The goals of sports nutrition can be outlined as follows:

It is important to note that although the macronutrient needs are similar among sports, there may be quantitative differences based upon the following variables: type of sport, gender, body composition, and age. In discussing the science of sports nutrition, it is important to understand the energy systems and energy substrates that fuel activity.

In order for any muscle to do physical work, adenosine triphosphate (ATP) is required. ATP is the energy catalyst, formed from the storage forms of carbohydrate and fat, that is, glycogen and fatty acids, and must be continuously formed, used, and reformed during physical activity. The other storage form of energy that can be used to fuel activity is creatine phosphate or phosphocreatine. Creatine is manufactured in the liver, kidney, and pancreas, and is stored in muscle. When energy demand increases due to exercise, the body relies on different types of energy systems to transfer stored energy to ATP to enable physical work to continue. There are three energy systems that will be used, depending on the duration and intensity of exercise. The phosphagen system is the first one that will be used when energy demand increases. This is an anaerobic energy system dependent upon ATP and creatine phosphate for high intensity, maximal outburst activity lasting less than 1 minute, such as a power-lift, a 6-second sprint, or a slam-dunk. As the duration of activity increases, the anaerobic
glycolysis system will allow ATP to be produced under anaerobic conditions for an additional 1 to 3 minutes after creatine phosphate levels diminish. This energy system will be used at the beginning of a road race; for a short duration, high-intensity event such as a 400-m sprint; in sports which are a combination of endurance and maximal outbursts, such as soccer, football, hockey, and basketball; and in the final sprint at the end of a road race. The aerobic system uses the three fuel substrates: carbohydrate, fat, and protein in the form of glucose, fatty acids, and amino acids for longer duration events such as distance cycling or a marathon.

To fuel the body optimally during activity, the body must have adequate stores of the macronutrients used for energy. Glycogen, the storage form of carbohydrate is stored in the muscle and liver. Most exercise is fueled by carbohydrate and fat, with protein providing a fuel source if carbohydrate stores are inadequate. Fuel usage is determined by the intensity and duration of activity as well as the level of training. During exercise, muscle glycogen is the major source, followed by liver glycogen and then blood glucose. Aerobic training and diet manipulation can significantly increase muscle glycogen stores (1). Muscle glycogen is used for intense, short duration activity and for endurance exercise. The rate of utilization of muscle glycogen is most rapid during the early part of exercise and is related to exercise intensity. Muscle glycogen declines with continued exercise, and is selectively depleted from the muscle(s) that are involved in physical work. As muscle glycogen stores decline, blood glucose and liver glycogen become important fuel sources. Plasma-free fatty acids will be used as a fuel source during endurance type activities through the process of adipose tissue lipolysis. Amino acids can be broken down to glucose to provide energy during activity, but only when carbohydrate stores are low.

Fatigue impairs performance. Subpar nutrition can be a significant cause of fatigue due to the following:

Fluid balance is essential for cardiovascular functioning, thermoregulation, injury prevention, optimal performance, and recovery from exercise. Fluid loss can be significant during exercise, in some cases up to 4 L/hour (2). In addition, exercise blunts the thirst mechanism, raising special challenges when encouraging the athlete to drink more. Fluid loss of more than 1.8% of total body weight can increase heart rate by 8 beats/minute thus impairing performance. In addition, mental functioning is impaired through a decrease in sustained attention, response time, and task accuracy, and an increase in error rate (3). Heat-related injuries increase with increased body water loss, as well as an increase in core body temperature. In addition, the perceived effort of exertion increases with dehydration, as does a difficulty in concentrating.

An athlete can become dehydrated due to changes in altitude, increases in training intensity and frequency, sudden climate changes, and long flights. The body cannot tolerate even slight dehydration, but unfortunately thirst sensation is dulled by exercise and voluntary fluid consumption is insufficient to meet fluid needs. Dehydration also reduces the gastric emptying rate, complicating the rehydration process (4). Curtailing fluid intake is a common practice in certain sports. The chronic dehydration that often accompanies weight class sports can impair the athlete’s ability to optimally train and compete.

The goal of fluid intake is to prevent dehydration. The American College of Sports Medicine (ACSM) published a position stand on fluid replacement in 1996 providing rationale and guidelines for hydration (5). The National Athletic Trainer’s Association published a similar position paper in 2000 (6). Athletes require a minimum fluid of 20 to 40 oz/hour of exercise, but most athletes consume only 8 oz/hour (5). A larger fluid intake during exercise leads to greater cardiac output, greater skin blood flow, lower core temperature, and reduced perceived effort of exertion (5). The overall goal is clear and copious urine as a sign that the body is well hydrated.

