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Are You Doing Thirst Right? The Science Says Probably Not

A look inside the great hydration debate.


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In black-and-white video reels captured during the 1912 Olympic marathon, runners in cotton kits and two-toed canvas shoes race through the streets of Stockholm, Sweden. Spectators, wearing Edwardian waistcoats and ruffled chiffon dresses, politely clap; once in a while, an enthusiastic fan gives a small-but-noticeable fist pump. It’s so restrained, so civilized—until the athletes hit the aid station. One by one, the runners take a cup of water, sip, and spit with such disgust you would think the beverage had been laced with vinegar. It looks like the runners are being poisoned and, in a way, they believe they are—by water.

Though the 1912 Olympians exhibited all the hallmarks of dehydration during the hot summer race (the temperature the day of the marathon was around 90°F), none would actually drink the cups of water. Instead, they simply sipped, swished, and spat to relieve dry mouth. Swallowing was off-limits, as it was believed a full stomach would slosh around during a run, creating nausea, making breathing difficult, and sandbagging mile splits.

For decades, endurance sports subscribed to the idea that to endure was to suffer. The more misery you could withstand in training, the better you’d perform on race day. Workouts, then, were focused on running as many miles as you could, as hard as you could, as often as you could, in the most miserable conditions you could find. Discomfort was ignored, fueling was not used in any context, and water was for the weak.

“Don’t get into the habit of drinking and eating in a marathon race,” advised the American Amateur Athletic Union in 1909. “It is not beneficial.”

King Water

That sentiment eventually changed and today water is king. Aid stations offer not only water, but a liquid buffet of sports drinks, flat cola, ginger ale, chicken broth, and sometimes even beer. Endurance athletes are encouraged to drink up before, during, and after workouts and races. We call it “hydration,” and we swallow every last precious drop.

But most of us still aren’t doing it right.

The details of hydration—what to drink, how much to drink, and when to drink it–remain bafflingly unclear. Some experts (both legitimate and self-proclaimed) advise drinking only when the tell-tale signs of thirst—dry mouth, dry skin, fatigue, headache—make themselves known. Others say if those signs appear, it’s already too late, as dehydration has already set in; athletes should drink proactively to stay ahead of ever feeling thirsty. Some running and triathlon coaches even carry forward the long-disproved ideas of 1912, encouraging athletes to train dehydrated and skip aid stations to shave time and avoid gastrointestinal distress. How did we get here?

In the 1950s, people noticed something: certain runners who bucked the swish-and-spit trend during races seemed to perform better than their competition. Suddenly, fluids were ergogenic aids, performance-enhancing and suspect. Well into the 1970s, official rules for marathons banned water consumption prior to the 10K mark to prevent water doping and to ensure fair competition. It wasn’t until the 1980s, when exercise physiologists conducted rigorous testing on athletes, that a clear picture emerged. Water wasn’t a performance-enhancing drug. Most runners were just seriously dehydrated and in dire need of fluids.

Even today, we haven’t quite corrected the course. Americans of all ages are still chronically dehydrated. After air, water is the most immediate need in the human body, yet we regularly fail to meet that need. We’ve gotten worse at recognizing the signs of dehydration for what they are: When we have a headache, we’re inclined to reach for a painkiller, not a glass of water. We lubricate dry skin with lotion after a post-workout shower. We lament how tired we are during a workout or race, not realizing how long it’s been since we last reached for a water bottle.

Even athletes who drink with intention can fall short. Years of conflicting information have led most to take a best-guess approach to how much they should drink. The results are hit-or-miss. In a study of half- and full-marathon runners, 70% experienced one or more incidents in which they believed dehydration resulted in a major performance decline and 45% perceived dehydration to have resulted in adverse health effects. These athletes aren’t necessarily wrong in their thinking. Science backs up the notion that hydration and performance are intertwined. What we drink matters—a lot. Though drinking water during a race is no longer thought of as cheating, hydration is, in fact, performance enhancing. You just have to know how to use it to your advantage.

