Most athletes, whether in training or competition, have been forced to perform in either hot and humid or cold conditions. Many athletes can lose over 3.8 L of fluid during one practice. Besides the obvious impact on sweat rates, fluid, and electrolyte losses, the heat can produce other physiological effects that influence performance. The 1996 decision to hold the Olympic Games in Atlanta created a lot of interest in the impact of heat and humidity on performance. Many scientific experts believed it would be impossible for some athletes to reach performance norms under those conditions. Cold temperatures, however, do not affect performance unless combined with wind chills or rain, which could result in hypothermia. Scientists have determined that for the endurance athlete, a temperature of 10°C-12°C is optimal. Less scientific data is available for strength and power sports, but it’s reasonable to assume these same optimal temperatures would apply.
Core temperature and fluid balance present the greatest concerns for athletes in the heat. Exertion during training and competition expend energy, which is dissipated in the form of heat. Dissipating heat from the body can take many forms including convection (using a fan to blow heat away from the body), conduction (using cold water towels on the back of the neck or sitting on a cool bench), and evaporation (sweating). Some of these forms of heat dissipation can be limited by equipment or gear worn by athletes (e.g., helmets, shoulder pads, pants). These types of equipment trap heat close to the body. As a result, many athletes rely heavily on sweating for thermoregulation. The loss of fluids results in drops in blood plasma and a subsequent drop in stroke volume, cardiac output, and maximal aerobic capacity. In addition, blood flow to working muscles declines. A secondary concern is that warm environments increase an athlete’s reliance on carbohydrates for fuel. Interestingly, while reliance on carbohydrates increases, it has not been shown to affect muscle glycogen levels at the point of fatigue, leaving scientists to conclude that decreased performance is not a result of fuel availability but thermoregulation. Hot conditions can also affect the brain. Recently, a theory has been developed related to the brain’s role as the central governor of the body.
Elevation of core and brain temperature could be responsible for preventing the body from working harder or longer in hot conditions. Other mechanisms could be related to neurotransmitters, such as serotonin, which act on sites in the brain and affect fatigue.
Athletes should focus on fueling strategies to improve thermoregulation and potentially affect mechanisms in the brain related to fatigue. Maintaining fluid balance and plasma volume during exercise are most vital for athletes. This is accomplished through proper fluid and electrolyte intake. In the US, American football players are restricted in thermoregulation because of the equipment (helmets, shoulder pads, etc.) they wear have been found to lose over 2 L of fluids/hour during practices in hot environments; this is compared to average losses by runners of 1.75 L/hour in hot temperatures, 1.6 L/hour by basketball players training indoors, and 1.25 L/hour by soccer players. When fluids are consumed, not 100% of intake is absorbed; therefore, it is recommended that athletes consume roughly 125% of losses. Both carbohydrates and electrolytes affect the absorption of fluid. Both assist in optimal absorption, and both can be obtained from food sources or from the beverage itself. Losses of electrolytes, specifically sodium, are highly variable and can range between 200-1,700 mg/L of sweat lost. This is typically greater than the amount commonly found in sports drinks. In addition, athletes prone to muscle cramps typically lose 2 times (or more) the amount of sodium as athletes less prone to cramping. Other electrolytes lost in sweat include potassium, calcium, and magnesium; the amounts of these nutrients lost in sweat are significantly less than those of sodium and chloride. Unless a dietary deficiency exists as a result of a poor diet, added supplementation of these electrolytes does not appear to be beneficial. While electrolytes are important, athletes should guard against overconsuming sodium in one dose. High intakes of electrolytes should be accompanied by large intakes of fluid. An intake of 500-700 mg of sodium/L (500-700 mg of sodium/1.1 qt) of fluid consumed is advised, much higher than that found in sports drinks. Athletes should guard against consuming small volumes of fluid containing large amounts of sodium or exceeding the amount of sodium per liter recommended as this can have a negative effect on restoring fluid balance.
Athletes should consider the following guidelines for fueling before, during, and after exercise to limit the impact of the heat on fluid balance and thermoregulation.
- Before exercise, consume 450 – 600 ml of fluid in combination with carbohydrates or electrolytes in the form of food or sport drinks 30-45 minutes before exercise.
- During exercise, consume 120-240 ml of fluid every 15 minutes of activity. Fluid intake should come from a combination of water and carbohydrate-electrolyte beverages.
- Carbohydrate intake during longer periods of exercise (>1 hour) is needed to optimize performance in the heat. A recommended intake is 22-60 g/hour of activity.
- Additional sodium should be provided for athletes prone to cramping or in extremely hot and humid conditions at a rate of 500-700 mg/L of fluid consumed, or roughly 500-700 mg/hour of activity. Extreme cases do exist where higher amounts of sodium are recommended.
In addition to protecting against issues of thermoregulation, certain dietary supplements or nutrition ingredients can affect serotonin and dopamine receptors regulating the brain’s involvement in the onset of fatigue especially in hot conditions. Table 1.0 includes a list of potential dietary supplements to consider.
Table 1.0 Supplement Options for Athletes Training in the Heat
450 – 600 ml prior to exercise; 500 – 1000 ml / hour of training; 20 oz/lb (0.59 L/0.45 kg) lost following exercise
Carbohydrate and electrolyte replacements (gels, fluids)
Prevents central nervous system (CNS) fatigue, fuel depletion; balances fluids
22-45 g carb/hour; 500 ml-1 L /hour of activity
|Sodium (Na)||Balances fluids||
500-700 mg/hour of activity in hot envn.
Prevents central nervous system (CNS) fatigue
75 mg/lb (75 mg/0.45 kg) of body weight 1 hour prior to exercise