With the Summer Olympic Games almost upon us, it is timely to consider what we know about nutrition and athletic performance. In the context of unprecedented claims about health foods and medical foods, this topic serves as an illuminating means of critically discussing the collision between nutrition and physiology. Just how significant is this collision in terms of clinical reality, or, in this case, athletic performance? That is the great and as yet unanswered question and a controversial topic.
There are scores of studies on nitrate supplementation; branched-chain amino acids, energy, free fatty acid oxidation, and muscle glycogen and recovery; probiotics, beta-glucan, and immunity; hydration; electrolytes; supplements; and the role and proportions of select macroand micronutrients (Gleeson, M., 2006; Jeukendrup and Randell, 2011; Kreider, R.B. et al., 2010; Nieman, D.C. et al., 2008).
The Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine (ACSM) assert that athletic performance and recovery from exercise are enhanced by optimal nutrition. Numerous position papers encourage “appropriate selection of foods and fluids, timing of intake, and supplement choices for optimal health, and exercise performance” (doi: 10.1016/j.jada.2009;10.020; doi: 10.1249/MSS. 0b013e318190eb86).
The most recent ACSM report underscores the importance of evaluating energy needs, assessing body composition, specific sport performance requirements, climate, and nutrient and fluid needs, special nutrient needs, the use of supplements and ergogenic aids, and adjustments for vegetarian athletes. Energy and macronutrient needs must be examined, especially carbohydrate and protein, and must be met during times of high physical activity to maintain body weight, replenish glycogen stores, and provide adequate protein to build and repair tissue. Fat intake should provide the essential fatty acids and fat-soluble vitamins as well as contribute calories for weight maintenance. Endocrine function and exercise performance can be affected by body weight and composition.
Athletes should be well [but not overly] hydrated before exercise and drink enough fluid during and after exercise to balance fluid and electrolyte losses. Sports beverages containing carbohydrates and electrolytes may be consumed to help maintain blood glucose concentration, provide fuel for muscles, and decrease risk of dehydration and hyponatremia (insufficient sodium). However, the composition and levels of carbohydrates and electrolytes in these products may affect rehydration processes and athletic performance (Lopez, R.M. et al., 2011).
One additional variable is noted, but not widely represented in the literature. As investigators and educators in the realm of clinical nutrition, we have come to respect the variability in individual nutritional requirements and preferences. Clinicians, coaches, and athletes themselves have long recognized the wisdom of “listening to one’s own body.”
In the same way that we have evidence-based and quite elegant understanding of the huge array of neuroendocrine appetite and satiety control mechanisms, we have a large volume of scientific data on nutrients and performance. But the question in each instance is one of clinical and performance significance. In other words, will a cognitive or emotional decision to over-ride these systems associated with appetite and satiety reduce their impact to statistical interest only and render their impact subtle at most? After all is said and done, we are faced with the question of statistical significance vs clinical relevance. Just how much faster, stronger, and more resilient will an athlete be if he/she has been maintained on “diet x” vs “diet y”?
With all the “grays,” there are some “black-and-white” principles of nutrition for athletes that most experts across specialties agree upon. It is worthwhile to review the core ACSM recommendations that relate to specific macronutrients, hydration, and electrolytes.
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• Carbohydrates. Carbohydrates remain the dietary essential for athletes. During endurance exercise, muscles rely on fats and carbs for fuel. The more intense the exercise, the more carbohydrates are required. For exercise of moderate to high intensity that could be sustained for one to four hours at a time, 50–60% of the energy fueling muscle contractions is coming from carbohydrates, and the rest from fat. It is important to be aware that even lean athletes have some fat stored away, but carbohydrate stores are in much shorter supply. Two to three hours of intense exercise can substantially deplete or even exhaust glycogen fuel reserves. Daily carbohydrate needs are based on body weight, and recommendations for athletes range from about 2.7–4.5 g carbohydrates per lb (6–10 g per kg) body weight. For a 150-lb (68-kg) athlete, that nets out at 405–675 g of carbohydrates each day (Kreider, R.B. et al., 2010).
• Protein. Most endurance athletes require in the range of 0.5–0.64 g of protein per lb (1.2–1.4 g per kg) body weight daily. For a 150-lb athlete, this amounts to about 82–95 g of protein daily. The majority of athletes get the protein they need. What they often do not understand is the importance of consuming adequate calories and carbohydrates along with protein. If athletes are short on calories and carbohydrates, they may be using the amino acids from the protein they eat as an energy source rather than for the making of new proteins to repair and build muscle tissue.
