As recently as the 1990s, Jodi Stookey, a nutrition consultant based in California, remembers hydration research being a very lonely field. The health chatter was all about fat and carbs; children routinely subsisted on a single pouch of Capri Sun a day. Even athletes were discouraged from sipping on fields and race tracks, lest the excess liquid slow them down. “I can’t tell you how many people told me I was stupid,” Stookey told me, for being one of water’s few advocates.
But around the turn of the millennium, hydration became an American fixation. Celebrities touted water’s benefits in magazines; branded bottles overran supermarket shelves. Academic research on hydration underwent a mini-boom. After ages of being persistently parched, we were suddenly all drinking, drinking, drinking, because we felt like we should. It was an aquatic about-face—and it didn’t make total scientific sense.
The importance of hydration, in the abstract, is indisputable. Water keeps our organs chugging and our muscles agile; it helps distribute nutrients through the body and maintains our inner thermostat. Take it away, and cells inevitably die. But the concrete specifics of adequate water intake are still, in large part, a mess. For hydration, “there are no clear numbers, or a threshold you have to maintain,” says Yasuki Sekiguchi, a sports-performance scientist at Texas Tech University. Experts don’t agree on how much water people need, or the best ways to tell when someone should drink; they differ on how to measure hydration, which beverages are adequately hydrating, and how much importance to attribute to thirst. They have yet to reach quorum on what hydration—a process that’s sustained life since its primordial inception—fundamentally is. The murkiness has left the field of hydration research, still relatively young and relatively small, rife with “vicious camps against each other,” says Tamara Hew-Butler, an exercise physiologist at Wayne State University.
[Read: Drink more water]
Forget, for instance, one of water’s most persistent myths: the oft-repeated advice to down eight 8-ounce glasses of water each day. No one can say for certain, but one theory is that the idea sprouted from a misinterpretation of a nutrition document from the 1940s, which stated that 2.5 liters of water a day (that is, approximately 10 8-ounce glasses) was “a suitable allowance for adults” in “most instances.” The guidance also noted, in the very same paragraph, “Most of this quantity is contained in prepared foods.” But the bigger issue is this: Probably no single number for water intake will ever suffice—not for a population of people with varying weights, genetics, diets, and activity levels, living in varying climates. Even within an individual, what’s best will change through a lifetime. The answer to How much water should I be drinking? is invariably Uh, it really depends.
Today’s hydration zeitgeist seems to hold that no amount of water is too much. The market teems with intake-tracking smartphone apps and time-stamped bottles that cheer drinkers toward hydration goals as high as a gallon a day—a quota astronomical enough to be stressful, even dangerous, should people flood their bodies all at once. But America’s hydration hype machine “has established a narrative that we are all walking around dehydrated, and need to drink more,” Hew-Butler told me. It’s no wonder that some people have reported legitimate anxiety over falling short on water intake.
No single source sold America on water. But a 2021 episode of the podcast Decoder Ring points to Gatorade as one of the first companies to pitch dehydration as a health problem—while simultaneously offering a cure. The company’s sports drinks were originally billed as thirst-quenchers, designed to stave off performance dips. But by the 1980s, Decoder Ring reported, the Gatorade Sports Science Institute was churning out data that supported the benefits of drinking before the mouth got parched. A decade later, the American College of Sports Medicine was recommending that athletes consume “the maximal amount” of water they could stand to keep down.
[Read: The organic Gatorade illusion]
Around the same time, during the fitness craze of the ’70s and ’80s, water was acquiring another identity: the enlightened socialite’s clean drink of choice. When European companies such as Perrier and Evian brought their bottled water to North America, they found a market among those wanting a high-end, calorie- and sweetener-free alternative to sodas, alcohol, and juice. Water “had this healthy, good-for-you halo,” says Michael Bellas, the chair and CEO of the Beverage Marketing Corporation. “There were no negatives.” In 2016, water became the U.S.’s leading bottled beverage, a title it has maintained since.
As water’s market share grew, so did its mythos. Companies hocked the illusion that their products could make people not just healthier but “sexier and more popular,” Peter Gleick, the author of The Three Ages of Water, told me. Hydration was so clearly vital to life that truth-adjacent ideas about its benefits, many of them pushed by prominent people, were easy to buy. Even concerns over single-use plastic bottles could not slow water’s roll: In response, the world cooked up eco-friendly Yetis, HydroFlasks, and Nalgenes, and made those trendy, too.
It’s not that water isn’t healthy. There’s just no evidence to show that guzzling tons of water can fix all our ailments. For people prone to kidney stones and UTIs, drinking more has been shown to cut down on risks; as a swap for sugary beverages, it can also help with weight loss. But for a variety of other issues—such as heart disease, metabolic issues, and cancer—the data is often “really mixed,” Hew-Butler told me. Although researchers have sometimes found evidence that dehydration may raise certain conditions’ risks, that doesn’t automatically imply the inverse—that extra water intake then lowers risk from a typical baseline. At very rare extremes, overdoing it on water can kill us, too.
