Christie Aschwanden’s new book, “Good to Go: What the Athlete in All of Us Can Learn from the Strange Science of Recovery,” is available this week. In it, she examines the latest recovery trends among athletes — including Tom Brady’s infrared pajamas, Sue Bird’s coffee naps and Michael Phelps’s “cupping” ritual. She also tests some of the most controversial methods herself, including cryochambers, float tanks and infrared saunas. Below, we’re publishing an excerpt of the book’s chapter on what science really tells us about what we should drink after we work out.

In the early 1990s, Gatorade ran a television commercial featuring Michael Jordan called “Be Like Mike.” It featured slam dunks by Jordan interspersed with footage of kids shooting hoops and, of course, Jordan and other happy people drinking Gatorade.

Stuart Phillips remembers that ad campaign well. As an aspiring athlete, he, too, wanted to be like Mike. “Michael Jordan drank Gatorade, so I drank Gatorade,” Phillips says. Despite guzzling the sports drink, Phillips never did make it to the pros, but instead grew up to become the director of the Centre for Nutrition, Exercise, and Health Research at McMaster University in Hamilton, Ontario. The Jordan ad taught him a lesson about the power of marketing, though: “If you can get an endorsement from an athlete that everybody recognizes, then who needs science?”

Scientific facts don’t sell products; stories do. Jordan was already a basketball superstar by the time Gatorade came calling, and the public was eager to experience something of his greatness. Enter Gatorade — Michael Jordan drank it, and young Stuart Phillips could too. To drink Gatorade wasn’t just to mimic a sports hero, it was to imagine a causal relationship — Jordan drank Gatorade and then made all those slam dunks, so the one must have had something to do with the other.

Psychologists call such thinking the “illusion of causality,” and it’s so powerful that it has spawned an entire genre of advertising — the celebrity endorsement. No one would care that a pro athlete uses a particular product if it didn’t somehow appear that the item played some role in that star’s success. The Irish have a saying, “An umbrella accompanies the rain but rarely causes it.” The same could be said of product endorsements and athletic greatness. Still, our minds are quick to connect the dots in the wrong direction.

The age of the athlete-endorsed sports drink began on a Florida football field in the mid-1960s. Back then, most coaches and athletes didn’t give much thought to fluid replacement during practice or competition. In some instances, athletes were even counseled to avoid drinking close to a workout lest they upset their stomach. But in 1965, a University of Florida football coach came to Dr. Robert Cade and his team of university doctors complaining that his players were “wilting” in the heat. (He also wondered why his players never urinated during games.) After some investigation, Cade and his colleagues concluded that two factors were causing the players to fall victim to the heat — they weren’t replenishing the fluids and salts they were sweating out, nor were they restoring the carbohydrates their bodies were burning for fuel.

In a stroke of genius, Gatorade turned the drink’s sodium, phosphorus and potassium into “electrolytes,” which is simply the scientific term for molecules that produce ions when dissolved in water.

Cade figured that he could solve the problem by helping players replace those lost resources, so he stirred together some sodium, sugar and monopotassium phosphate with water to create a drink soon dubbed Gatorade, after the University of Florida’s nickname: the Gators. Legend has it, the drink turned the struggling Gators football team around. It finished the season with a winning record, and in 1967, the team won the Orange Bowl for the first time in school history. Other teams took notice of the newfangled beverage, and in 1967, Cade and the University of Florida signed an agreement with canned goods company Stokely-Van Camp to produce Gatorade commercially. Orders for the drink poured in.

What followed was a national campaign to sell the public on the idea that exercise caused dehydration, the cure was Gatorade’s specially developed drink, and this tonic was critical for sports performance — it was created by a doctor and tested in studies, after all. One of the brand’s early print advertisements boasted that Gatorade was absorbed 12 times faster than water (a claim walked back in 1970, after Ohio State team doctor Robert J. Murphy challenged it at a meeting of the American Medical Association).