An athlete tosses a cup of water Ironman Maryland in 2018. Photo: Patrick Smith/Getty Images for Ironman

A Thirst Trap

About half of the human body is made of water. The exact amount fluctuates from person to person, based on factors like biological sex, age, and body composition, but an abundance of water is universally needed for every body to function. Just about every cell in the body relies on water in some way; studies show hydration is critical for regulating body temperature, keeping joints lubricated, preventing infections, delivering nutrients, and keeping organs functioning properly. Being well-hydrated also improves sleep quality, cognition, and mood. When training and racing, hydration is especially critical to reduce fluids lost through sweat, keep the heart rate at a sustainable level, maintain plasma volume, and reduce heat stress and heat exhaustion.

Because we have so much water within the body, many people assume a dip in hydration levels isn’t a big deal. We treat water reserves like gasoline in a car, where things hum along just fine so long as we top off the tank before it empties out. In reality, though, bodily function declines with even minor dehydration. A constant supply is needed to replenish fluids lost through respiration, urination, and sweating. Endurance sports, which involve increased respiration and sweating, require increased water uptake to maintain balance.

This is why we get thirsty. Unlike the gas tank of the car, where an alert chimes only when the tank is close to empty, our brain is constantly reminding us to keep the tank topped off. It used to be assumed that thirst was a fairly straightforward signal—if you’re thirsty, drink, and the problem will be solved. Only recently have scientists learned that thirst is much more complicated than that.

There are so many times you drink during the day even when you’re not necessarily thirsty.

“Thirst is essentially the very primordial emotion of needing to drink water,” said Dr. Claire Gizowski, who studies the mechanisms of thirst at the Knight Laboratory at the University of California in San Francisco. “For most people, you think of thirst as this actual conscious decision—‘Oh, I’m thirsty, I could go for a drink’—but there are so many times you drink during the day even when you’re not necessarily thirsty. But if I asked you why you were drinking, you wouldn’t know.”

This is what Gizowski studies: the “why” of our unconscious desires to drink. By looking at the tiny neural pathways of the brain, which transmit information between brain and body to maintain optimal functioning, Gizowski has found that thirst is more than just a response to a lack of water in the body; it’s also an anticipatory process. The body is constantly calibrating itself, gauging not only what it needs at the moment, but what it will need in minutes or hours. In her research studying mice, Gizowski discovered that the body’s circadian rhythms (which ensures we feel alert during daylight hours and sleepy at night) drives water intake prior to sleep. Even when we aren’t dehydrated, we become thirsty in the hours before bedtime because our body anticipates we’re about to go several hours without water.

Red and blue rendering of neural pathways
Neural activity in hydration
Photo: Claire Gizowski

“Before your body even knows that you’re thirsty, your brain sends out these signals, and before you know it, you’re drinking water,” explains Gizowski. “It’s not just about when you have a deficit, it’s also about anticipating that deficit. Your brain is basically saying we’re going to do everything we can to avoid a deficit or not create a larger deficit.”

In addition to anticipating the dry hours of our sleep, the brain’s specialized cells also respond to stimuli that can eventually lead to dehydration. Studies show that being exposed to heat triggers unconscious thirst pathways in the brain almost immediately, well before there are actual changes to core body temperature or hydration levels or obvious external signs of thirst, like dry mouth. Basically, the body knows it’s hot and that it’s about to start sweating; it needs water now, early on, to prevent and mitigate any impact of the heat. The same is true with foods containing high levels of salt or caffeine. When external factors threaten the body’s delicate fluid balance, we’re driven to drink.

It’s a tightly-regulated system and incredibly well-optimized, right down to the type of beverage we need. To maintain homeostasis in the body, water works in tandem with electrolytes (minerals that conduct electricity: sodium, chloride, potassium, magnesium, and calcium) to control fluid balance, regulate blood pressure, help muscles contract, and maintain the correct acidity of the blood. Recent research has found that our brain cells are so specialized they can send signals calling for different types of fluid to maintain this water-electrolyte balance, resulting in a craving for pure water or a beverage that contains salt and minerals (like a sports drink).

