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How Much Water Should You Drink?

If you change your relationship with water using one simple rule, you can improve your health and possibly lose weight.

Biwa Studio/Getty Images
Biwa Studio/Getty Images,

by Sarah Mahoney

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By now, you’ve probably heard that the old rule—drink eight 8-ounce glasses of water a day—is basically baloney. The trouble is, experts say, people are still overdoing, in the name of making a “healthy” change. Case in point: The 30-day water challenge has gone viral on social media, urging people to drink a gallon of water a day (128 fluid ounces).

This has experts worrying about rising rates of hyponatremia, a dangerous—even fatal—condition that develops from drinking too much water in too short a time.

“Hyponatremia was once only seen (and rare) in athletes competing in very long endurance races,” says Tamara Hew-Butler, associate professor of exercise science at Oakland University. “Now, we’re seeing it in shorter races, among canoeists, cyclists, and even in people taking Bikram yoga.”

Why do smart people make bad mistakes about hydration, often drinking more than is good for them? What’s led us from believing that if healthy hydration is good, more water must be better? Hew-Butler says part of the problem is that we’ve been listening to the wrong experts.

“A little bit of marketing goes a long way,” she says, “most of the hydration advice we get comes from companies trying to sell products, whether it’s fruit juice, soda, sports drinks or bottled water. Unlike scientific studies, advertisements are not regulated, so companies can say whatever they want."

A marathoner herself, Hew-Butler’s research has focused on groups most at risk, “who feel compelled to drink beyond thirst for fear of dehydration.” She says that some of the most vulnerable people include women who are just beginning to run marathons, and conversely the fastest-finishing triathletes and ultra-long distance runners.

Because exercise stimulates an anti-diuretic hormone, overzealous guzzling (for small women, that can mean anything more than a liter per hour) is a risk.

It’s more than a little scary, she says. “Even though there’s no one-size-fits-all guideline, there are still people out there making recommendations about drinking more and more water for health, as well as weight loss. Yet most people are unaware that drinking too much water can be fatal.”

There are still people out theremaking recommendations about drinking more and more water. Yet most people are unaware that drinking too much water can be fatal.

How to Figure Your Water Rules

The best way to know how much to drink? Listen to your body. “The vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide,” say guidelines from the Institute of Medicine. The recommendations are deliberately vague—about 2.7 liters (91 ounces) of total fluids each day for women, and for men an average of approximately 3.7 liters (125 ounces). That includes all fluids—water, other beverages, and even food, which typically provides up to 20 percent of our fluid intake.

The American College of Sports Medicine warns people not to drink more than 1 quart, or 32 ounces, in an hour.

Hew-Butler says you really can trust the old noggin on this one. “Your brain monitors blood sodium concentration and circulating blood volume continuously,” she explains. So when the concentration of sodium begins to rise and it’s time to take in more fluid, our brains tell us we’re thirsty. Thirst, she says, is the most trustworthy sign that we might be approaching dehydration.

Your body will tell you when to step away from the water bottle, too. “If you drink too much water, you may feel the sensation of sloshing of fluid in the stomach, and feel puffy.” Other symptoms of overdoing it include nausea, vomiting clear fluid, severe headaches and muscle twitching.

Many people swear they can gauge proper hydration when their pee is the color of lemonade, rather than darker or transparent. But that can be misleading. “Urine that appears dark and concentrated may just mean that the body is conserving water to maintain appropriate sodium levels,” she says. Thirst is more important than the color of urine.

She takes an equally dim view of calculating your “sweat rate” by subtracting your post-exercise weight from your pre-exercise weight (adjusting for how much you drank and peed during your workout). “That works fine in the lab, where everything from climate to stress to exercise intensity is controlled,” she says. “But in real life, there’s lots you can’t predict.”

All that, she says, brings us back to the one simple rule: Drink when you feel thirsty.

Water and Weight Loss

While many of the wonders attributed to water don’t hold up under scientific scrutiny—drinking more won’t make your skin look better or shorten the length of a cold—there are many known risks from drinking too little water. While dehydration poses big risks for the elderly and small children, a recent study from the University of Arkansas finds that even mild levels of dehydration can impact heart health.

So giving your body the sips it asks for can change your health for the better, as long as you obey your thirst and don’t go overboard. A recent study from the University of Illinois at Urbana-Champaign found that increasing consumption of plain water by small amounts resulted in fewer calories consumed, as well as less saturated fat, sugar, cholesterol and sodium.

The study, which looked at 18,300 adults in the U.S., found that when people drank just 1 percent more water, they ate 8.6 fewer calories per day. Adding one to three cups of water per day cut calorie intake by 68 to 205 calories. And people who drank that much ate 5 to 18 fewer grams of sugar.

The findings were mostly consistent regardless of body weight, age, ethnicity or education levels, “but larger among males and young/middle-aged adults than among females and older adults,” says Ruopeng An, assistant professor of kinesiology and community health, and lead researcher.

His advice? Learn to love water, “in replacement for caloric beverages, especially sugar-sweetened beverages as a way to reduce total daily caloric intake.” Drink according to your thirst, favoring water as your beverage of choice.