In a stroke of genius, Gatorade turned the drink’s sodium, phosphorus and potassium into a special selling point by rebranding these ordinary salts with their scientific name — “electrolytes,” which is simply the scientific term for molecules that produce ions when dissolved in water. Your body maintains some reserves of these vital ions that it can tap into as needed to keep your body’s fluid and salt balance in check. We do lose electrolytes through sweat, but even when you exercise continuously for many hours, you will simply correct any losses via your normal appetite and hunger mechanisms. (You’ve already experienced this if you’ve ever had a hankering for a salty snack.) One small study of cyclists and triathletes found that it didn’t really matter whether they drank plain water, a sports drink or a milk-based beverage after an hour of hard exercise. As long as they drank some liquids along with a meal, they restored their fluid levels just fine.

Gatorade may not have been the first to use this term, but they’re the ones that landed electrolytes in the public lexicon. In 1985, the Gatorade Sports Science Institute was founded to promote the study of hydration and nutrition for athletes, research that also happened to make for great marketing. Conveniently, the studies that came from the GSSI could be used to support the product’s claims. A 1990 magazine ad read: “We test Gatorade in laboratories. We test it at major universities, with sports science experts, on sophisticated scientific equipment with names that are longer than this sentence. What does it prove? Gatorade works.”

Early advertisements presented thirst as the problem that Gatorade was designed to solve, but as the GSSI’s research program progressed, the emphasis moved to a more clinical concept of hydration and the notion that thirst was not a good indicator of whether an exerciser was drinking enough. “Unfortunately, there is no clear physiological signal that dehydration is occurring, and most athletes are oblivious to the subtle effects of dehydration (thirst, growing fatigue, irritability, inability to mentally focus, hyperthermia),” wrote GSSI co-founder Bob Murray in one report. Instead, athletes were advised to drink according to scientific formulas. A Gatorade ad that ran in Northwest Runner in 2001 depicted the glistening torso of a runner with the race number 40 pinned to her shorts and the words, “Research shows your body needs at least 40 oz. of fluid every hour or your performance could suffer.” That’s the equivalent of five 8-ounce glasses of liquid, which means that a runner finishing a marathon in a fast three hours would need to drink 15 glasses of fluid along the way. Gulp.

Gatorade wasn’t alone in promoting the benefits of drinking before, during and after exercise. Other sports drink manufacturers, such as the drug company GlaxoSmithKline (Lucozade Sport), also pointed to science when marketing its products. Lucozade, for example, established a “sports science academy” to promote its drink. Together, these campaigns fostered the idea that exercise depletes your fluids and electrolytes (which, remember, is just a fancy name for salts) and that special measures are required to make things right again.

It was no longer sufficient to simply drink some water and eat a meal after exercising. The idea these marketing campaigns fostered was that physical activity created extraordinary nutritional needs and that these specially formulated beverages were the best way to meet them. This was science speaking.

The Limited Science Behind Hydration Advice

Sports doctors were also urging athletes to drink. The American College of Sports Medicine (ACSM), a professional organization of sports science experts (which receives financial support from Gatorade), put out a consensus statement in 1996 recommending that “during exercise, athletes should start drinking early and at regular intervals in an attempt to consume fluids at a rate sufficient to replace all the water lost through sweating (i.e., body weight loss), or consume the maximal amount that can be tolerated.” The message coming from experts was that athletes needed to replace the fluids they lost during exercise lest their performance and health suffer.

In the wake of all this promotion, sports drinks have become a multimillion-dollar business. But when a team of medical researchers trained in the evaluation of scientific findings had a look at the research underpinning the boom in sports drinks, they reached a startling conclusion. “As it turns out, if you apply evidence-based methods, 40 years of sports drinks research does not seemingly add up to much,” Carl Heneghan and his colleagues at the University of Oxford’s Centre for Evidence-Based Medicine wrote in a 2012 analysis published in the British medical journal BMJ. When Heneghan’s team gathered and examined all of the available evidence on sports drinks (it even consulted sports drink manufacturers to ask them for their supporting studies, though not all complied), they found what amounted to a bunch of preliminary or inconclusive evidence packaged as more definitive proof.