Despite the intricate knowledge of the most minute structures of the brain (the structures that Gizowski studies are approximately 300 micron large, or one-third of a millimeter), there’s still a lot we don’t know about thirst signals. This is particularly true when considering the needs of endurance athletes, who engage in activities that drain water from the body—quite literally, in the form of sweat—and frequently override signs of discomfort, including thirst. When you ignore the body’s signals long enough, you eventually learn to accept thirst and dehydration as a normal state of being. In the long run, that could mean the difference between a PR and DNF.

Stavros Kavouras in his hydration lab at ASU. Photo: Andy DeLisle

More Equals Better

The more you exercise, the hotter you get. The hotter you get, the more you sweat. The more you sweat, the more water you lose. And the more water you lose, the harder it becomes to sustain any sort of physical effort.

“Even a small degree of dehydration has a negative impact on exercise performance,” said Dr. Stavros Kavouras, lead researcher at the Hydration Science Lab at Arizona State University. “One percent of dehydration—a super, super, super small deficit—makes us slower.”

Until recently, it was hard to draw a direct causal link between dehydration and exercise performance. Most studies would tell athletes to come into the lab for two cycling or running time trials on two separate days; before the first, they were instructed to come in dehydrated, and then for the second they were told to drink optimal amounts. These studies indeed found that dehydration made study subjects slower, but critics suggested the effect was psychological, not physical. When athletes know they’re participating in a study about how dehydration impacts performance, they’re primed to do their worst. This self-fulfilling prophecy is well-documented in studies of self-talk: If you think you’re going to suck, you probably will.

Everything—everything—gets sacrificed when you’re dehydrated.

“Long story short, when you expect to do your worst, you feel worse, and you do worse,” said Kavouras. “People said these studies were biased, and we took that very seriously.” The gold standard of science is what’s known as a blind test, where the subject does not know when or even if they are exposed to the stimulus. But how do you hydrate (or dehydrate) a person without their knowing? The act of drinking from a bottle or cup is an obvious one, and delivering hydration through an IV line can create a sensation under the skin, which tips off the recipient of the fluids. For a truly blind test, Kavouras had to get creative.

“We took tubes and put them from the subject’s nose down to their stomachs,” Kavouras said of his ultimate study design. “We connected those tubes to IV pumps, where we could put the fluid directly into their stomach. And to prevent them from feeling anything, we warmed up the water, so they couldn’t feel any temperature effect.”

These subjects were then instructed to exercise at moderate intensity for two hours, then do a 5K time trial, all while wearing the nasogastric tube. All subjects sipped a very small amount of water every five minutes from a bottle, and some also received additional hydration through their tube. Kavouras hypothesized those who received extra fluids would perform better during intense exercise efforts. And, ultimately, he was right. The difference between the hydrated and dehydrated groups was stark. The cyclists who were well-hydrated completed the 5K time trial an average of one minute faster and produced a higher power output than those who were dehydrated.

“When you look at exercise performance, when you look at the body’s ability to dissipate heat, your body temperature, blood flow, oxygen supply to the muscles, everything—everything—gets sacrificed when you’re dehydrated,” Kavouras said. “We are confident our findings are real. If you drink a small amount of water during exercise, your performance improves. If you drink more water, performance improves more. And if you drink even more than that, it improves even farther.”

But Too Much Equals Bad

But is it possible to have too much of a good thing? In 2012, Tim Noakes made waves in the endurance sports world with his book Waterlogged, which claimed athletes were overhydrated to great detriment. His message was that drinking plain water to thirst, instead of drinking to a predetermined hydration plan or consuming additional electrolytes in sports drinks, was the best approach for endurance athletes. He also asserted that much of our belief about being wildly dehydrated was created by the sports drink industry. The advice is lauded for its simplicity—after all, endurance sports are complicated enough. However, thirst is not so straightforward, especially in the context of hours upon hours of fluid loss during a marathon or Ironman.

