The more energy an athlete expends, the greater are his fluid needs. Heat is a byproduct of energy production, and excess heat must be transferred from the body to the environment to maintain normal body temperature. Evaporation of sweat from the athlete's body is the primary way of dissipating excess heat. Sweat loss varies among athletes and can exceed 1.5 liters per hour. Consuming 1.0 to 1.5 ml of fluid for each kilocalorie that is expended is optimal.
Fluid consumption includes all liquids plus water in foods.
Dehydration impairs temperature regulation and both physical and mental performance. The human body cannot adapt to dehydration. Decreased blood volume due to dehydration reduces blood flow to the skin and impairs heat dissipation. When sweating slows, cooling slows. Exercise combined with hyperthermia places an extreme stress on cardiovascular function.