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How Much Water Should I Drink?

What experts think about the ‘8 glasses a day' rule.

by Karin Klein
May 22 2018, 5:39pm

Marc Tran/Stocksy

Water is having its moment as a panacea. We’re told that by drinking at least eight glasses (or half a gallon) a day—or, another version goes, enough ounces to equal half our weight—we’ll moisturize our skin, flush toxins from our bodies, lose weight, and keep our minds sharp.

The truth, though, goes more like: no, not really, sort of yes, and nuh-uh.

If we’re going to name a magic health elixir, certainly it’s better to pick water than most trendy drinks. In the United States, water is a fairly pure and safe product (at least if you don’t live in Flint). No calories, no artificial sweeteners, no contributions to obesity or diabetes.

But as with most things that are good for us, more isn’t necessarily better. For the most part, doctors are scratching their heads about why bottles of water have become constant companions in so many Americans’ lives, to the point where college students assiduously carry bottles from class to class, and office workers make sure their desks are never without.

Perhaps they’ve heard that even mild dehydration can impair cognitive function. One of the most-quoted studies on this topic found that in young adults, a water loss amounting to 1 percent to 2 percent of body weight can lead to reduced alertness and concentration.

Think about that for a moment. In a 150-pound adult, 1 to 2 percent of body weight would mean losing one and a half to three pounds of weight from water alone. In the medical world of dehydration, that’s considered mild, says Rachel C. Vreeman, associate professor of pediatrics at Indiana University School of Medicine, but in everyday life, it’s a lot of water loss, well past the point where most people would feel thirsty and down some fluids.

Believe in your thirst, Vreeman and other doctors say. It’s a finely attuned mechanism that keeps healthy people as hydrated as they need to be—which doesn’t usually amount to needing a half-gallon of water a day.

“Your body tells you that you’re thirsty pretty early on,” says Vreeman, who’s researched hydration beliefs and co-authored a book busting medical myths including the eight-glasses maxim.


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Something else to consider: Who’s funding or conducting the research? Many of the studies that give credence to the idea that we are a sadly underwatered country are funded by the bottled water industry that markets the same message, Vreeman says. “All of it is the industry that’s grown up around drinking bottled water,” she adds.

And though you hear all about the research cautioning that your thinking skills might be hampered, study results are far more mixed. Some studies haven’t found reduced cognitive ability in people who appear to have lower water levels in their bodies. Some have found that people reported feeling more alert after drinking water, but their actual performance was the same as those who hadn’t taken some sips. “The results of these studies are inconsistent, preventing any conclusion,” according to a 2011 article in the journal Nutrients.

For that matter, the very definition of hydration is a matter of debate. How much water do we need each day to be considered hydrated? Is it eight, eight-ounce glasses a day, which has become the traditional rule? Or should we use another often-repeated calculation: As many ounces per day as our body weight in pounds for active people? Or half that for sedentary people?

In truth, Vreeman said, there’s no evidence for any of these numbers. In fact, it’s not even known how these ideas got their start, much less how they became commonly accepted rules.

In Don’t Swallow Your Gum!, the book of health myths that Vreeman wrote with her Indiana University colleague Aaron E. Carroll, the authors surmise that it could have come from people lopping off the end of some advice from the National Research Council in 1945.

The council “stated that adults should take in about 2.5 liters a day and that most of this is contained in prepared foods,” the authors wrote. “If you ignore the last part of that statement, you would interpret this statement as a mandate to drink eight glasses of water per day.” That’s a little more than eight ten-ounce glasses, but somehow we arrived at eight ounces.

In 2004, the Institute for Medicine suggested an even higher total water intake—2.7 liters, or 91 ounces, from foods and beverages for women, and 3.7 liters, or 125 ounces, for men. but strangely, it concluded that the correct amount wasn’t really known. Its ultimate word on the subject? “The vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide.”

The problem, Vreeman says, is that people have grown so used to these arbitrary goals that they have overridden their thirst signal and no longer have a good sense of when they need to drink something. They can get that back, though, by paying closer attention instead of automatically drinking a certain amount of water.

Hydration hysteria has birthed various corollary myths: That plain water is the only drink that will hydrate you. That it’s a bad sign if urine is any darker than pale yellow. That if you’re thirsty, you’re already dehydrated.

That last one is true only if you define “dehydrated” as meaning you have less water in your body than you did before, says Stanley Goldfarb, a kidney specialist at the University of Pennsylvania who has been outspoken about our dehydration fears. This is the purpose of thirst—to tell you that you’re not at your usual water level and you might want to drink something. It doesn’t mean you’re growing weak with water loss or unable to solve math equations.

Coffee, tea, juice, soda, and milk also hydrate you, Goldfarb says. Those drinks might contain other things you personally want to avoid, such as caffeine or empty calories. But they will all put water into your body, the same as the stuff that comes out of the tap.

And although really dark urine could be a sign of serious trouble such as liver disease, there’s a lot of room for personal variation, Vreeman says. Medium to dark yellow doesn’t mean anything bad in a healthy person, or even a color coming close to the look of apple juice.

When it comes to “flushing toxins,” that’s a natural function that occurs in anyone with healthy kidneys, Vreeman and Goldfarb say. Extra pints of water beyond your basic need won’t somehow search out and eliminate extra toxins and leave your body purer.

As for moisturizing our skin, we simply can’t drink enough water to make the remotest difference. “The amount of water that goes to your skin is in similar proportion to a man standing next to the Eiffel Tower,” Goldfarb says. Vreeman advises moisturizers rather than ineffectually trying to prevent dryness from the inside out.

There’s more reasonable evidence on weight loss, Vreeman says. In one study, people who drank a pint of water 30 minutes before meals were found not only to eat less at that meal, but to lose more weight. Apparently, once the water had been eliminated, they still weren’t hungry enough to chow down an extra snack. But the research isn’t decisive and some studies haven’t come to the same conclusions.

None of this is to say that our bodies don’t need to take in water in one form or another. And some extra water probably isn’t going to hurt us, as long as it’s not taken to extremes. In addition, people with various health conditions might need to drink more than others—or less, Vreeman says.

But healthy people can forget about the eight glasses or the 2.7 liters for women or 3.7 liters for men. Contrary to the word on the well-watered street, Americans are not stumbling around in a parched fog. And a little bit of thirst does not mean we’re toxic-laden prunes.

“If you look at the studies, you do not see large numbers of people who are walking around hugely dehydrated,” Vreeman says.

So before you swig from that bottle, you might want to check in with that small voice inside to see if you’re actually thirsty.

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