No doubt about it, water is critical. In fact, it constitutes more than two-thirds of your body weight. However, you might not need to work as hard as you thought to get enough. Here are answers to the most common questions about staying hydrated.

Q: Do I really need to drink eight glasses every day?

A: No. According to a key review in the "Journal of Physiology" by Dr. Heinz Valtin, a hydration expert and professor emeritus at Dartmouth Medical School in Hanover, N.H., there is 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 ounces a day, and men need 125 ounces — a good deal more than the 64 ounces (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 Dr. Stephen Rice, a sports medicine specialist at the Jersey Shore Medical Center in Neptune, N.J.

Vegetables and fruits are the most hydrating (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, a professor of kinesiology at the University of Connecticut. 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, Boston-area sports nutritionist and author of "Nancy Clark's Sports Nutrition Guidebook" (Human Kinetics, 2003). Hydration is measured by blood concentration (the amount of sodium in your blood). The higher the concentration, the more dehydrated you are. When this concentration increases by just 2 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 5 percent," says Valtin.

What about "storing" water (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."

Q: Are sports drinks better than water?

A: Sometimes. "Sports drinks are designed to be taken during exercise that lasts for more than an hour," says Clark. "They are particularly helpful for athletes because they contain a little sugar to fuel the muscles and the brain, as well as a little sodium to enhance fluid absorption and retention."

Q: Are coffee, tea and other caffeinated drinks dehydrating?

A: Absolutely not, says Casa. "They provide fluids just like any beverage. A slightly greater percentage of the ingested fluid may be urinated, but it's still providing water." In fact, "People who are used to drinking caffeinated beverages get accustomed to the caffeine and do not urinate more fluid than they consume via their coffee or tea," adds Clark.

Q: Is water an effective appetite suppressant?

A: "There's no real evidence. However, people often mistake thirst for hunger, which means you could be eating food when you actually don't even feel the need," advises Clark. How can you tell the difference? Finish a tall glass of water when you feel a snack attack coming on, and then decide if you still need food. Nevertheless, extensive research by Barbara Rolls, a professor of nutrition at Penn State, found that soup and other liquid-based foods at the beginning of a meal reduce hunger.

Charles Stuart Platkin is a syndicated health, nutrition and fitness writer.