How Much Water Do You Really Need to Drink?

The well-known “8 x 8” rule — you should drink eight 8-ounce glasses of water a day — is not only daunting, it’s unfounded. In fact, nobody is sure where the idea came from, and science doesn’t support it. “It has no basis in fact,” says Michael Farrell, a professor at Monash University in Australia, who studies how the brain responds to thirst and other sensations. Likewise, the old advice to “drink before you’re thirsty” is countered by the latest research, as scientists finally figure out how the brain knows when you’re thirsty, and when you’ve had enough.

The human body is 55–60% water, varying by individual (muscle has more water than fat). Blood is 83% water, and 70% of your brain is all wet. Water aids digestion, clears toxins from the liver and kidneys, removes excess sodium from the bloodstream, regulates body temperature and blood pressure, protects skin and other tissues, and keep joints lubricated.

A person can survive weeks without food, but seldom more than a few days without water. Even mild dehydration, within hours, can affect mood, cognitive function, and physical performance, studies show.

We lose water constantly, by breathing, sweating, and using the toilet. But water loss is highly variable. On a cool day when a person isn’t active, eight glasses of water could be “well in excess of need, in which case a lot of water will be excreted” along with vital substances like sodium, Farrell says. Alternately, a person exercising on a hot day might need more than eight.

Fluid figures

There is no official U.S. government recommendation for how much water to drink. But there are guidelines for total fluid intake from independent groups. The average adult woman should consume about 11.4 cups of fluid per day (a cup equals 8 ounces) and men should consume 15.6 — be it straight from the tap, in other beverages, or in food, according to a widely cited 2004 report from the Health and Medicine Division of the National Academies. People get about 20% of their water from food, the report states. Fruits and vegetables are particularly water-laden — both tomatoes and watermelon are 90% or more water.

Subtracting the 20% of water consumed through food, that means the average woman should drink about 9.1 cups of fluid daily, and a man should drink about 12.5. That’s almost exactly what the group Dieticians of Canada advises for non-food fluid intake (nine for women and 12 for men).

Those estimates can be off for some people, however, considering different body sizes and types, varying ambient temperature, and activity levels. And there are other caveats. Women who are pregnant or nursing need more than they would otherwise.

Meanwhile, older people are particularly prone to drinking less than what’s advised. A 2017 study published in the journal Nutrition and Healthly Aging found that 56% of seniors drank fewer than six glasses of fluid a day, and 9% drank fewer than three. Other research has shown that somewhere between 6% and 30% of people over 65 who are hospitalized are dehydrated.

Along with regular water, milk, juice, and other non-alcoholic beverages count towards your fluid intake, according to the Mayo Clinic and other experts. Even coffee counts (the idea that caffeine dehydrates the body is a myth).

That said, health experts stress the value of plain water, the original zero-calorie beverage. Research published earlier this year in the journal JAMA Pediatrics found about 20% of children and young adults drank no regular water on a given day, and those same people consumed 93 more calories a day, on average, and got 4.5% more of their calories from sweetened drinks. It has long been known that sweetened beverages are linked to weight gain and a slew of problems — from depression to diabetes and heart attacks. Drinking more plain water leads to lower total calorie intake, according to a study of 18,300 adults published earlier this year in the Journal of Human Nutrition and Dietetics.

Knowing when you’re thirsty

You feel thirsty when your blood gets too salty, a signal picked up by specialized thirst neurons in the brain, explains Christopher Zimmerman, a grad student in neuroscience at the University of California, San Francisco. Clear urine is a sign you’re well hydrated. When urine turns yellow, and especially if it becomes dark, dehydration has begun. Other symptoms include infrequent urination and dry or blue lips, along with blotchy skin, rapid breathing, fatigue, fever, and dizziness.

Under normal circumstances, the body and brain work together to let you know you need water before any of that happens, Zimmerman says. Though he adds that “every person’s threshold for ‘feeling thirsty’ is probably a bit different, similar to pain thresholds. So in dangerous circumstances, like during extreme heat or exertion, it is a good idea to drink water even if you aren’t feeling thirsty yet to guarantee that you don’t become dehydrated.”

“This is an absolutely new way to look at thirst, as we show that it is throat and the gut that play an active role in quenching thirst much before your blood gets diluted by the ingested water.”

