Temperature regulation during marathon competition.

In resting man, deep body temperature is maintained at about 37-380C. The existence of such a steady temperature indicates that the rates of metabolic heat production is balanced -by the rate of heat loss to the environment. At rest the main challenge to the regulation of body temperature comes from changes in environmental conditions. At low ambient temperatures, especially if this is coupled with a high rate of air movement across the body surface, the rate of heat loss will exceed the rate of heat production and body temperature will fall. At high ambient temperatures, heat will be gained from the environment and body temperature will rise unless protective measures are initiated. During hard physical exercise, metabolic rate may rise 10 or 15-fold, and this rate of heat production may be sustained for several hours. For the exercising individual, therefore, cold exposure does not normally represent a serious challenge to the body's homeostatic mechanisms, but the problems of heat loss when exercising at a high ambient temperature may be acute.