Maintaining normal body temperature during exercise is essential. Heat is a byproduct of muscular work, and excess heat must be dissipated. The higher the intensity of the exercise and the longer the duration of the work, the more the athlete sweats to cool the body. Water lost by sweating needs to be replaced during exercise or the athlete's core temperature will rise and cause his body to overheat. Mental and physical fatigue result from dehydration.

As the duration of high intensity work continues, muscle glycogen levels diminish and blood glucose is increasingly relied on to supply fuel for the brain and the muscles. For endurance athletes, carbohydrate feedings are recommended during exercise lasting longer than one hour.