How does heat harm the athlete and what to do about it?

Exercise in the heat posses a considerable challenge to an athlete’s ability to maintain body temperature within tolerable limits. Where ambient temperature is high, heat is gained by physical transfer from the environment and is added to the metabolic heat production. The only available mechanism of heat loss is by evaporation of sweat secreted onto the skin surface. This is effective in promoting heat loss, but will result in hypohydration: if not corrected, this can impair exercise performance.
It is clear that high temperatures, in excess of about 42o C, cause damage to most body tissues as a result of protein denaturation. Functional capacity, however, is generally impaired before this level is reached. The brain seems to be particularly sensitive to high temperatures and it has been proposed that exercise performance is limited by the ability to keep brain temperature below about 40o C. There are, of course, problems in measuring brain temperature reliably, and various sites are used as surrogate measures of core temperature, including the rectum, oesophagus and the inner ear. The mechanism by which an elebated brain temperature impairs exercise performance is not clear, but both direct effects on the motor cortex and indirect effects on motivation have been proposed. There is some indirect evidence to suggest that changes in the release of key neurotransmitters including dopamine and serotonin (5-hydroxytryptamine) may occur during exercise in the heat.
The athlete who has to compete in the heat can adopt a variety of strategies to reduce the potentially negative effects of heat on performance. Effective strategies include prior acclimatization, modification of the pre-exercise worm-up routine to attenuate any rise in core temperature, pre-exercise cooling, modification of clothing, and ingestion of fluids. In some sporting situations, such as soccer and other ball games, the amount of aerobic activity can be reduced by adopting a different tactical approach and relying more on a passing game.
Acclimatisation has a number of beneficial effects, including a more effective sweating mechanism (earlier onset of sweating, greater sweating sensitivity, and better distribution of sweating over the body surface), a reduction in pre-exercise core temperature, and an increased exercise tolerance in the heat. Effective acclimatization requires a number of exposures to heat, each lasting about 60-100 minutes, carried out on a daily basis or at intervals of not more than a few days. The response is proportional to the heat strain, so exercise in the heat is more effective than passive heat exposure. Prior dehydration can reduce exercise performance, though some events may benefit, especially where body mass must be moved against gravity (as, for example, in high jumping). Athletes in endurance events should be well hydrated before exercise begins. Once exercise begins, sweat loss is inevitable if exercise is sufficiently strenuous and prolonged, and sufficient fluid should be ingested to maintain body mass within about 1-2% of the pre-exercise mass.