Water has the capacity to regulate the internal temperature of the body in response to the external temperature.
Sweat is the main means by which water prevents the human body overheating when the temperature outside it is very high(1). The evaporation of sweat brings a loss of calories, in the form of heat(2). This release of energy enables our internal temperature to remain constant. Without this mechanism it would rise in conditions of hot weather or fever. To maintain stable body temperature, we have to both sweat and allow the sweat to evaporate!
Don’t forget to drink!
We lose about 450 ml of water per day via perspiration in a temperate environment. When exercising, the amount of water lost by sweating increases significantly. For 1h of cycling, we can lose on average 0.8 liter of water while it goes up to 2 liters for 1 h of rugby! It is recommended, therefore, to drink regularly before, during and after exercise.
Sources: (1) Montain S.J., Military Medicine, 1999; (2) Wenger, Journal of Applied Physiology, 1972; (3) Rerher and Burke, Australian Journal of Nutrition and Dietetics, 1996