"By the time you're thirsty, you're already dehydrated," says Jeanne Grant, a registered dietitian at the University of Washington Medical Center's Nutrition Clinic. To compensate for dehydration, the body increases heart rate due to the decreased blood return to the heart. Restricted blood flow leads to constriction of blood vessels in the skin; this constriction reduces your body's ability to dissipate heat.
So drink early and often -- about 2 quarts of water a day. Before exercise, which tends to blunt thirst even more, drink 1 to 2 cups of water. During long hikes and bike rides, try to drink another cup or two every 20 minutes. Avoid alcohol and caffeine, which are diuretics and can dehydrate the body. Sports drinks are okay if you are exercising longer than an hour, Grant says, but in most cases water is fine.