Strength-training athletes generally need a bit more protein than endurance athletes. The recommended ACSM intake range is 0.5–0.8 g per lb (1.2–1.7 g per kg) body weight daily. For a 150-lb athlete, this equates to about 82–116 g of protein daily. Here again, pairing protein with sufficient calories is crucial if muscle growth is the goal. A higher protein intake plus adequate calories is especially important early in strength training, when the largest gains in muscle mass are likely to occur.
The protein needs of vegetarian athletes are estimated to be about 0.6–0.82 g per lb (1.3–1.8 g per kg) body weight daily. For a 150-lb athlete, this nets out at about 88–122 g of protein daily.
• Hydration. Staying hydrated is crucial for athletes to perform optimally. Performance can be negatively affected by a 2% drop in body weight due to fluid loss during exercise. This is just 3 lbs (1.4 kg) in a 150-lb athlete—a fluid loss that is common during a long practice session or competition, especially in hot weather. Dehydration also increases the possibility of a potentially life-threatening heat stroke.
The conventional wisdom is that athletes begin drinking fluids about 4 hours before a workout or competition. A good pre-exercise hydration rule of thumb is to drink about 2 fl oz (60 ml) for every 25 lbs (11 kg) body weight.
During exercise, the hydration goal is to prevent a water loss of more than 2% of body weight. As noted above, state-of-the-science hydration calls for each athlete to have an individualized hydration protocol based on their own fluid losses; sweat rates can vary considerably between athletes and from one day to the next.
• Electrolytes. Sodium is the key electrolyte lost in sweat and through insensible losses, such as breathing. To replace the sodium lost during exercise, ACSM recommends consuming a sports drink that contains sodium. Another option may be to consume an energy gel that contains sodium and to pair it with water. Consuming sodium during exercise not only replaces some of the sodium being lost, it also stimulates the drive to take in more fluids and helps with the retention of the fluids consumed.
Together with sodium, potassium is involved in a daunting spectrum of the body’s processes such as fluid balance, protein synthesis, nerve conduction, energy production, muscle contraction, synthesis of nucleic acids, and control of heartbeat. In many of its roles, potassium is opposed by sodium, and these two positive ions are jointly balanced by the negative chloride ion, also a crucial electrolyte requiring repletion following fluid losses during athletic effort.
It is worth mention that intracellular magnesium, another positive ion, is correlated with intracellular potassium, and plays vital roles in DNA biochemistry and cellular energy. Finally, unlike sodium, potassium, and magnesium, chloride is the principal negatively charged ion in the body. It is critical in maintaining acid-base balance in the body, and supports kidney and gastric functions. Balanced oral rehydration solutions contain appropriate concentrations and bioavailable forms of these four necessary water-soluble electrolytes.
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Athletes who have a rest day or two between long and exhausting workouts can usually recover adequately with normal meals and fluid intake. However, athletes who engage in long, intense daily workouts or morning and afternoon workouts need to maximize recovery between workouts. This requires purposefully replenishing carbohydrate stores, repairing and building muscle tissue, and replacing lost fluids and sodium.
ACSM recommends consuming 0.5–0.7 g carbohydrates per lb (1.0–1.5 g carbohydrates per kg) body weight within 30 minutes of finishing exercise, and then again at 2-hour intervals for up to 4–6 hours. Consuming protein along with carbohydrates after exercise will help to stimulate a fast recovery and provide the amino acids necessary for muscle tissue repair and building.
Finally, many athletes will have some degree of net fluid loss after exercise. To replace that fluid loss, it is recommended that the athlete gradually drink, between meals, 16–24 oz (450–675 ml) of fluid for every lb (0.5 kg) of body weight lost. The sodium and other electrolytes in sweat need to be replaced.
Clearly, physical activity and athletic performance require critical attention to proper training regimen and to many dietary factors. Those dietary factors include appropriate levels of the macronutrients, balance of electrolytes, and adequate hydration.
References cited in this column are available from the authors.
Peter Pressman, M.D.,
CDR, Medical Corps, U.S. Navy, Director Expeditionary Medicine, Task Force for Business & Stability Operations
Roger Clemens, Dr.P.H.,
Chief Scientific Officer, Horn Company, La Mirada, Calif.