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The maintenance of body temperature is a major concern for the athlete, particularly during exercise in hot, humid conditions. Evaporation of sweat from the skin provides a major mechanism of heat dissipation, with the athlete’s sweat rate being determined by exercise intensity, the state of heat acclimation, and the prevailing environmental conditions. Sweat rates as high as 2-3 L/hr have been reported in some athletes exercising at high power outputs in hot and humid conditions. However, during more prolonged, moderate-intensity exercise such as running and cycling, sweat rates in most athletes are closer to 1.0-1.2 L/hr. Unless this fluid is replaced, the athlete will eventually become dehydrated.

Dehydration of as little as 2% of an athlete’s body mass has been shown to significantly reduce high-intensity exercise capacity. Furthermore, the effects on exercise response appear to be directly related to the degree of dehydration, and the athlete cannot acquire a tolerance to dehydration as is popularly believed in some sports. It appears that the effects of dehydration on exercise performance are related to the type of event or sport being undertaken. While aerobic exercise, particularly in the heat, is impaired at such low levels of dehydration, events requiring strength and power do not seem to be affected by such small fluid losses. However, minimal dehydration may negatively impact on mental function and should therefore have a greater impact on team and racquet sports which involve skill and decision making processes, than endurance sports such as running and cycling. Dehydration has also been shown to reduce the rate of gastric emptying which may further compromise exercise performance. For these reasons, the athlete should aim to minimize net fluid losses during all types of exercise.

In terms of optimal fluid balance, the athlete might be advised to consume fluids to keep pace with sweat losses; or at least 80% of sweat loss rate. However, in most competition and training situations, athletes are limited to drinking what is practical rather than optimal. This appears to 400-800 ml of fluid per hour under most sports conditions. Whether this is done at aid stations, at formal breaks between quarters or halves of a game, or from drink bottles carried by the athlete will vary according to the sport. However, athletes should be encouraged to establish a drinking routine that takes into account their sweat losses and their opportunities to drink fluid (see Table 2). This may not be optimal when sweat losses greatly exceed the general rate of gastric emptying (about 1 litre per hour), but the athlete should aim to minimize dehydration. The athlete should drink early and frequently (e.g. 150-250 ml every 15-20 minutes) to prevent rather than try to reverse dehydration. Special consideration should be given to athletes who need to consume fluids during events, literally ‘on the run’ (e.g. marathon runners, cyclists, cross country skiers, triathletes). These athletes may need to balance their intake against the possibility of gastrointestinal discomfort or upset, as well as the time lost while eating/drinking (i.e. slowing down to approach an aid station or to handle fluids/food). Fluids that are palatable are likely to be consumed in larger quantities; for this reason, cool and sweet-tasting drinks are promoted.