Enough with drinking water
Healthy people don't have to drink nearly as much as they think
By Cathy Gulli
When it comes to water, there’s nothing clear about how much we need to drink or even what good it does us. Still guzzling eight 8-oz. glasses a day? There’s no scientific proof everyone requires so much. Urine should be colourless? That’s a sign you’ve chugged too much. Thirst means you’re already dehydrated? Not even close.
“I want to squash that notion. It’s baloney,” says Heinz Valtin, professor emeritus of physiology at the Dartmouth Medical School in Hanover, N.H., in a recent podcast produced by the American Physiological Society. He should know. His seminal 2002 study, “ ‘Drink at least eight glasses of water a day.’ Really? Is there scientific evidence for the ‘8 x 8’?” is often cited by other researchers investigating how much water we should consume daily. Now, many physiologists are debunking the most common assumptions about water intake. Valtin’s conclusion: healthy people who live sedentary lifestyles in temperate climates don’t have to drink so much.
So how did this belief get so widespread? One theory suggests it was a misinterpretation of the 1989 “recommended dietary allowance” (RDA) data produced by the Institute of Medicine, says Samuel Cheuvront, principal investigator at the U.S. Army Research Institute of Environmental Medicine. Those stated that for every calorie expended, one millilitre of fluid was to be consumed. Assuming everybody kept a 2,000 calorie diet, which is the reference often used to calculate RDAs, says Cheuvront, that equals two litres, which is 64 fluid ounces—or eight 8-oz. glasses.
The recommendations “never said that was what everyone required,” says Cheuvront, but people just “latched on.” Over the last two decades, there has been “a major cultural change” in our obsession with good hydration, adds Mark Knepper, chief of the kidney and electrolyte metabolism lab at the National Institutes of Health in Bethseda, Md. “Somehow we all survived without carrying around water and chugging every 10 minutes,” he says. Toting water, wrote Valtin in his study, has become akin to a “security blanket.”
That’s misguided, experts say, and they’re worried we’ve developed tunnel vision when it comes to how we replenish our bodies. “It’s a myth that in order to hydrate ourselves we need [only] plain water as opposed to water found in any other food or beverage,” says Susan Barr, a professor of nutrition at the University of British Columbia, who was part of an Institute of Medicine panel that established 2005 water intake recommendations. She is a proponent of plain water because it contains no calories, but Barr says that all kinds of fluid count, including juice, pop, beer and even mild diuretics such as coffee and tea. There’s even water in foods such as chicken and bread, Barr adds.
Rather than slavishly choking down eight glasses of water, these experts say we should take a more enlightened, individualized approach to hydration. How much we each need depends on factors such as our diet, level of daily physical activity, how hot it is, where we live, our size and even our personal health issues. Fortunately, when we need to drink, our bodies send us a signal far more clear and accurate than any formula: thirst. “When the salt level goes up in your blood, so does your thirst,” says Knepper. The most useful way to know the state of your water balance, he explains, is by taking a sip. “Everyone has had that experience [where] you get some cold water and boy, does that taste good. So you can tell if you need that water by how it tastes when you try it.”
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