Feature Archive

Water, Water, Everywhere

How Much Water Do You Really Need to Drink?

WebMD Feature

April 16, 2001 -- You'd think we were suffering a nationwide drought, the way Americans go around clutching bottles of water these days. Forget American Express cards: The one thing many of us would never dream of leaving home without is our bottled water.

By all rights, that should be good news. For years nutritionists have been warning us about the dangers of dehydration. Quaff at least eight 8-ounce glasses of water, the common wisdom goes, or you'll suffer the consequences: flagging energy, dry skin, lowered disease resistance, even constipation.

And don't count the coffee, tea, or other caffeinated beverages you drink. Anything with caffeine, we've long been told, actually increases the risk of dehydration because it flushes water out of the system.

Nor can you rely on thirst. By the time you're thirsty, you're well on your way to being dehydrated.

There's only one problem with all these warnings. Almost none of them hold water. Here's why:

Myth No. 1: We need to drink at least eight 8-ounce glasses of water a day

Researchers aren't sure where this familiar advice came from, but most agree there's very little solid scientific evidence to support it. The average adult loses only about 1 liter of water a day through sweating and other bodily processes -- the equivalent of only four 8-ounce glasses. We typically get that much water just in the foods we eat. Drinking an additional eight tall glasses of H20 is probably more fluid than most of us need.

What about older people? For years, experts have warned that elderly people are especially prone to dehydration because they lose their sense of thirst. But even this may be overstated, according to a report in the July 2000 Journal of Gerontology. Robert Lindeman, MD, professor emeritus of medicine at the University of New Mexico, surveyed fluid consumption among 833 elderly volunteers.

"People who drank less than four glasses of water a day were no more likely to show signs of dehydration than those who drank six or more," says Lindeman. "We found absolutely no difference between those who drank a little and those who drank a lot when we looked at all the standard markers for dehydration."

Of course, that doesn't mean you shouldn't drink plenty of water a day. In fact, there's at least one reason to think it's a very good idea. In a 1999 study published in The New England Journal of Medicine, researchers found that the more liquids men consumed, the lower their risk of bladder cancer. Men who drank more than 10 8-ounce servings of fluids had a 49% lower incidence of the disease than those who drank only half that much.

Myth No. 2: Caffeinated beverages make you dehydrated

Not true.

"For years, newspaper and magazine articles have repeated the notion that caffeine is dehydrating as if it's absolute fact," says University of Nebraska researcher Ann Grandjean, EdD. But in a study published in the October 2000 Journal of the American College of Nutrition, Grandjean and her colleagues at the Center for Human Nutrition showed that it's pure fantasy.

The researchers looked at how different combinations of water, coffee, and caffeinated colas affected hydration levels in a group of 18 men between the ages of 24 and 39. During one phase of the experiment, the only fluid the volunteers consumed was water. During another, 75% of their intake was caffeinated.

"Using almost every test ever devised to measure dehydration, we found no difference at all," says Grandjean.

Myth No. 3: By the time you feel thirsty, you're already becoming dehydrated

Maybe if you're an elite athlete running a marathon or a hotshot tennis player sweating in the noonday sun -- but not if you're going about your everyday activities.

Thirst is, in fact, a very sensitive mechanism for regulating fluid intake, according to Barbara Rolls, PhD, a nutrition researcher at Pennsylvania State University. In a 1984 study in Physiology and Behavior, she and a group of colleagues at Oxford University followed a group of men as they went through their normal day. Left to their own devices, the volunteers became thirsty and drank long before their hydration levels showed any signs of dipping.

Says Rolls, "If people have access to water or other fluid beverages, they seem to do a very good job of maintaining hydration levels."

Myth No. 4: Drinking plenty of water can help you lose weight

This idea makes sense, since water contains no calories. The trouble is, drinking a glass of water doesn't do anything to take the edge off hunger.

"Water sneaks right past without triggering satiety signals, the cues that tell your body when you're full," says nutritionist Barbara Rolls, author of Volumetrics.

Surprisingly, adding water to the food you eat, on the other hand, does seem to tame hunger. In a study reported in the October 1999 American Journal of Clinical Nutrition, Rolls found that women who eat a bowl of chicken soup feel fuller than those who eat chicken casserole served with a glass of water, even though both meals contain exactly the same ingredients. The soup eaters also tended to be less hungry at their next meal -- and to eat consume fewer calories -- than those who ate the casserole.

There is one way that drinking water could help you lose weight, however: if you drink it in place of beverages that contain a lot of added sugar. Like water, sugary beverages fail to trigger a sense of fullness, which means you can consume a lot of calories without taking the edge off hunger.

Peter Jaret is a freelance writer in Petaluma, California, who has written for Health, Hippocrates, and many other national publications.

©1996-2005 WebMD Inc. All rights reserved.
Last Editorial Review: 1/30/2005 11:16:09 PM