The connections between hydration and health are shaky enough that health authorities have been reluctant to push a strict recommended daily allowance, like the ones that exist for various vitamins. Instead, the National Academy of Medicine proposes a tentative “adequate intake”: 3.7 liters of total water intake for men, and 2.7 for women (both including hydration from food). Recently, Abigail Colburn, a physiology researcher at Yale, and her colleagues ran an analysis that concluded those figures were sound. Still, the numbers came from population surveys, published in the early aughts, of the amounts that Americans were already drinking—a reflection of how things were, but not necessarily how they should be. And they represent medians within a huge range. Over the years, multiple studies have documented people living, by all appearances healthfully, on daily water budgets that span less than a liter to four, five, or six—sometimes more.
If researchers don’t agree on how much water is good, they also differ on how little water is bad: the point at which dehydration starts to become a problem—or how long people can linger at that threshold without raising long-term health risks.
A bit of water loss should be completely fine. Fluid status is, by design, “a constantly changing state,” Colburn told me. When the body doesn’t take in enough water to recoup the liquid it’s lost—as it naturally does throughout the day, via sweat, urine, and breath—the brain releases a hormone called vasopressin that prompts the kidneys to hold onto fluid. The urine gets darker and less voluminous; eventually, blood-salt levels rise, and the mouth and throat ache with thirst. The goal is to get the body to excrete less water out and take more in so we don’t wring our vital tissues dry. Life forms have evolved to tread carefully down this cascade of steps, and the flexibility is built in—much like a rubber band that snaps back after being stretched and released.
[Read: Why ‘drink more water’?]
But some researchers have started to worry about repeatedly asking the body to compensate for less than optimal hydration—stretching the band over and over again. The issue isn’t chronic dehydration, Colburn told me, but a subtler precursor state called underhydration, which occurs after a lack of water intake has prompted the body to conserve but before the appearance of signals such as thirst. It’s not clear how worrying teetering on that precipice is. In the same way a rubber band is “designed to stretch,” our fluid balance is built to bounce back, says Evan Johnson, a hydration expert at the University of Wyoming. Over time, though, wear and tear could add up, and resilience could drop.
Tracking those outcomes gets even more complicated when researchers try to quantify how dehydrated individual people are—another thing that experts can’t agree on. “We really don’t have a gold standard for measuring the all-encompassing term of hydration,” Johnson told me, especially one that’s both simple and cheap, and can account for body water’s constant flux. Which leaves scientists with imperfect proxies. Broadly speaking, there’s a urine camp and a blood camp, Stookey told me. Those in the pee camp tend to be hydration conservatives. A change in urine color or volume, they argue, is an early sign—well in advance of thirst—of impending dehydration. The blood-camp crew is more laissez-faire. Diet, medications, and supplements can all alter the shade of urine, making it a fickle clue; Hew-Butler for instance, defines true dehydration as what happens when the plasma’s gotten saltier than usual, to the point where cells have started to shrink—a sign that retaining water is no longer sufficient, and that the body needs to drink.
Which camp researchers fall into influences how bad they think America’s hydration problem is. “When you draw blood, most people are within a normal range if they’re not thirsty,” Hew-Butler told me. But Stookey, who’s firmly in the pee camp, contends that a majority of Americans are “walking around dehydrated” and should be drinking far more. Colburn, too, would rather err on the side of heeding urine’s warning signs. By the time thirst kicks in, “you’re already in a dangerous zone,” she told me.
There can be a middle ground. Sekiguchi, of Texas Tech, told me that for most young, healthy people who are spending plenty of time in the air-conditioned indoors—as so many Americans do—it’s probably fine to just drink when thirsty. (That advice works less well for older people, because the sensation of thirst tends to dull with age.) When specific circumstances shift—a stint of heavy exercise, a week of toasty days—people can take notice, and adjust accordingly.
But guidelines for typical water intake, under typical conditions, are quickly going out the window as heat waves get more frequent and intense. When temperatures skyrocket and humidity makes otherwise-cooling sweat stick stubbornly on skin, our bodies need more water to keep cool and functional, beyond what thirst alone might dictate. Part of the problem is that thirst vanishes more quickly than the body rehydrates, Sekiguchi told me, which means that people who drink until they think they’re sated tend to replace only a fraction of the fluids that they’ve lost.
“We’re never going to be able to tell people an exact number,” Colburn told me, for how much to drink. But in reality, many of the healthy people most worried about fine-tuning their hydration to a perfect level are probably among those that least need to fret. The dangers of water tend to happen not in those middle grounds, but at its extremes—especially when failing infrastructure hampers access to water, or contamination makes it undrinkable. Many of the populations that are most vulnerable to dehydration’s effects also happen to be the same groups that probably aren’t getting enough to drink, Johnson told me. While bottled-water markets boom, plenty of pockets of the U.S. still lack consistent access to safe, reliable water from the tap. And the situation is even worse in many places abroad. Perhaps nothing reminds us of water’s power like dramatic deficit: Water, simply, is what keeps us alive.