Athletes may ask for a recommendation for the types of fluid to consume. It is important to consider the sport, duration, calorie needs, and taste preferences. Water is a noncalorie fluid that works well for short duration activities, but it not as beneficial for exercise lasting longer than 60 minutes and/or shorter duration but more intense exercise. Juices can provide calories and carbohydrate but contain fructose, which has a decreased absorption rate and may cause gastrointestinal distress and are generally not advised before exercise. Carbonated beverages before activity may cause gastrointestinal distress, and oftentimes confer a feeling of fullness before fluid needs have been met. High levels of caffeine in the energy drinks may have a stimulatory effect, and the concentration of carbohydrate may slow gastric emptying. In addition, some of the protein powders contain diuretic herbs such as lovage, buchu, saw palmetto, and nettle. Alcoholic beverages have a diuretic effect, causing the body to lose valuable fluids before activity begins. Sports drinks can be an appropriate option for longer duration sports, and are certainly extremely popular with young athletes. Sports drinks contain a dilute glucose solution which stimulates water and sodium absorption so that more fluid is absorbed than from plain water (7,8). Because sports drinks have a fairly low carbohydrate content, they empty more rapidly from the stomach than a more concentrated beverage. With the flavor, athletes are also inclined to drink more than they would of plain water alone.

To achieve optimal gastric emptying, fluids should be cold or cool. A large volume of fluid empties more rapidly than smaller amounts. One liter of fluid empties from the stomach and will be absorbed by the intestine within 1 hour (5). This can be an advantage during exercise, when the athlete does not want to have a “full” feeling in the stomach while physically active. Athletes need to practice drinking during training to determine a comfort level, and to learn to drink proactively, instead of reactively. The recommended fluid intake is 2 to 3 quarts/day for basic needs plus 1 L of fluid for every 1,000 calories
expended (5). Strategies for fluid replacement pre, during, and post exercise are listed here (8):

For children younger than 10 years, the goal is to drink to satisfy thirst plus an additional 3 to 4 oz. Older children and adolescents should also drink to satisfy thirst plus an additional 8 oz of fluid (9).

Some athletes may inquire about the use of glycerol for hyperhydration. Glycerol is a three-carbon molecule which is the structural core of triglycerides and phospholipids. Glycerol ingestion increases blood osmolarity, decreasing urine production and increasing fluid retention. Although glycerol is easily absorbed, the increased weight may be a disadvantage, and glycerol loading can cause headaches, dizziness, bloating, and nausea (10,11). To encourage optimal hydration, coaches, athletic trainers, parents, and other health care professionals can assist the athlete in the following ways:

In addition to the fluid loss that accompanies exercise, electrolyte loss occurs as well. An athlete working out for 2 to 3 hours can lose up to 4 to 5 quarts of sweat. One pound of sweat contains 80 to 100 mg of potassium and 400 to 700 mg of sodium. In a 2- to 3-hour exercise session, an individual can lose up to 300 to 800 mg of potassium and 1,800 to 5,600 mg of sodium. The answer is not sodium or potassium tablets, but foods and fluids that provide the electrolytes (see Table 4.1). Encourage athletes to be more liberal with the use of salt or salty foods before working out, and to make an effort to include more high-potassium foods daily.

If calorie needs are not met, the body will fatigue earlier and performance will be curtailed. Some athletes skimp on calories, increasing the likelihood of early fatigue and risk of injury. Others eat in excess of need, resulting in excess stores of adipose tissue, which can adversely affect performance. Calorie needs are higher for an athlete, than a nonexercising individual, and need to be individualized according to gender and weight. An athlete who has been injured may require additional calories early in the recovery process to aid tissue repair, but oftentimes will require fewer calories as the frequency and intensity of activity declines. Athletes who retire from their sport need to learn to eat less than in their playing days or suffer the consequences of carrying around excess weight. The Institutes of Medicine Food and Nutrition Board have recently revised calorie guidelines as follows (12):

For Male athletes 30 years and older:

For Female athletes 30 years and older:

For males and females between 19 and 30 years:

For males and females older than 30 years:

For children and teens younger than 19 years, use the following guidelines to estimate calories:

The goal is to achieve a balance in the diet through a mix of carbohydrate, protein, and fat. This has become especially challenging in light of the popular eating plans recommending that entire categories of foods be limited or avoided. Whereas endurance athletes are more apt to rely on carbohydrate as the mainstay of diet, strength-trained athletes may be more likely to have been advised to consume a high-protein diet. Although the overall amount of food consumed may vary, every athlete should aim to include protein, carbohydrate, and fat at every meal/snack.

