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What does 8 cups of water a day do for your digestive system?

By S. Jhoanna Robledo, undergoing doctor review.

Kerri Carpenter, a Los Angeles buyer for an e-commerce Web site, begins her workday by grabbing a cup of water and setting it by her computer. After she downs it, she gets up to refill it again, repeating this routine throughout the day.

It's a habit she's had for as long as she can remember, religiously following the adage that drinking at least eight cups of water a day is good for your health. "If I don't get enough, I feel like I'm depriving myself of something vital that I really need," Carpenter says.

She's right. Health-care providers say water nourishes the entire body. "Water is a life-sustaining beverage," says Leslie Bonci, a registered dietician and author of The American Dietetic Association Guide to Better Digestion. "Every organ in the body needs water."

Keeping the GI tract in fine working order

For water to get where it's needed, it must be absorbed through the gastrointestinal tract. When you drink water, it travels quickly down the esophagus, through the stomach and into the intestines, where it's partially diffused into the bloodstream, hydrating the body's cells. The kidneys and the bladder make use of some of the water, and what's left goes into the large intestine to move fecal matter.

"Water helps flush the system," says Dr. Donald F. Kirby, a gastroenterologist and chief of the nutrition section at Virginia Commonwealth University Medical Center in Richmond, Va. When there aren't enough fluids in the colon, patients suffer from constipation, a common gastrointestinal ailment.

And one solution to constipation — eating a diet rich in fiber, which can be found in fruits, vegetables and whole grain — can't work without adequate intake of fluids. "Fiber draws water from all sources in the body to make stools softer and easier to pass through," says Bonci.

Water may also play a role in preventing colorectal cancer. The Mayo Clinic cites a study that showed that women who drank more than five glasses of water a day had a risk of colon cancer that was 45 percent less than that of those who drank two or fewer glasses a day.

"If you're drinking enough fluids, you're moving things more efficiently and quickly so toxins won't be sitting in your gut for too long," says Bonci.

Are eight cups of water enough?

Although long accepted as conventional wisdom, the eight-cups-a-day recommendation is now being questioned by some health-care providers. "It's not clear to me that the advice is based on medical science," says Kirby. "That's just a number that may ensure adequate hydration."

Bonci suggests that body weight may be a better gauge. "Take a third of your weight in pounds and that's how many ounces you need to drink daily," she says. A 150-pound woman, for example, will need 50 ounces, which translates to a little more than six cups of water.

Mark Glen, a registered dietitian at the Mayo Clinic in Rochester, Minn., prefers to recommend anywhere from 64 ounces (8 cups) to 96 ounces (12 cups) of fluid each day. For smaller-framed patients, eight may be enough; those with a medium build can rest easy with 10, while those who are larger-framed may require up to 12 cups of fluid. You may need to drink even more on particularly hot days.

Activity level may increase requirements, too. "People who exercise will need more than the minimum," says Bonci, who also serves as director of sports nutrition at the University of Pittsburgh Medical Center. "They must drink two to three cups an hour before exercise, one to two bottles of water during, and at least three cups of fluid afterwards."

Experts also say it doesn't matter if all those cups are filled with water. Beverages such as juice and milk also count. Although not ideal because they contain caffeine and sugar, coffee and soda can also help you reach your daily fluid intake goals. Alcohol, however, is another story; although technically a liquid, it can cause dehydration and a host of other health problems.

Signs of Dehydration

To stay fully hydrated, Bonci suggests keeping a beverage by your side at all times and drinking early and often. If you find you haven't been drinking enough based on what experts recommend, up your intake gradually. Start by adding a cup to your daily regimen during the first week, and then incorporating more as needed. The point is to meet your minimum as soon as you're able, and not to wait until you're thirsty to drink. "If you're already thirsty, you're already behind the eight ball," says Kirby. You'll know you've had enough to drink if your urine is clear or pale yellow.

When the body is not optimally hydrated, all the organs have to work harder, says Bonci. The body tries to create fluids by doing things like breaking down muscle, which isn't ideal.

Symptoms of Dehydration

Signs of severe dehydration, a potentially life-threatening condition, include lethargy, headaches, dry lips, decreased urination, a burning sensation in the stomach, an abnormally fast heart rate, inability to concentrate and fatigue. If you experience any of these symptoms, reach for a sport drink, says Kirby, which will help replenish the water and salt your body needs. If symptoms don't resolve within minutes, though, call your doctor immediately.

S. Jhoanna Robledo Wade is a freelance journalist based in New York.

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Updated: Dec 21 2013