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Eight glasses of water per day? There's no evidence that shows a benefit

Coming Up Empty

8 x 8

Recognize that equation? It’s the widely-touted recommendation for water intake: at least eight glasses per day, eight ounces per glass.

But when a couple of University of Pennsylvania researchers reviewed studies that investigated myths surrounding the 8 x 8 advice, they were surprised to find that there’s no known source for the recommendation.

And they’re not the first to discover a cold trail.

A few years ago, Heinz Valtin, M.D., of Dartmouth College went looking for scientific evidence to support the 8 x 8 dogma. In the August 2002 issue of American Journal of Physiology, Dr. Valtin wrote: “I have found no scientific reports concluding that we all must ‘drink at least eight glasses of water a day.’ On the contrary, there are publications that state the opposite.”

The U. Penn team took Dr. Valtin’s work a step further. Actually, four steps further …

Four busted water myths

All quotes below are taken from the U. Penn report, published in the Journal of the American Society of Nephrology. (For those who skipped medical school, nephrology is the branch of medicine that focuses on kidney health.)

MYTH ONE: Increased water intake helps remove toxins from the kidneys.

FACT: Water does have some impact on substances cleared from the kidneys, but “current data are insufficient to assess the clinical significance of these observations.” One study found that increased water intake actually lowered a type of filtration rate used to measure kidney function.

MYTH TWO: Increased water intake curbs appetite by increasing satiety.

FACT: “There is surprisingly little evidence regarding this issue. None of these studies makes clear whether drinking a large volume of fluid over the course of a day will decrease the number of ingested calories.”

MYTH THREE: Increased water intake improves skin tone.

FACT: “One study suggested ingestion of 500 ml of water increases indices of capillary blood flow in the skin. To summarize the conclusions of other more exhaustive reviews: There is no clear evidence of benefit from drinking increased amounts of water.”

MYTH FOUR: Increased water intake prevents headaches.

FACT: Only one trial was found that examined the effects of water on headaches. When 15 migraine patients were randomly assigned to increase water intake or placebo for 12 weeks, patients in the water group had a small but statistically insignificant drop in the number of hours with headache. Based on this result, the authors note that the water/headache-prevention link deserves further research.

(Side note: Wouldn’t you like to know what was used as a placebo for water in this study? Where do you find a clear, tasteless liquid that does NOT do all the things water does? How did they pull THAT off?)

Back to the source

Meanwhile, Dr. Valtin believes he knows where the 8 x 8 myth got started.

The Food and Nutrition Board of the National Research Council recommends an intake of one milliliter of water for each food calorie ingested. For most people, this adds up to about 64 to 80 ounces of water per day. But the FNB recommendation also states that most of that water intake is achieved when eating prepared foods. So when the FNB recommendations were originally released, maybe someone missed that additional detail and jumped to the 8 x 8 conclusion.

That’s Dr. Valtin’s theory, and he’s sticking to it.

“Just Add Water” Dan Negoianu and Stanley Goldfarb, Journal of the American Society of Nephrology, published online ahead of print, 4/2/08,