Achieving and maintaining optimal carbohydrate intake is important for training intensity, preventing hypoglycemia during exercise, serving as a fuel substrate for working muscles, and assisting in postexercise recovery. Carbohydrate use increases with increased exercise intensity, but decreases with increased exercise duration. The goal of carbohydrate ingestion is to fill carbohydrate stores in the muscles and liver. The higher the initial glycogen the longer an athlete can exercise at a given intensity level. Eating increases glycogen stores whereas exercise depletes glycogen stores. Glycogen depletion can occur in sports requiring near maximal bursts of effort.

Athletes who do not optimally refuel may experience gradual and chronic glycogen depletion that can ultimately decrease endurance and performance. A tip-off is the athlete who experiences sudden weight loss, a consequence of training glycogen depletion. To maintain optimal glycogen stores, carbohydrate needs must be estimated on the basis of the number of hours the athlete trains daily. Carbohydrate requirements are always higher for training than for competition. Some athletes may consume inadequate amounts of carbohydrate due to calorie restriction, avoidance of certain foods such as sugar, fad diets, sporadic infrequent meals, and poor nutrition knowledge of good carbohydrate sources versus marginal choices. Needs can be estimated as per values shown in Table 4.2 (15):

There has been much discussion as to which type of carbohydrate is better for sports, simple versus complex. The distinction is not that clear, and what matters most is the total amount of carbohydrate consumed on a daily basis.
Athletes can use the “Nutritional Facts” panel on a food label to quantify the amount of carbohydrate ingested. Some athletes are more comfortable ingesting carbohydrates in a liquid form, such as Gatorade Energy Drink or UltraFuel. Table 4.3 lists the carbohydrate content of common foods.

Recently, athletes have begun experimenting with manipulating the type of carbohydrate consumed at various points during exercise according to the glycemic index of the food (16). The glycemic index indicates the actual effects of carbohydrate-rich foods and fluids on blood glucose and insulin levels. The glycemic index ranks foods on the basis of the blood glucose response following ingestion of a test food that provides 50 gm of carbohydrate compared with the blood glucose response from a reference food. The response reflects the rate of digestion and absorption of a carbohydrate-rich food. Foods are classified into three categories—high, moderate, and low glycemic index foods as outlined in Table 4.4.

Although manipulating the meal choices based on the glycemic index may enhance carbohydrate availability and may improve athletic performance, the results will vary from athlete to athlete. Ingesting low glycemic index foods before competition may be used to allow for sustained availability of carbohydrates during exercise and to prevent an insulin surge and subsequent decrease in blood glucose. This may be most useful for the athlete who experiences hypoglycemia during competition or fatigue early. Athletes who are not thrilled with the food items on the low glycemic index list may opt to wait to consume carbohydrates a few minutes before exercise, in a liquid form. Once exercise begins, the rise in the hormones—epinephrine, norepinephrine, and growth hormone—inhibits insulin release and the blood glucose lowering effect of insulin. Athletes who are not sensitive to changes in blood glucose may benefit from consuming high glycemic index foods 1 hour before exercise, especially the athlete who has a morning event after an overnight fast; but this regime would not be beneficial for athletes participating solely in anaerobic events. Athletes will need to experiment to find out which foods work well, and more importantly do not cause gastrointestinal distress. Consuming dried beans or lentils before exercise may be fine for a cyclist, but may not be desirable for a runner.

Athletes have also experimented with the concept of carbohydrate loading or muscle glycogen supercompensation, which combines tapering of exercise with a high carbohydrate intake to top off muscle glycogen stores. The original method called for a depleting exercise protocol coupled with a low carbohydrate diet, followed by 3 days of rest with an extremely high carbohydrate diet. This often
made the athlete feel exhausted during the low carbohydrate intake phase, and very heavy during the carbohydrate loading phase. Current guidelines recommend 3 to 5 days of carbohydrate loading to attain maximal glycogen levels and the exercise done to lower glycogen stores must be the same as the athlete’s competitive event (17). Carbohydrate loading is advantageous only for endurance athletes whose event lasts longer than 90 minutes. Carbohydrate loading before a 10-K event is not helpful and may actually make the athlete feel heavier and stiff.

Carbohydrate needs for activity are divided into three distinct time periods: pre, during, and post exercise. The goal of pre-exercise carbohydrate is to provide energy for the athlete who exercises heavily in excess of 1 hour. Pre-exercise carbohydrate will also help prevent the feelings of hunger, which can be distracting, especially in a competition. The pre-exercise carbohydrate will also elevate blood glucose levels to provide energy for the exercising muscles. Current guidelines recommend 1.8 gm of carbohydrate/lb of body weight within 3 to 4 hours pre-exercise, and 0.5 gm/lb 1 hour pre-exercise (18,19). Examples are listed here.