“‘Drink when you’re thirsty’ is quite simplistic. But it doesn’t work like that,” said Kavouras. “It’s like if you’re a runner, and you say, ‘You know, we’ll make running simple, every runner wears one size shoe. Size 10. Man, woman, tall, short, big-footed, small-footed, everyone gets size 10.’ That’s it. If we did that, it would be simple, but it wouldn’t be effective.”

Just like each athlete has a unique foot size and shape, so too does an athlete have a unique sweat rate. Some sweat gallons over the course of a workout or race, while others lose only a few liters of fluid. The makeup of one athlete’s sweat might be nearly pure water, while another’s contains high amounts of essential electrolytes—in endurance circles, these people are colloquially known as “salty sweaters,” who can usually be identified with powdery white streaks on their clothing or skin during workouts. Some feel thirsty no matter how much they drink, while others never really notice a strong thirst signal. Almost all of us are bad at guessing what we really need.

It’s very common with endurance athletes that if you tell them having one of anything is good for you, they assume ten must be even better, and a hundred would be fantastic.

“I have never met an athlete that intuitively knew how much they sweat and therefore how much they need to replace. They do not know how much fluid they lose, and some have no idea they lose fluids at all, especially if they live in a dry heat where sweat evaporates quickly,” said Kavouras.

There’s also the need to maintain electrolyte balance, which water alone cannot do. Too much water consumption without adequate electrolytes can sometimes lead to a medical condition called hyponatremia. When athletes ingest significantly more fluid than they lose in sweat, they can become over-hydrated, which dilutes sodium levels in the body. They’ll then experience headaches, nausea, impaired balance, and, in extreme cases, seizures or death. And, yes, a lot of those symptoms are the same as dehydration—so how do athletes know which is leading to their problems?

“It’s very common with endurance athletes that if you tell them having one of anything is good for you, they assume ten must be even better, and a hundred would be fantastic,” said Kavouras. “But a triathlon is not a drinking race. You’re not racing on how much water you drink, you’re racing to prevent significant dehydration. Studies on marathon runners show that the people who develop hyponatremia gain an average of seven to twelve pounds in a marathon. They are drinking twelve pounds over what they lose through pee or sweat during the race. You really have to try very, very, very hard to drink this much.”

Trying to find the sweet spot in between dehydration and hyponatremia can be complicated, especially since physiology varies widely from one person to the next. Fortunately, new developments in hydration science are making it easier to unlock the perfect fluid formula for every athlete.

The GX sweat patch. Photo: Gatorade Sports Science Institute

The Future of Thirst

One day in the not-so-distant future, you’ll wake up and get a smart notification that you need to drink X amount of fluid. Your smart refrigerator might even dispense that exact amount of fluid, calibrated to the temperature you prefer, into a smart bottle, which will track how much you consume with every sip. Throughout the day, you’ll be prompted to drink at various intervals to maintain the perfect level of hydration in your body. Knowing what you need during a long run workout or at an Ironman aid station may very well be as easy as looking down at the personalized advice on your watch.

We’re not quite there yet, but technology is evolving to give us more thirst clues. Researchers like Dr. Lindsay Baker at the Gatorade Sports Science Institute (GSSI) have spent years obsessing over sweat—specifically, the wide variations in the way we sweat. Her expertise has been honed over years of sweat testing thousands of athletes in every sport, from volleyball teams to ultramarathon runners. What she’s found is that sweat is not as predictable as you might think. Some people lose sweat at a rate of 0.2 liters per hour, while others lose up to 2.5 liters per hour. She’s seen large football players under heavy gear with a lower sweat rate than Ironman triathletes running around in spandex, and runners who can sweat out gallons of thin, clear fluid while running alongside other athletes who are caked in salt residue.

“It’s a huge variability,” said Baker. “It can be difficult to know how much a person needs to drink and what they need to drink unless they actually measure it.”

