Water comprises 60 percent of the body and 80 percent of the muscles. It is the primary component of the blood, helping to transport oxygen and nutrients through the body and waste from the body.

Water also helps to regulate temperature through sweat. For every quart of sweat, the body dissipates roughly 600 calories of heat. A person may sweat between 1 to 2 quarts every hour, depending on his or her weight, intensity of exercise and environmental conditions.

People generally feel thirsty after losing just 1 percent of their body weight through sweat. If water loss reaches 2 to 4 percent, performance may be affected as symptoms of dehydration set in.

Do not wait until thirsty before drinking. By then, it is unlikely the body will be able to replace all the lost fluid as exercise continues. This may cause the exerciser to quit. Water is a perfect choice for fluid replacement. It moves quickly from the stomach into the bloodstream, and people can buy it bottled or get it free from a fountain.

Fitness experts recommend drinking water before working out, even if the exerciser plans to consume a sports drink during exercise. Drink one or two cups of water [8 to 16 ounces (240 to 480 milliliters)] two hours prior to the workout, and another one or two cups a half-hour before exercising. For exercise sessions that last longer than 30 minutes, experts agree that 3 to 6 ounces fluid should be taken every 15 to 20 minutes. Some experts recommend ranges as high as 6 to 12 ounces every 15 to 20 minutes, depending on the activity's intensity.