The first, almost universal, problem among these studies was that they were too small to produce meaningful results. “Small studies are known to be systematically biased toward the effectiveness of the interventions they are testing,” Heneghan and his colleagues wrote. Out of the 106 studies they analyzed, only one had more than 100 subjects, and the second-largest study used only 53 people. The median sample size? Nine.

“Worryingly, most performance tests used to assess sports drinks have never been validated.”

Another common shortcoming was that the studies were often designed in a way that almost assured that they’d find a benefit from sports drinks. Deborah Cohen, an investigations editor at the BMJ who was involved in the project and wrote a summary of the findings, recalls a study in which volunteers who fasted overnight were divided into two groups, one whose members were given a sports drink containing water, salts and sugar and another whose members received water. “People who were given the sports drink fared better,” she says. “Well, no shit.” If you haven’t had any food in 12 hours and then you get a bit of sugar, of course you’ll perform better than the people still running on empty. But to say that this means the sports drink is superior to whatever a normal person would consume leading up to or during exercise just isn’t generalizable, she says. “Who starves themselves overnight and then goes to perform some exercise?” And yet the BMJ investigation found that this type of study design is surprisingly common among tests of nutritional products.

Some of the dazzling powers that sports drinks display in the studies touted by their makers may be nothing more than the placebo effect. When people volunteer for a study to test a new sports drink, they come to it with an expectation that the product will have some performance benefit. Studies use a placebo group to factor out such effects, but a placebo only controls for these expectations when it’s indistinguishable from the real deal. So it’s telling, Cohen says, that studies using plain water for the control group found that the sports drink had positive effects, while the ones that used taste-matched placebos didn’t.

The BMJ analysis also concluded that many of the measures made in these studies may not matter for real-world performance. “Worryingly, most performance tests used to assess sports drinks have never been validated,” Heneghan and his colleagues write, and some of them are known to produce highly variable results that may not be reproducible.

Heneghan and his team concluded that claims about sports drinks rely on small studies with comparison groups that favor the products being studied, a lack of rigorous blinding so that participants were likely nudged to perform better while taking in the sports drinks, and measurements of effectiveness that might not be meaningful in real life. Add to that statistical sleights of hand that inflate the benefits of the drinks (for instance, one study increased the benefit of carbohydrate drinks from 3 percent to 33 percent by excluding a segment of the test from the analysis), and sports drinks don’t come out looking so impressive.

When Heneghan’s and Cohen’s reports came out, some sports science experts blasted it as unnecessarily rigid, because they set their standards based on the conventions of clinical medicine rather than sports science, where, for instance, small sample sizes are common. Which standards and methods should be used for assessing evidence is an important debate that is gaining attention within the sports science community. In the meantime, the emphasis on hydration has created another problem to address.

Hydrate Till You Drown

Exercise scientist and physician Tim Noakes was a believer in the dangers of dehydration until two separate experiences left him questioning what he thought he knew.

First, Noakes was involved in a study examining participants in a four-day canoe race. During a particularly rough day, one of the paddlers lost all of his drinking water when it washed overboard as he went through some breakers. Despite having canoed about 50 kilometers without drinking, the paddler’s body temperature hadn’t become elevated, as the dehydration theory would have predicted. “We weighed him, and he’d lost about eight or nine pounds, but his body temperature was normal and I thought, oh my gosh — body weight loss has nothing to do with body temperature,” Noakes says. This was a lightbulb moment, because conventional wisdom held that one of the reasons that dehydration was (supposedly) so dangerous was that it put people at risk for heatstroke, and this finding contradicted that assumption.

The canoe study prompted Noakes to reconsider the idea that maintaining full hydration was essential to staving off heatstroke. Then, in 1981, a runner wrote to Noakes describing a strange experience she’d had at that year’s Comrades Marathon — a famous 90-kilometer ultramarathon in South Africa. It was the first time that the event had provided drink stations every mile of the 56-mile course, he says, and the runner wrote to say that she’d begun feeling really strange about three-quarters of the way through the race. Her husband pulled her off the course and delivered her to the medics. The first responders assumed she was dehydrated and gave her two liters of intravenous fluid, after which she lost consciousness. She had a seizure on the way to the emergency room.

