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 2009 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.”
Another way to gauge hydration is by our urine colour. Valtin suggests it should be a “moderate yellow” like lemonade. But Barr says that hues can be tricky indicators because urine colour can be affected by what we consume. Multivitamins, for instance, can make it golden. Cheuvront says that the first urination of the day can be quite accurate though. Meanwhile, colourless urine signals that you’re drinking more water than you need, says Knepper.
Over-hydration isn’t usually a problem unless you’re under extreme stress, like a soldier or a marathon runner might experience. Under normal circumstances, when we’ve consumed too much water, an antidiuretic hormone in our bodies called vasopressin goes down. As it falls, the kidneys excrete the water and we relieve ourselves. But in rare and extremely stressful situations, vasopressin can get inappropriately high, even as a person drinks plenty of water. The kidneys hold onto the excess fluid, which dilutes the salt in our blood. “So you keep filling the tub, but you don’t open the stick-it to let the water out,” says Knepper. In the worst cases, this turns into hyponatremia, which causes swelling in the brain, and can actually kill you.
But for most of us, who are healthy, have easy access to water and food, and don’t often face extreme conditions, the risk of over-hydration or dehydration is low. Even when we first feel thirsty, we’re a long way off from dehydration. That parched sensation sets in when our blood concentration is about two per cent, says Valtin, and dangerous dehydration doesn’t occur until the blood is concentrated at five per cent. “So,” he says, “there’s a big leeway.”