For years we were told to drink eight glasses of water a day. That recommendation just isn’t right. It originated from a 1945 Food and Nutrition Board paper that said humans require roughly 2.5 liters per day, but the paper also noted we get most of the fluid we need from food and other beverages (fruits, veggies, soup, juice, even beer — all loaded with water). Yet for some reason, the second half of that message got glossed over, while the “drink more water” part became emblazoned in people’s minds. As a result, experts now fear we’ve become so focused on staying hydrated that we’re guzzling water to life-threatening extremes.

That’s not to say dehydration isn’t dangerous. If your body loses more fluids than you put in it — say, you’re barfing your brains out, sweating excessively, or running a high fever and drinking nothing at all — you’ll likely feel dizzy and get a headache. And if dehydration gets really bad, your blood pressure can plummet and your heart rate can speed up. But a vast majority of the time, if you just drink when you’re thirsty, there’s no need to freak out about dehydration.

And yet, because of this 70-year-old myth, many people, especially athletes, worry about running short on fluids. They’ll pound water and sports drinks whether their body really needs them or not — and that can lead to the opposite problem: overhydration.

“When you drink fluids to excess, the amount of water in your body goes up while the sodium level in your blood falls,” says Dr. Mitchell Rosner, a kidney specialist at the University of Virginia. Usually, the kidneys can control this balance by excreting extra water, he explains. “But if you take in too much fluid too quickly, the kidneys can’t keep up, and your sodium concentration dips to dangerous lows.” Even sports drinks, which contain sodium, don’t deliver enough of the mineral to keep this balance in check.

By itself, overhydration isn’t necessarily life-or-death. At its onset, Rosner says you’ll probably feel nauseous or disoriented and your head may start to pound, not unlike what happens when you’re dehydrated. But if overhydration occurs while you’re working out, it can quickly spiral into a dangerous condition called exercise-associated hyponatremia (EAH). When this happens, the brain cells swell, which can quickly knock you unconscious or cause you to drop dead.

In recent years, there’s been an alarming uptick in EAH deaths among marathon runners, football players, and other athletes. “These deaths were completely preventable,” says Rosner. “These were people who were otherwise in prime health but drank to excess trying to prevent dehydration and wound up in trouble.”

It’s time to change the message on how much water we should be drinking, Rosner says, especially before, during, and after exercise. The prescription is actually pretty simple: Drink when you’re thirsty. “Thirst is the body’s physiological cue that you need more fluids,” Rosner says. “Listen to it.” The idea that you only become thirsty when you’re dehydrated is another myth, he insists. So is the notion that you can “get ahead of dehydration” by pounding a bunch of water before you go for a run.

Your fluid needs will vary depending on the climate and your activity level. “If you’re exercising outside on a hot summer day, you’ll obviously sweat more and likely need to drink more water than when you’re sitting on the couch in the winter,” says Rosner. But let your thirst guide you, and you should be fine. Strap on a hydration belt when you run so you’ll have water when you need it. Hit the drinking fountain between reps at the gym if you feel thirsty. Keep a reusable water bottle at your desk at work to sip on throughout the day.

And in sweaty situations, in which you’re not able to take water breaks every time thirst kicks in, it’s not a big deal if you get slightly dehydrated. “Generally, mild dehydration, which means losing up to two percent of your bodyweight, won’t affect your performance,” Rosner says. Just be sure to drink up as soon as you get a chance.

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