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

Deep Sixed

When society gets an idea deeply planted in its collective consciousness, it’s very hard to shake it.

For instance: “8 x 8”

No doubt you’ve heard many times the recommendation to drink eight eight-ounce glasses of water each day. Why? You’ll flush toxins from the body, we’re told. And you’ll keep skin looking youthful, while helping maintain general good health.

A new review of these widely held beliefs exposes them for what they are: completely unsubstantiated.

Coming up empty

University of Pennsylvania researchers Dan Negoianu and Stanley Goldfarb recently published a review of water research that investigated some of the prominent myths surrounding the 8 x 8 recommendation.

Their guiding light is Dr. Heinz Valtin, M.D., of Dartmouth College. A few years ago, Dr. Valtin 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.”

Negoianu and Goldfarb take Dr. Valtin’s work a step further – in fact, four steps further with four busted myths. (All quotes are taken from the authors report in the Journal of the American Society of Nephrology.)

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.

Do not apply directly to the forehead

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? How did they pull THAT off?)

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, did someone simply miss that additional detail and jump 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, asn-online.org
“The Water Shortage and How to Fix It” William Campbell Douglass III, M.D., Daily Dose, 1/21/08, douglassreport.com