Water is a basic component of the human body. The average adult body is 55–75% water equaling 10–12 gallons of water. Physical activity leads to an increase in your body's production of heat. The body gets rid of some of the heat through sweat. To prevent dehydration, water has to be replaced at a faster rate than it is lost. Dehydration reduces muscle strength, endurance and coordination, and increases the risk of cramps, heat exhaustion, and life-threatening heat stroke.
Drink 2 cups of fluid about 2 hours before your event, followed by 2 cups 15–20 minutes before endurance activities. If it is hot and humid, frequent, small servings (4–6 ounces of plain cool water or sports drink) every 15 minutes throughout the activity will help keep you hydrated. Some people may lose up to 6–8 pounds of sweat per hour during strenuous activity. Measure body weight before and after training sessions. Each pound lost should be replaced with 1 pint of fluid.