Like many hydration experts, Baker recommends athletes use an individualized hydration approach, which is tailored to each athlete’s individual fluid and sodium loss. By knowing exactly what they’re sweating out, they can go about replacing it in an informed, methodical way. Multiple studies have found that individualized hydration plans improve performance in athletes in all sports, and multiple professional organizations (including the American College of Sports Medicine) recommend it, yet most athletes don’t employ this strategy. The formula for calculating sweat rate is not complicated and a simple test can be done at home with some ingenuity, math, and a scale, but it can be intimidating. That’s why GSSI has such a large database of athlete sweat rates. Their scientists are often called to perform sweat testing for teams and athletes who don’t feel confident carrying it out themselves.

“It’s a slow, labor-intensive process,” said Baker. “We send a team of our scientists out, put gauze patches on the athletes to collect their sweat, weigh them before and after exercise, keep track of all their fluid intake so that we can do the calculations to figure out their sweat rate. We squeeze out their gauze and analyze the fluid to determine their sweat and electrolyte concentration. It takes a lot, and for the average person, it’s not feasible to do that, so it’s unlikely it will actually happen.”

For years, Baker’s research team fantasized about what it would be like to bring the testing within the reach of the everyday athlete. The opportunity to make this a reality presented itself in 2017, when Northwestern University bioengineering group Epicore Systems approached GSSI with new technology for second-skin wearables capable of measuring sweat biomarkers in real time. The low-cost patches were capable of analyzing small droplets of sweat directly from the skin to measure fluid loss during a workout, hourly sweat rate, and the amount of sodium lost through sweat. After identifying the ideal location for accurate sweat collection (the inner arm won out over ten other locations on the body), Baker and her team confirmed the patch data matched the data gathered in traditional sweat testing. Confident in the validity of the patch, GSSI developed a mass-market product, Gx Sweat Patch, and a coordinating app that helps users interpret their data. The recommendations then account for physiology, effort, and conditions. In theory, an athlete can determine their specific sweat rate for hard workouts or easy runs, for hot or cold or windy or humid environments, and they can then calibrate their hydration plan to replace what they’ll lose in a workout or race. It might not be perfect, but it’s another step in the hydration evolution.

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The Best You Can Be

GSSI is not the only company working on technology to help users fine-tune optimal hydration practices. Multiple companies, including General Electric and Biometrica, are working on wearable sweat monitoring technology and devices to analyze hydration through at-home urine testing. At first blush, some athletes may wonder if it’s overkill to bring so much technology to something as simple as drinking water, but the same was once said about wearing step trackers and 24/7 heart rate monitors, which are now standard features on just about every smartwatch.

“I think we’re going to see this soon,” said Kavouras. “There will be accurate real-time tracking to tell us when we need to increase our water intake and really assess hydration, not just during exercise but all day long.” Kavouras hopes this technology will simplify hydration in a way that allows athletes to get exactly what they need to perform in workouts and races.

In the meantime, a sweat test—whether using old-school methods or new technology—can be a useful tool for athletes who are keen to dial in their specific hydration strategy. When athletes know how much fluid they lose per hour, they can develop a plan to replace those fluids in real time. Individualized data can also help athletes choose fluids or supplements with a similar electrolyte makeup to what is lost in their sweat. Striking the right balance can be a true performance boost: One study found that athletes who added a salt supplement to their usual hydration routines during a half-Iron distance triathlon completed the race an average of 26 minutes faster than those who only used sports drinks. But that doesn’t mean salt pills are free speed for all, for others too much salt can cause GI distress and cramping. A single product or one-size-fits-all approach to hydration is never going to work for all athletes in all scenarios. But it does underscore what the larger body of sweat testing reveals: what athletes think they need is often quite different from what they actually need. Most of us could stand to tweak our hydration plan, whether it’s drinking more water or adding extra electrolytes to a bottle.

“There’s so much conflicting information out there. In the time of social media, every person becomes an expert. Everybody that drinks fluids is an expert on hydration, so that makes every single human being on this planet,” said Kavouras. “People think it’s too scientific or technical to figure out an individual hydration plan, so they go with the one that sounds the best, which is usually the easiest one. But I don’t think it’s any more technical to figure out what to drink during exercise than it is to figure out what your best running shoe is or to get a bike fit to your body. It’s all part of the process of being the very best athlete you can be.”