If drinking eight glasses of water a day has you wearing a path to the bathroom, set down your cup and relax. Drinking water can be a good substitute for high-calorie beverages. But there's no proof we all need to drink those eight glasses of water each day, according to physiologist Heinz Valtin.

"Proponents of the 'eight glasses theory' say that every one of us, including couch potatoes, should drink that much," says Valtin. "And it's just not true." We have a finely tuned system for regulating water flow, including an antidiuretic hormone that tells the kidneys when to keep water in the body, and thirst, which normally kicks in well before we dehydrate. Juice works as well as water, and caffeinated drinks count too. Nearly one-half of the body's water needs are derived from food. Some people do need more water, including patients with kidney stones, people who exercise hard, and those in hot climates. For most, drinking more won't hurt, but trust your body. "If you drink when you're thirsty," says Valtin, "you should be safe."