requirements by consuming additional protein in the form of shakes, powders, and protein bars. Table 5.3 lists the protein recommendations for athletes.
TABLE 5.3
PROTEIN RECOMMENDATIONS FOR ATHLETES
Specific Athlete Group
Recommendation in g/kg of Body Weight
Endurance athletes
1.2–1.4
Strength athletes
1.6–1.8
Vegetarian athletes
1.3–1.8
Energy-restricted athletes
1.5–1.7
Application for Vegetarian Athletes
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Vegetarian athletes appear to meet or exceed their protein requirements; however, since plant proteins have less bioavailability, vegetarian athletes should increase their protein intake by approximately 10%.
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A disadvantage of being a vegetarian athlete is low levels of muscle creatine, a nitrogenous compound found in muscles, which phosphorylates and produces phosphocreatine. Phosphocreatine then donates a phosphate group to adenosine diphosphate (ADP) to become ATP.
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Several studies have found that vegetarian athletes tend to have lower levels of muscle creatine compared with their omnivore counterparts; in addition, vegetarian athletes who consume creatine supplements had a greater potential for resistance training and greater lean tissue mass compared with a placebo group.
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Vegetarianism has several benefits for athletes as vegetarian athletes gain a greater proportion of their energy needs from carbohydrates, including more fruits and vegetables, which may minimize their potential for free-radical damage, which may be advantageous to an athlete’s training and health.
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However, there is the potential for nutritional concerns for vegetarian athletes and include vitamin B 12 , iron, zinc, and calcium deficiencies; deficiencies in any of these nutrients can result in poor athletic performance.
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Considering the variability of a vegetarian diet, a sports dietitian can play a key role in educating vegetarian athletes regarding menu planning, cooking, and food preparation in order to maximize their athletic potential.
Fat
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The role of fats in the body is to supply a source of energy as well as essential fatty acids and fat-soluble vitamins that play important roles in an athlete’s diet. Most athletes often meet and/or exceed their recommendations of dietary fat usually because of increased caloric intake.
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High-fat diets are not recommended for athletes primarily because of their adverse health effects. Conversely, athletes who consume very-low-calorie diets (<15% of total calories from fat) do not exhibit any additional performance benefits.
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Athletes who have increased abdominal fat and or a predisposition for diabetes and cardiovascular disease should consume healthier fats such as monounsaturated and polyunsaturated fatty acids and lesser saturated fatty acids and trans fats, along with the addition of more whole grains, fruits, and vegetables.
Fat Recommendations
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In general, fat recommendations for athletes follow public health guidelines, such as the Dietary Guidelines for Americans, which recommends fat intake in the range of 20%–35% of total caloric intake; however, similar to carbohydrates, fat intake should be customized based on training levels and body composition.
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Some athletes believe that following a high-fat, low-carbohydrate diet can enhance rates of fat oxidation and improve endurance performance. Available literature suggests that high-fat, low-carbohydrate diets may match exercise capacity at moderate intensity but impair exercise at higher intensities.
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While high-fat diets may impair exercise intensity, low-fat diets (<20% of calories from fat) are not recommended because they tend to restrict intake of nutrients such as fat-soluble vitamins and essential fatty acids, particularly omega-3 fatty acids. In addition, individuals who restrict their dietary intake of fats to <20% of their calories for weight-loss purposes found no advantage of further weight loss following a very – low-fat diet.
Micronutrients
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Micronutrients play a specific role in facilitating the release of energy from food and in tissue synthesis.
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In general, if an athlete meets overall energy intake and incorporates a wide variety of nutrient-dense foods, vitamin and mineral recommendations can be met through the diet itself.
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Moreover, athletes tend to consume more calories owing to their energy expenditure during competition and training; therefore, they can meet their nutrient requirements with wise food choices. In addition, they may consume highly fortified “sport” foods and drinks, which provide several micro- and macronutrients.
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Certain athlete populations may be at risk of low or marginal intake of micronutrients. Athletes who compete in lean (distance running, gymnastics, swimming, and diving) or weight-restricted sports (wrestling and crew) are often at risk of low or marginal intake of micronutrients owing to their low-caloric diets.