The science behind satiation

A drink of water enters the stomach quickly, but then it must flow into the intestines, then be absorbed by veins that flow through the liver, before it can pass on into the rest of the bloodstream, Zimmerman explains. Considering it takes about 10 minutes to begin changing the body’s overall hydration, scientists have long wondered why a gulp of fresh water is so immensely and immediately thirst-quenching and even pleasurable.

It’s been known since the 1990s, from a small study on people, that the mouth and stomach both send thirst-satiation signals to the brain. But the mechanism was a mystery. Three years ago, Zimmerman and study leader Zachary Knight wired up some mice, whose thirst mechanisms are similar to humans. When thirsty mice drank water, sensory signals raced to the brain’s hypothalamus, which regulates appetite and other vital functions, shutting down thirst neurons immediately, the researchers reported in the journal Nature.

“This fast signal from the mouth and throat appears to track how much you drink and match that to what your body needs,” Zimmerman says. But how does the sensory system know straight water from, say, salt water?

In the researchers’ latest study, detailed earlier this year in Nature, mice drank salt water. Their brain’s thirst neurons went quiet, same as before. But then they switched back on, indicating a contrarian signal was received. Next, the scientists injected fresh water directly into the rodents’ stomachs, and sure enough, the thirst neurons were deactivated. Salt water injections did not deactivate the neurons.

The brain rewards the drinking of water, but then there’s a gut check, the researchers concluded.

Other research in mice finds that gulping water — specifically the physical motion in the throat, which is different from the motion of swallowing food — also sends satiation signals to the brain’s thirst neurons. That study, by Caltech assistant professor of biology Yuki Oka and graduate student Vineet Augustine, was published in Nature last year.

“When the mice sipped water, these neurons did not get activated,” Augustine says, “very clearly showing that gulping is necessary.” Their research also showed, like Zimmerman’s, that the gut weighs in on whether the liquid that came down is sufficient.

“This is an absolutely new way to look at thirst, as we show that it is not just your brain but your throat and the gut that play an active role in quenching thirst much before your blood gets diluted by the ingested water,” Augustine says.

Drinking water also triggers the brain to release dopamine, the chemical that makes people feel good about everything from sex and drugs to gambling, according to Oka’s latest research, published May 29 in the journal Neuron.

The upshot: Drinking water doesn’t just satiate, it satisfies.

The brain can be fooled, however. A study back in 1997 found that after exercising, people drank less water the more carbonated it was. Other research showed sparkling water hydrates as well as regular water, if a person drinks an equal amount. But that isn’t what typically happens.

A 2016 study in the journal PLOS ONE found differences in voluntary fluid intake among very thirsty people depending on both carbonation and water temperature. Nintey-eight healthy adults were deprived of water for 12 hours, then they all drank 13.5 ounces of water that was either refrigerated or at room-temperature, and either carbonated or not. Then all four groups were allowed to drink as much additional room-temperature water as they wished, to measure how thirsty they remained. The people who initially drank cold water were less thirsty than those who drank warm water, and the people who drank cold, carbonated water were even less thirsty, meaning they drank the least overall. “Thirst signals a physiological need, but the cessation of thirst is the result of sensory information being integrated in the brain,” said senior author Paul Breslin, of the nonprofit Monell Chemical Senses Center.

Exactly why carbonated water causes people to drink less isn’t known, but one idea is that the bubbles create a sense of fullness in the stomach that discourages further drinking.

Finding the right balance

Drinking too much water can be deadly. Water intoxication, called hyponatremia, dilutes the body’s salt level, causing cells to swell. The condition is rare, but at least 14 athletes are known to have died from it, according to a 2015 study published in the Clinical Journal of Sport Medicine, which issued this new advice to athletes: Drink when you’re thirsty.

The good news is, if you’ve had enough water, your body will tell you. Research led by Farrell, the Monash University professor, finds that when people drink plenty of water and don’t feel thirsty, swallowing more water requires more effort — three times as much, people in the study said. The researchers dubbed it a “swallowing inhibition” — the body’s reaction to excess intake.

Bottom line: While the elderly and anyone exercising intensely or dealing with extreme heat may need to stay ahead of their hydration, by and large your body and brain are on the case. “The message is, do what comes naturally,” Farrell says. “Drink when you want to, and chances are this behavior will keep your fluid balance on an even keel.”

This story is part of The Elemental Guide to Water, a five-part special report on the health benefits of water, the science behind seltzer, the truth about fancy H2O, the safety of tap water, and how much water you really need to drink.

Illustrations by Shuhua Xiong