Researchers Say Evidence Is Lacking for Benefits of Drinking 8 Glasses of Water a Day
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Reviewed By Louise Chang, MD
April 2, 2008 — The health benefits of drinking water, at least for already healthy people, may have been oversold, according to a new report. The findings will likely disappoint water-bottle-toting Americans and relieve those who can never seem to down those eight glasses of water a day, widely recommended for our health.
But there is nothing magical about those eight glasses, at least when it comes to proven health benefits, according to a new report. "There is no clear evidence of benefit from drinking increased amounts of water," writes Stanley Goldfarb, MD, professor of medicine at the University of Pennsylvania, Philadelphia, and the senior author of an editorial on the topic in the Journal of the AmericanSociety of Nephrology.
On the other hand, he adds, "There is also no clear evidence of lack of benefit." There's a general lack of evidence either way.
Those doctors and others who have been recommending drinking eight glasses of water aren't basing it on anything scientific, according to Goldfarb. He concludes that most healthy people don't have to worry about drinking eight glasses every day.
He emphasizes he is talking about healthy people with kidneys that function well. And he points out that people who live in hot, dry climates do need to drink more water to avoid dehydration, as do those who engage in vigorous exercise.
Health Benefits of Drinking Water: Search for Evidence
Goldfarb was curious about where the longstanding recommendation about eight daily glasses of water originated. "In my mind it wasn't that drinking this extra water would hurt you, but that you might not have to."
So he combed through medical literature dating back to the early 1970s, trying to find the science to back up the advice.
Turns out, there is no single study and no single outcome that led to the recommendation becoming popular, he says. Somehow, it took on a life of its own.
Goldfarb and his University of Pennsylvania colleague, Dan Negoianu, MD, next examined some popular claims about the health benefits of drinking water, trying in each case to find scientific evidence.
"We looked at the evidence of some of the so-called urban myths that have grown up about drinking water," Goldfarb says.
Claim No. 1: Drinking Water Helps Excrete Toxins
Drinking lots of water is widely thought to help improve kidney function and boost the clearance of toxins. One way it could do this, Goldfarb says, is by a mechanism called glomerular filtration, a measure of the kidney's ability to filter and remove waste products.
But in one study the researchers looked at, increased water intake by 12 young and healthy people actually decreased their glomerular filtration rate. And in another study, the rate did not change over time during a six-month period in which older men drank more water to try to improve bladder function.
In other research, increased water intake was found to affect the clearance of many substances by the kidneys, including sodium. But the studies don't prove any sort of clinical benefit, Goldfarb says.
"What almost certainly happens is, any toxins the kidney is responsible for excreting simply get diluted when the person is drinking a lot of water," Goldfarb says.
Claim No. 2: Drinking Water Helps Your Organs Work Better
Water is retained in various organs, so the thinking goes, and they work better with more water in them.
But Goldfarb and Negoianu say how much water is retained varies with the speed with which the water is taken in. If it's sipped, it's more likely to stay in the body than when gulped.
Even so, they could find no studies documenting that increased water intake helped the organs.
Claim No. 3: Drinking Water Reduces Food Intake and Helps You Lose Weight
Drinking more water is widely encouraged to help weight loss, the theory being that the more water you drink, the fuller you will feel and the less you will eat. "The [medical] literature on this is quite conflicted," Goldfarb says.
"Drinking before a meal might decrease intake [according to one study], but another study found [it did] not."
Even so, Goldfarb calls this claim one of the most promising for further study.
Claim No. 4: Drinking Water Improves Skin Tone
"From a quantitative sense, this doesn't make sense," Goldfarb says. The water you drink will be distributed throughout the body. "Such a tiny part of it would end up in the skin," he says.
"It turns out one small study showed there might be an increase in blood flow in those who drink [a lot of] water, but no one has ever looked scientifically [to see if it improves skin tone]."
Claim No. 5: Drinking Water Wards Off Headaches
Headache sufferers often blame water deprivation. But Goldfarb could only find one study that looked at this. The study participants who boosted their water intake had fewer headaches than those who did not, but the results were not statistically significant, meaning they could have been chance findings.
Second Opinion: Health Benefits of Drinking Water
The report provides interesting — and sometimes surprising — information, says David Baron, MD, a family physician and chief of staff at Santa Monica-UCLA Medical Center & Orthopaedic Hospital, Calif., who reviewed it for WebMD.
The most surprising finding, he says, was the lack of a scientific link found between drinking a lot of water in order to eat less. "I thought [the suggestion that] filling up your stomach with water might help lose weight makes sense," he says.
The report isn't dismissing the need to drink a healthy amount of fluids, he says. It simply showed no scientific basis to the recommendation to drink eight glasses of water daily.
"There is a lot of individual variation" in exactly how much water or fluid people need," he says.
Most of us, he says, are OK "by trusting our instincts" about how much to drink. "If you have a normal heart, normal kidneys, and normal thirst mechanism, it's not likely you will get dehydrated if there is a sufficient supply of fluids available," he says, and drink when thirsty.
Drinking Water: A Placebo Effect?
Might drinking a lot of water make us think we feel better, look better, and function better? Could there be a placebo effect to those eight daily glasses?
"I'm certain there is," Goldfarb says. "The placebo effect is very strong."
And if you're still convinced lots of water does your body good? No problem. "People say they feel stronger and healthier if they drink more water," he says. "That's fine. If they enjoy that benefit, so be it. [But] those who don't feel that way shouldn't feel obligated to drink the eight glasses."
SOURCES: Stanley Goldfarb, MD, professor of medicine and associate dean for curriculum, University of Pennsylvania School of Medicine, Philadelphia. Negoianu, D. Journal of the American Society of Nephrology, published online April 2, 2008. David Baron, MD, family physician; chief of staff, Santa Monica-UCLA & Orthopaedic Medical Center, Calif.
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