Nutrition and Athletic Performance

Nancy R. Rodriguez, PhD, RD, CSSD, FACSM; Nancy M. DiMarco, PhD, RD, CSSD, FACSM; Susie Langley, MS, RD, CSSD

Disclosures

March 01, 2010

In This Article

Hydration

Being well hydrated is an important consideration for optimal exercise performance. Because dehydration increases the risk of potentially life-threatening heat injury such as heat stroke, athletes should strive for euhydration before, during, and after exercise. Dehydration (loss of >2% body weight) can compromise aerobic exercise performance, particularly in hot weather, and may impair mental/cognitive performance.[83]

The American College of Sports Medicine's (ACSM) Position Stand on exercise and fluid replacement[83] provides a comprehensive review of the research and recommendations for maintaining hydration before, during, and after exercise. In addition, ACSM has published position stands specific to special environmental conditions.[84,85] The major points from these position stands are the basis for the following recommendations.

Fluid and Electrolyte Recommendations

  • Before exercise

At least 4 h before exercise, individuals should drink approximately 5-7 mL·kg−1 body weight (~2-3 mL·lb−1) of water or a sport beverage. This would allow enough time to optimize hydration status and for excretion of any excess fluid as urine. Hyperhydration with fluids that expand the extra- and intracellular spaces (e.g., water and glycerol solutions) will greatly increase the risk of having to void during competition[83] and provides no clear physiologic or performance advantage over euhydration. This practice should be discouraged.[83]

  • During exercise

Athletes dissipate heat produced during physical activity by radiation, conduction, convection, and vaporization of water. In hot, dry environments, evaporation accounts for more than 80% of metabolic heat loss. Sweat rates for any given activity will vary according to ambient temperature, humidity, body weight, genetics, heat acclimatization state, and metabolic efficiency. Depending on the sport and condition, sweat rates can range from as little as 0.3 to as much as 2.4 L·h−1.[83] In addition to water, sweat also contains substantial but variable amounts of sodium. The average concentration of sodium in sweat approximates 50 mmol·L−1 or approximately 1 g·L−1 (although concentrations vary widely). There are modest amounts of potassium and small amounts of minerals such as magnesium and chloride lost in sweat.

The intent of drinking during exercise is to avert a water deficit in excess of 2% of body weight. The amount and rate of fluid replacement is dependent on the individual athlete's sweat rate, exercise duration, and opportunities to drink.[83] Readers are referred to the ACSM position stand for specific recommendations related to body size, sweat rates, types of work, etc., and are encouraged to individualize hydration protocols when possible.[83]

Consumption of beverages containing electrolytes and carbohydrates can help sustain fluid and electrolyte balance and endurance exercise performance.[83] The type, intensity, and duration of exercise and environmental conditions will alter the need for fluids and electrolytes. Fluids containing sodium and potassium help replace sweat electrolyte losses, whereas sodium stimulates thirst and fluid retention and carbohydrates provides energy. Beverages containing 6%-8% carbohydrate are recommended for exercise events lasting longer than 1 h.[83]

Fluid balance during exercise is not always possible because maximal sweat rates exceed maximal gastric emptying rates that in turn limit fluid absorption, and most often, rates of fluid ingestion by athletes during exercise fall short of amounts that can be emptied from the stomach and absorbed by the gut. Gastric emptying is maximized when the amount of fluid in the stomach is high and reduced with hypertonic fluids or when carbohydrate concentration is greater than 8%.

Disturbances of fluid and electrolyte balance that can occur in athletes include dehydration, hypohydration, and hyponatremia.[83] Exercise-induced dehydration develops because of fluid losses that exceed fluid intake. Although some individuals begin exercise euhydrated and dehydrate over an extended duration, athletes in some sports might start training or competing in a dehydrated state because the interval between exercise sessions is inadequate for full rehydration.[82] Another factor that may predispose an athlete to dehydration is "making weight" as a prerequisite for a specific sport or event. Hypohydration, a practice of some athletes competing in weight-class sports (i.e., wrestling, boxing, lightweight crew, martial arts, etc.), can occur when athletes dehydrate themselves before beginning a competitive event. Hypohydration can develop by fluid restriction, certain exercise practices, diuretic use, or sauna exposure before an event. In addition, fluid deficits may span workouts for athletes who participate in multiple or prolonged daily sessions of exercise in the heat.[84]

Hyponatremia (serum sodium concentration less than 130 mmol·L−1) can result from prolonged, heavy sweating with failure to replace sodium, or excessive water intake. Hyponatremia is more likely to develop in novice marathoners who are not lean, who run slowly, who sweat less, or who consume excess water before, during, or after an event.[83]

Skeletal muscle cramps are associated with dehydration, electrolyte deficits, and muscle fatigue. Non-heat-acclimatized American football players commonly experience dehydration and muscle cramping particularly during formal preseason practice sessions in late summer. Athletes participating in tennis matches, long-cycling races, late-season triathlons, soccer, and beach volleyball are also susceptible to dehydration and muscle cramping. Muscle cramps also occur in winter-sport athletes such as cross-country skiers and ice hockey players. Muscle cramps are more common in profuse sweaters who experience large sweat sodium losses.[83]

  • After exercise

Because many athletes do not consume enough fluids during exercise to balance fluid losses, they complete their exercise session dehydrated to some extent. Given adequate time, intake of normal meals and beverages will restore hydration status by replacing fluids and electrolytes lost during exercise. Rapid and complete recovery from excessive dehydration can be accomplished by drinking at least 16-24 oz (450-675 mL) of fluid for every pound (0.5 kg) of body weight lost during exercise. Consuming rehydration beverages and salty foods at meals/snacks will help replace fluid and electrolyte losses.[83]

Special Environmental Conditions

Hot and Humid Environments. The risk for dehydration and heat injury increases dramatically in hot, humid environments.[84] When the ambient temperature exceeds body temperature, heat cannot be dissipated by radiation. Moreover, the potential to dissipate heat by evaporation of sweat is substantially reduced when the relative humidity is high. There is a very high risk of heat illness when temperature and humidity are both high. If competitive events occur under these conditions, it is necessary to take every precaution to ensure that athletes are well hydrated, have ample access to fluids, and are monitored for heat-related illness.

Cold Environments. It is possible for dehydration to occur in cool or cold weather.[85] Factors contributing to dehydration in cold environments include respiratory fluid losses and sweat losses that occur when insulated clothing is worn during intense exercise. Dehydration can also occur because of low rates of fluid ingestion. If an athlete is chilled and available fluids are cold, the incentive to drink may be reduced. Finally, removal of multiple layers of clothing to urinate may be inconvenient and difficult for some athletes, especially women, and they may voluntarily limit fluid intake.[86]

Altitude. Fluid losses beyond those associated with any exercise performed may occur at altitudes >2500 m (8200 ft) consequent to mandatory diuresis and high respiratory water losses, accompanied by decreased appetite. Respiratory water losses may be as high as 1900 mL·d−1 (1.9 L·d−1) in men and 850 mL·d−1 (0.85 L·d−1) in women.[87,88] Total fluid intake at high altitude approaches 3-4 L·d−1 to promote optimal kidney function and maintain urine output of ~1.4 L in adults.[87]

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