At the hospital, doctors discovered that her blood sodium concentration was dangerously low. The ultimate diagnosis was a medical condition called “water intoxication” or hyponatremia — too little sodium in the blood. Contrary to what the medical crew at the race had assumed, the runner wasn’t dehydrated— she was overhydrated. She’d drunk so much fluid that her blood sodium had become dangerously diluted. Low blood sodium causes cells in the body to swell, and when it happens in the brain, the results can be deadly.

Noakes has built a reputation as a loud contrarian on a variety of issues. He is perhaps most famous for his theories about exercise fatigue and has made a career out of pushing against conventional scientific wisdom, some say to his own detriment. So it’s not surprising that he was one of the first and loudest voices on overhydration (the guy wrote a whole book about it).

Yet Noakes is far from alone in worrying that the rush to prevent dehydration may have put exercisers at risk of the far more serious condition of water intoxication. In 1986, a research group published a paper in the Journal of the American Medical Association describing the experience of a medical student and a physician who’d become stuporous and disoriented during an ultramarathon. The men were diagnosed with hyponatremia, and they concluded that they’d developed the condition by drinking too much.

There’s never been a case of a runner dying of dehydration on a marathon course, but since 1993, at least five marathoners have died from hyponatremia they developed during a race. At the 2002 Boston Marathon, researchers from Harvard Medical School took blood samples from 488 marathoners after the finish. The samples showed that 13 percent of the runners had diagnosable hyponatremia, and three had critical cases of the condition. German researchers similarly took blood samples from more than a thousand finishers of the Ironman European Championship over multiple years and found that 10.6 percent of them had hyponatremia. Most of the instances were mild, but nearly 2 percent of the finishers had severe or critical cases. Although the findings indicate that hyponatremia is still a rare condition, what makes them especially concerning is that the early symptoms of hyponatremia are very easily confused with those of dehydration — weakness, headache, nausea, dizziness and lightheadedness.

The problem with this model of hydration is that it overlooks basic physiology.

How did hyponatremia become an affliction of athletes? In retrospect, it may come down to an error of shifted priorities. In the wake of Gatorade’s massive success, sports drink makers turned to science to promote their products, and researchers focused on things that were easy to measure — body temperature and sweat losses. Based on an idea that dehydration must be a risk factor for heatstroke, attention moved to replenishing fluid loss.

The problem with this model of hydration is that it overlooks basic physiology. It turns out, your body is highly adapted to cope with losing multiple liters of fluid, especially during exercise. When you exercise, you lose fluid and salts through sweat, and that translates into a small change in what’s called your “plasma osmolality” — the concentration of salts and other soluble compounds in your blood. You need enough fluid and electrolytes in your blood for your cells to function properly, and this balance is tightly regulated by a feedback loop, says Kelly Anne Hyndman, a professor of medicine at the University of Alabama at Birmingham and leading expert on kidney physiology.

When you sweat, your brain senses the corresponding rise in plasma osmolality and directs the release of antidiuretic hormone (ADH), which prods the kidneys to activate aquaporins, which are like tiny straws that poke into the kidneys to draw water back into the blood. “It’s a pathway to conserve water,” Hyndman says. As your body reabsorbs water, your plasma osmolality returns to normal, your brain senses the change, and it shuts down ADH. This feedback loop is finely tuned to keep plasma osmolality in a safe range. Even a tiny drop in electrolytes will activate this system to keep your fluid balance in check. “People always worry they’re going to be dehydrated when the reality is, it’s much easier to over- hydrate because our bodies are so good at conserving water,” Hyndman says. “Being a little dehydrated is not a bad thing. Our bodies can handle it.”

Athletes who develop hyponatremia during exercise usually get there by drinking too much because they’ve been conditioned to think they need to drink beyond thirst, says Tamara Hew- Butler, a professor of exercise science at Oakland University and the lead author of several papers on hyponatremia. Even if you don’t drink anything (which she does not recommend), your blood sodium levels will rise in response to sweat losses, and as a result, your body will shift fluid into the blood to maintain your fluid balance, Hew-Butler says.

