Q: Do I really need to drink eight glasses every day?
A: No. According to a key review in the Journal of Physiology by Heinz Valtin, M.D., a hydration expert and professor emeritus at Dartmouth Medical School in Hanover, N.H., there's no evidence to support drinking eight glasses of water each day.
So how much water do you really need? According to the Institute of Medicine of the National Academies, women should consume 91 oz. a day, and men need 125 oz.—a good deal more than the 64 oz. (8 cups) generally recommended.
Here's the catch: We get most without heading for the tap or uncapping a bottle of Evian even once. The main reason? We get the water we need from a variety of sources, including food and other liquids.
"Approximately 45 to 50 percent of daily water intake comes from drinking fluids, about 35 percent from eating food and the rest from metabolism," says Stephen Rice, M.D., Ph.D., M.P.H., a sports medicine specialist at the Jersey Shore Medical Center in Neptune, N.J.
Vegetables and fruits are the most hydrating (e.g., lettuce is 95 percent water). But we also get a lot from meat, as well as soup, juice, soda, milk and even coffee.
Q: How long can I go without any liquids?
A: "It depends on a myriad of factors including body size, sweat rate, amount of activity and environment," says Douglas J. Casa, Ph.D., a professor of kinesiology at the University of Connecticut at Storrs. But just to give you some idea, according to Valtin, a person can die in one day without water in a desert but could last as long as two weeks in a hospital.
Q: If I'm thirsty, am I already dehydrated?
A: No. "You are underhydrated, not totally dehydrated. Thirst is a signal that your body would like more fluid," says Nancy Clark, M.S., R.D., Boston-area sports nutritionist and author of Nancy Clark's Sports Nutrition Guidebook (Human Kinetics, 2003).
Hydration is measured by blood concentration (e.g., the concentration of sodium in your blood)--the higher the concentration, the more dehydrated you are. When this concentration increases by just two percent, you get thirsty.
"Thirst is a warning mechanism, letting you know that dehydration is lurking around the corner, but to escalate to actual dehydration, the blood concentration must rise by five percent," says Valtin.
What about "storing" water (i.e., drinking a lot before you go out and lose fluids)? "That doesn't work," says Valtin. "Assuming we're healthy, all liquids we drink will be out of our bodies within a half-hour. So you can't store up your liquids."