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The 64-ounce myth: How much water is enough?

February 20, 2019

All Posts by MinuteClinic

We've all repeatedly heard the advice to drink eight 8-ounce glasses a day to stay hydrated. But there's no evidence behind it.

If you're like many Americans, you probably have a love affair with your reusable water bottle. It's your trusted companion, toted around whether you're meeting at work, shopping at the mall or taking the kids to the playground.

It's easy to explain this collective cultural obsession: Americans constantly hear the advice to drink at least eight glasses of water a day.

There's no doubt that hydration is crucial to a healthy human body. But there is no evidence behind the 64-ounce quota. In fact, the best advice for how much water to drink may surprise you.

The myth of the '8 x 8'

The good news is that you can stop anguishing about drinking eight 8-ounce glasses of water a day. There's no science behind this prevalent advice.

Dartmouth Medical School researcher Heinz Valtin completed extensive research nearly two decades ago trying to find the origin of this recommendation. In an invited review published in 2002 in the American Journal of Physiology, he offered two possible theories.

One potential origin, Valtin says, was a Nutrition for Good Health, a 1974 book by nutrition experts Fredrick J. Stare and Margaret McWilliams. Another was the 1945 publication of the recommended dietary allowances (RDAs) by the Food and Nutrition Board of the National Academy of Sciences. Either one of these sources could have given way to the eight-glasses-of-water-a-day requirement.

The 1974 book authors, for instance, only casually mentioned drinking six to eight glasses of water a day, and stated that other beverages, fruits and vegetables are also good water sources. Likewise, the National Academy of Sciences' recommendation—2.5 liters of water (10.5 glasses)—supported that most of that quantity is contained in foods.

The National Academy of Sciences' Institute of Medicine has updated its recommendations since then, with the most recent research published in 2004. Its recommended intake for adults is 3.7 liters for men and 2.7 liters for women (active people and pregnant or nursing women need more). This sounds even more overwhelming than 64 ounces—except that it's not just plain water.

“What many people don't realize is that a lot of that fluid can come—and does come—from our food intake, in addition to our beverage intake," says Hallie Zwibel, DO, Director of Sports Medicine and Assistant Professor at the NYIT College of Osteopathic Medicine in Old Westbury, New York.

“Some fruits and vegetables contain a lot of water. We don't consider them hydration, but they help toward the body's water intake," Dr. Zwibel continues.

Even your coffee or beer counts.

“The diuretic effect from these is not very large," says Michael Sutters, MD, MRCP, a nephrologist at Virginia Mason Health System in Seattle, Washington. “I think it's very reasonable to say that you can include alcoholic and caffeinated drinks into your overall fluid intake. They're not self-negating because of their diuretic properties."

Inexact science

Unlike other recommended daily allowances, scientists can only go as far as recommending an "adequate intake" specifically for water. It's because the available research on water needs was based largely on self-reported numbers from the National Health and Nutrition Examination Survey (NHANES) rather than stronger research like trials.

“The reason we couldn't get to a recommended dietary allowance for water was that we didn't have enough strong evidence," says Stella Volpe, PhD, RDN, ACSM-CEP, FACSM, and one of 10 members of the Food and Nutrition Board's committee that researched and published the 2004 “Dietary Reference Intakes for Water, Potassium, Sodium, Chloride and Sulfate.

“We had to go with an adequate intake, which at least guides people to what they need per day," says Dr. Volpe, currently a professor and chair of the Department of Nutrition Sciences at the Drexel University College of Nursing and Health Professions in Philadelphia, Pennsylvania.

Dr. Zwibel says men need more water because their metabolism is higher than women's and they also typically consume more calories.

“You're not getting energy from water, but it helps regulate metabolic processes that require calories, so you're running through more water when your metabolism is higher," he says.

Drink to thirst

Dr. Sutters says that the human body has “exquisite osmo-regulatory feedback mechanisms" to regulate its extracellular osmolality—the ratio between water and sodium in the body. That's why healthy people are unlikely to become severely dehydrated in the short term unless their environment has extreme variations.

“We tell people who are perfectly normal, going about their everyday lives, to drink if they're thirsty, and not worry too much about drinking a lot," he says.

You can learn to recognize the symptoms of thirst.

“Slight headaches, tiredness and dry mouth may be clues that you need to drink," Dr. Sutters says.

The problem is that some individuals don't drink enough because they get too busy or suppress their thirst. In the workplace, for example, there's pressure to perform or perhaps there's lack of bathroom access. So people learn to ignore the clues.

“Our body is excellent at regulating our hydration," Dr. Zwibel says. “The larger issue is that most of us aren't very good at listening to our body's cues."

That's why in addition to drinking when you're thirsty, Dr. Zwibel recommends keeping an eye on urine color.

“Assuming you're healthy and don't have any issues with your kidneys, if your urine is darker yellow, that means you're not adequately hydrated," he says. “Your body is concentrating the urine and keeping as much water as possible because you're not putting enough in."

Dr. Zwibel says light yellow to clear indicates an excellent saturation. Dr. Volpe prefers light yellow—she believes clear urine could be a sign of over-drinking.

“People need to be aware that you can over-consume water," she says. “If you have healthy kidneys, it's not going to hurt you, but in certain situations, you can get things like hyponatremia [abnormally low sodium concentration in the blood]."

How to stay hydrated

So why drink water after all, when you could be drinking coffee, juice or wine all day?

“The thing about the other liquids that we're sold or encouraged to drink is they contain a lot of different things, mostly things we either don't need or are actively harming us," Dr. Sutters says.

Sugar is of special note; he says it would be “absolutely horrifying, quite sobering" to see the amount of sugar in a soda can illustrated as a solid pile.

“You don't want to be drinking your calories," Dr. Zwibel agrees. “You can be equally hydrated from uncaffeinated tea or seltzer, but we recommend water because it's the easiest thing—it's universal and it's calorie-free."

That means carrying around your reusable water bottle is a good habit after all.

“By simply having it around, you're more likely to get a subconscious cue that you're thirsty," he says.

One way to boost fluids intake is through foods that are high in water, everything from soups to spinach and watermelon. The national nutritional survey used by the National Academy of Sciences showed that 22 percent of the estimated total water intake came from food, but you can increase that with fruits and vegetables that have high water content.

“People think they need eight 8-ounce glasses of water a day and it overwhelms them," Dr. Volpe says. “We still need close to that, but we don't need to drink all that water because we can get fluids from a variety of sources."

Thirst can also masquerade as hunger.

“If you feel hungry, try drinking water first, to find out if that satisfies before grabbing a snack," Dr. Sutters adds.

Dr. Zwibel notes it's a good idea to start every meal with water.

“Research shows that when you're hungry, if you drink water or a beverage, you'll consume less," he says. “So you'll take in less calories if you start with a glass of water."