The same feedback loop that calls in the aquaporins also activates your thirst. “You don’t have to drink above thirst — you’ll be fine!” she says. Just as sleepiness is your body’s way of telling you that it’s time to sleep, thirst is how your body ensures that you seek fluids when you need them. No one tells you to sleep before you’re tired, and unless you’re in a situation where you can’t drink for a prolonged period, there’s no sense in drinking before you feel thirsty either. Your body is a finely tuned machine that that is capable of adapting to changing conditions, and it’s not usually necessary to try to outsmart it.

You can also forget those pee charts that look like paint swatches for urine, and ignore anyone who says that yellow pee is a sign that you need to drink more water. If you think about hydration from the standpoint of what’s going on inside your body, it’s easy to see why urine hue isn’t helpful. The color of your pee is essentially just a measure of how concentrated your urine is. If it contains more waste than water, it looks dark, and if it’s mostly water, it’s light or almost clear. But that’s not what’s important. What you really want to know is what’s going on in your blood, and your urine can’t tell you that. Dark pee might mean that you’re running low on fluid, but it could also mean that your kidneys are keeping your plasma osmolality in check by conserving water. Very light or clear urine just means that you’ve drunk more water than your body needs, and that’s not necessarily a good thing, especially right before an athletic event.

Because of the way the body adapts to fluid loss, the common advice to drink a lot in advance of a big event like a marathon may actually backfire. If you drink a bunch of excess water leading up to a competition, you prime your body to become less adept at holding on to precious fluids, says Mark Knepper, chief of the Epithelial Systems Biology Laboratory at the National Heart, Lung and Blood Institute. When you’re very hydrated, your body doesn’t need to activate many aquaporins, and over time, it reduces the number in reserve, meaning that you’ll have fewer of these water straws at the ready when you need them.

Yet everywhere I look, it seems that people are telling me to drink more water. In his best- selling 2017 book, “The TB12 Method,” New England Patriots quarterback Tom Brady presents his magic hydration formula — drink at least one-half of your body weight in ounces of water every day. “At 225 pounds, that means I should be drinking 112 ounces a day, minimum,” he writes. (Brady also contends that “the more hydrated I am, the less likely I am to get sunburned,” a claim disputed by scientists.) If our bodies are so good at adapting to moderate fluid loss and letting us know when we need to drink, why are there still so many messages out there urging us to drink before we feel thirsty?

An obvious explanation for this is that most of what we hear about hydration comes from companies and researchers with a vested interest in making it all seem complex and highly scientific. The current guidelines from the ACSM and the National Athletic Trainers’ Association have been updated to warn about hyponatremia, but they still promote the ideas that thirst is a poor indicator of hydration and that more than a 2 percent body weight loss should be avoided. The ACSM, NSCA and NATA all receive funding from sports drink makers, as do some of their members. If staying hydrated were as simple as just drinking to thirst, you wouldn’t need expert advice or scientifically formulated products like Gatorade.

From a biological perspective, it’s hard to imagine that the human body is so delicate that it can’t function properly without scientists (or football stars) swooping in with calculators to tell us how to keep it running properly. “You have to trust your body,” Knepper says. Humans have evolved to survive exercising without chugging water or sports drink on some rigid schedule. “You get clues about what you need if you listen to your own body,” he says. “You don’t have to know chemistry to survive.”

After examining the science, I can’t help thinking we’ve made hydration unduly complicated. I take my dog running with me most of the time, and I’ve never measured the color of her pee or forced her to drink (as if I could). I make sure she has regular access to water, but she doesn’t always take it. At times, she won’t drink at all during a long run, and on those occasions, she always goes straight to her water dish when we get home and slurps until she’s satisfied. I’ve never had to give her an emergency IV for low fluid levels. If drinking to thirst is good enough for her, it’s probably good enough for me too.

Reprinted from “GOOD TO GO: What the Athlete in All of Us Can Learn from the Strange Science of Recovery” by Christie Aschwanden. Copyright © 2019 by Christie Aschwanden. With permission of the publisher, W. W. Norton & Company, Inc. All rights reserved.