Claim:   The average person needs to drink eight glasses of water per day to avoid being "chronically dehydrated."

Status:   False.

Example:   [Collected via e-mail, 2001]

75% of Americans are chronically dehydrated. In 37% of Americans, the thirst mechanism is so weak that it is often mistaken for hunger. Even mild dehydration will slow down one's metabolism as much as 3%. One glass of water shut down midnight hunger pangs for almost 100% of the dieters studied in a U-Washington study. Lack of water is the number one trigger of daytime fatigue. Preliminary research indicates that 8-10 glasses of water a day could significantly ease back and joint pain for up to 80% of sufferers. A mere 2% drop in body water can trigger fuzzy short-term memory, trouble with basic math, and difficulty focusing on the computer screen or on a printed page. Drinking 5 glasses of water daily decreases the risk of colon cancer by 45%, plus it can slash the risk of breast cancer by 79%, and one is 50% less likely to develop bladder cancer. Are you drinking a healthy amount of water each day?

Origins:   "You need to drink eight to ten glasses of water per day to be healthy" is one of our more widely-known basic health tips. But do we really need to drink that much water on a daily basis?

In general, to remain healthy we need to take in enough water to replace the amount we lose daily through excretion, perspiration, and other bodily functions, but that amount can vary widely from person to person, based upon a variety of factors such as age, physical condition, activity level, and climate. The "8-10 glasses of water per day" is a rule of thumb, not an absolute minimum, and not all of our water intake need come in the form of drinking water.

The origins of the 8-10 glasses per day figure remain elusive. As a Los Angeles Times article on the subject reported:

Consider that first commandment of good health: Drink at least eight 8-ounce glasses of water a day. This unquestioned rule is itself a question mark. Most nutritionists have no idea where it comes from. "I can't even tell you that," says Barbara Rolls, a nutrition researcher at Pennsylvania State University, "and I've written a book on water."

Some say the number was derived from fluid intake measurements taken decades ago among hospital patients on IVs; others say it's less a measure of what people need than a convenient reference point, especially for those who are prone to dehydration, such as many elderly people.

The consensus seems to be that the average person loses ten cups (where one cup = eight ounces) of fluid per day but also takes in four cups of water from food, leaving a need to drink only six glasses to make up the difference, a bit short of the recommended eight to ten glasses per day. But according to the above-cited article, medical experts don't agree that even that much water is necessary:

Kidney specialists do agree on one thing, however: that the 8-by-8 rule is a gross overestimate of any required minimum. To replace daily losses of water, an average-sized adult with healthy kidneys sitting in a temperate climate needs no more than one liter of fluid, according to Jurgen Schnermann, a kidney physiologist at the National Institutes of Health.

One liter is the equivalent of about four 8-ounce glasses. According to most estimates, that's roughly the amount of water most Americans get in solid food. In short, though doctors don't recommend it, many of us could cover our bare-minimum daily water needs without drinking anything during the day.

Certainly there are beneficial health effects attendant with being adequately hydrated, and some studies have seemingly demonstrated correlations between such variables as increased water intake and a decreased risk of colon cancer. But are 75% of Americans really "chronically dehydrated," as claimed in the anonymous e-mail quoted in our example? Many of the notions (and dubious "facts") presented in that e-mail seem to have been taken from the book Your Body's Many Cries for Water, by Fereydoon Batmanghelidj. Dr. Batmanghelidj, an Iranian-born physician who now lives in the U.S., maintains that people "need to learn they're not sick, only thirsty,'' and that simply drinking more water "cures many diseases like arthritis, angina, migraines, hypertension and asthma." However, he arrived at his conclusions through reading, not research, and he claims that his ideas represent a "paradigm shift" that required him to self-publish his book lest his findings "be suppressed.''

Other doctors certainly take issue with his figures:

[S]ome nutritionists insist that half the country is walking around dehydrated. We drink too much coffee, tea and sodas containing caffeine, which prompts the body to lose water, they say; and when we are dehydrated, we don't know enough to drink.

Can it be so? Should healthy adults really be stalking the water cooler to protect themselves from creeping dehydration?

Not at all, doctors say. "The notion that there is widespread dehydration has no basis in medical fact," says Dr. Robert Alpern, dean of the medical school at the University of Texas Southwestern Medical Center in Dallas.

Doctors from a wide range of specialties agree: By all evidence, we are a well-hydrated nation. Furthermore, they say, the current infatuation with water as an all-purpose health potion — tonic for the skin, key to weight loss — is a blend of fashion and fiction and very little science.

Additionally, the idea that one must specifically drink water because the diuretic effects of caffeinated drinks such as coffee, tea, and soda actually produce a net loss of fluid appears to be erroneous. The average person retains about half to two-thirds the amount of fluid taken in by consuming these types of beverages, and those who regularly consume caffeinated drinks retain even more:

Regular coffee and tea drinkers become accustomed to caffeine and lose little, if any, fluid. In a study published in the October issue of the Journal of the American College of Nutrition, researchers at the Center for Human Nutrition in Omaha measured how different combinations of water, coffee and caffeinated sodas affected the hydration status of 18 healthy adults who drink caffeinated beverages routinely.

"We found no significant differences at all," says nutritionist Ann Grandjean, the study's lead author. "The purpose of the study was to find out if caffeine is dehydrating in healthy people who are drinking normal amounts of it. It is not."

The same goes for tea, juice, milk and caffeinated sodas: One glass provides about the same amount of hydrating fluid as a glass of water. The only common drinks that produce a net loss of fluids are those containing alcohol — and usually it takes more than one of those to cause noticeable dehydration, doctors say.

The best general advice (keeping in mind that there are always exceptions) is to rely upon your normal senses. If you feel thirsty, drink; if you don't feel thirsty, don't drink unless you want to. The exhortation that we all need to satisfy an arbitrarily rigid rule about how much water we must drink every day was aptly skewered in a letter by a Los Angeles Times reader:

Although not trained in medicine or nutrition, I intuitively knew that the advice to drink eight glasses of water per day was nonsense. The advice fully meets three important criteria for being an American health urban legend: excess, public virtue, and the search for a cheap "magic bullet."

Last updated:   31 December 2005

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  Sources:

    Batmanghelidj, Fereydoon.   Your Body's Many Cries for Water.

    Global Health Solutions, 1995.   ISBN 0-962-99423-5.

    Carey, Benedict.   "Hard to Swallow."

    Los Angeles Times.   20 November 2001   (Health; p. 1).

    Foreman, Judy.   "The Water Fad Has People Soaking It Up."

   The Boston Globe.   11 May 1998   (p. C1).

    Hoolihan, Charlie.   "Body Needs Plenty of Water to Work."

    The [New Orleans] Times-Picayune.   31 May 1998.

    CNN.com.   "Americans Need to Shake Salt Habit."

    11 February 2004.

    Los Angeles Times.   "All That Water Advice Just Doesn't Wash."

    15 January 2001   (Health; p. 7).

    Los Angeles Times.   "Readers Take Issue with Article About Water Consumption."

    25 January 2000   (Health; p. 5).

    The Toronto Star.   "Distilling Water Facts from Water Fiction."

    21 March 1999.