Iron
Functions : Iron is a part of oxygen-carrying transport proteins such as hemoglobin and myoglobin. Iron is also a component of cytochromes that carry electrons to molecular oxygen in the electron transport chain.
Effects on performance : Iron deficiency with or without anemia can impair performance and limit work capacity. Some athletes, at the start of training, experience sports anemia, wherein blood volume expands before the synthesis of red blood cells increases. This expansion results in dilution of the blood and a decrease in hemoglobin. Sports anemia is not detrimental to performance but is hard to differentiate from true anemia.
Etiology : Potential causes of iron-deficiency anemia can vary; as observed in the general population, female athletes are most susceptible to low iron status owing to monthly menstrual losses. Special diets followed by athletes, such as low-energy and vegetarian, particularly vegan, diets are likely to be low in dietary iron. Distance runners should pay special attention to iron intake because their intense workouts may lead to foot-strike hemolysis or increased iron loss in sweat, feces, and urine, which can negatively influence the iron status.
Important interventions:
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Screening: Distance runners and all female athletes should have their iron status checked at the beginning of the season and midseason.
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Serum ferritin : Currently, there is no consensus on a specific serum ferritin level that corresponds to a possible level of iron depletion/deficiency in athletes. Suggested values range from >10 to <35 ng/mL.
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Iron intake : Athletes who are at risk should aim for an iron intake of greater than the RDA (>18 mg for women and >8 mg for men) ( Table 5.4 ). Athletes at risk must follow dietary strategies that increase food sources of iron, such as eating iron-fortified breakfast cereals with a source of vitamin C to enhance iron absorption and cooking in iron skillets.
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Supplements : Decisions regarding iron supplementation are best made on an individual basis. It may take 3–6 months to reverse iron-deficiency anemia. While there is some evidence suggesting that iron supplements can improve performance in athletes with iron depletion without anemia, indiscriminate use of iron supplements is not advised and may cause gastrointestinal distress and toxicity.
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Calcium
Function: Calcium plays a major role in the formation and maintenance of bone. Athletes, particularly women trying to maintain a lean profile, can have marginal or low intakes of dietary calcium, particularly if they restrict their caloric intake or avoid calcium-rich foods. Of still greater concern are female athletes who suffer from menstrual disturbances, low energy availability, and bone loss/osteopenia. Low calcium intake in such female athletes contributes to an increased risk of bone fractures during training and competition.
Important interventions:
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Screening : Female athletes should be asked about current and past intake of calcium-rich foods, calcium supplementation, as well as menstrual history.
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Calcium intake : The RDA for calcium is 1300 mg/day for female athletes aged 19–18 years and 1000 mg/day for athletes aged 19–50 years. Female athletes should be educated about highly bioavailable dietary sources of calcium and how to incorporate these sources in their daily diet ( Table 5.5 ).
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Supplements : Calcium supplementation should be determined after a nutritional assessment of dietary calcium. In general, 1500 mg/day of calcium and 1500–2000 IU/day of vitamin D are recommended for athletes with compromised bone health owing to low energy availability and/or menstrual dysfunction.
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Hydration Guidelines
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Fluid loss of >2% of total body weight can compromise athletic performance and cognitive function, particularly in hot environments. Fluid loss ranging 3%–5% of body weight further disturbs athletic performance, particularly in sports that require specific skills. Additional fluid loss of 6%–10% of body weight has profound effects on physiologic functions, including cardiac output, blood flow, and sweat production, and increases the risk of exertional heat illness.
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Thirst is a late sign of dehydration and is not a reliable indicator of an athlete’s need to replace fluid during exercise. An athlete who drinks only when thirsty is likely to take 48 hours to replenish fluid loss.
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To identify optimal fluid replacement strategies, athletes should know their individual sweat rates ( Box 5.2 ); athletes should weigh themselves before and after exercise to identify sweat loss from exercise. The rule of thumb is for every pound lost during exercise, 2–3 cups of fluid should be consumed. Athletes should aim for a fluid intake that prevents weight loss during exercise as well as avoid consuming too much fluids resulting in weight gain during exercise.
Box 5.2Related posts:
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