Hot Stuff: The History and Science of Heat Acclimation

There were only 400 yards to go at the 2016 International Triathlon Union’s Grand Final race in Cozumel, Mexico when the wheels fell off for Jonny Brownlee. The 26-year-old British triathlete and Olympic silver medalist was leading the race, almost certain of the win, when his smooth run form suddenly devolved into a drunken stagger, then came to a full stop. Brownlee lurched to the side of the road, falling onto the shoulder of a race official. Running in third place was his older brother, Olympic champion Alistair Brownlee, who draped Jonny’s arm over his shoulder and supported his brother’s limp body for the final steps before pushing him across the finish line, where Jonny immediately collapsed.

The dramatic moment became (and remains) one of the most iconic images in endurance sport, joining a highlight reel of race-day implosions and finish-line failures: Julie Moss crawling across the finish line at the 1982 Ironman World Championships; Chris Legh’s collapse at the same race in 1997; Gabriela Andersen-Schiess’s zombie walk through the Los Angeles Coliseum during the 1984 Summer Olympic marathon. These moments of sheer depletion show what happens when athletes are bested not by the competition, but by their own bodies.

Races aren’t always won by the fastest athlete, but sometimes by the one who can best manage the conditions of the day.

Endurance athletes, by definition, like to find their supposed limits and then push through them. The quest to go faster, farther, and harder is mostly fueled by the prevailing belief that just about anything can be overcome with the right mix of grit and tenacity. But despite wishing it were otherwise, the human body does indeed have actual finite limits. Nowhere is this more clear than in the examples of Jonny Brownlee, Julie Moss, Chris Legh, and Gabriela Andersen-Schiess. The common thread running through all of these cases—and in so many spectacular blow-ups on the race course every year—is heat illness, which occurs at the confluence of effort and environment.

Heat is the bane of the endurance athlete’s existence, laying bare the simple truth that races aren’t always won by the fastest athlete, but sometimes by the one who can best manage the conditions of the day. Since the 1970s, sport scientists have tried to crack the code of heat management, hoping to uncover the exact mechanisms that will allow someone to run just as fast on a sunny summer day in New Orleans as they do on an overcast autumn morning in New York City. To accomplish this requires an interdisciplinary approach, combining the fields of anatomy, physiology, kinesiology, medicine, genetics, and meteorology. As such, endurance sport researchers have found some strange bedfellows, including the U.S. military. Many of today’s discoveries in running science, especially those pertaining to extreme environmental conditions, can be traced back to research done at the U.S. Army Research Institute of Environmental Medicine (USARIEM) laboratory in Natick, Massachusetts. That’s because both groups are trying to answer the same core question: Is there any way to hack the heat?

From Mine to Military

Wars are not usually fought in temperate climates. Often, the elements can be more dangerous than the actual enemy. Dense jungle heat, unrelenting desert sun, and suffocating rainforest humidity can sabotage the efforts of even the most elite soldiers. To prepare troops for the rigors of war, the United States Surgeon General established the USARIEM in 1961. Their only task: figure out ways to help soldiers endure punishing conditions better than their enemies.

Though an Olympic running competition is nowhere close to the rigors of war, there is a lot of overlap between human performance in combat and competition. Both require endurance and stamina, and in the heat, both are diminished. In the early 1980s, coaches and physiologists working with the U.S. Olympic track and field team contacted USARIEM to inquire about strategies for managing intense efforts in hot conditions. The military researchers shared their protocol for heat acclimation, which had been successful in preparing soldiers for intense physical efforts in hot and humid conditions.

The heat acclimation protocol, which involved gradual exposure to exercise in increasingly warmer conditions (using a climate-controlled chamber), was first used in South African gold mines. The high occupational workloads and temperatures approaching 140 degrees Fahrenheit made the mining environment uniquely challenging in terms of thermal stress, and workers often succumbed to illness and even death from extreme heat. Mine owners were not so distressed by the loss of life as they were by the loss of labor—when workers experienced heat illness, they weren’t very productive. To increase profit, they needed to find a way to build their workers’ heat tolerance. This came in the form of acclimation: putting new workers through a process where they spent four hours a day stepping up and down on blocks in a climate-controlled tent. Each day, the temperature in the tent would gradually increase, until the protocol was completed on day eight. “The method seems to have considerable economical, physiological, and psychological advantages,” Cyril Wyndham, lead investigator, wrote in 1965. Indeed, heat illness in the mines decreased, and productivity increased. Heat acclimation, when done methodically, seemed to reduce the strain on the body, improve physical work capabilities, and protect the brain, liver, and kidneys, from heat injury.

The United States Army got wind of those heat acclimation studies in South Africa, and began developing its own version for use in training soldiers. When the U.S. Olympic team came calling, the established processes proved helpful. While experimenting with athletes like marathon runner Alberto Salazar, researchers were then able to develop a heat acclimation protocol for all Olympic runners, giving the United States an edge over their competition. As a result, the 1984 United States Olympic team topped the medal count at the Olympics for the first time since 1968, winning a record 83 gold medals and surpassing the Soviet Union’s total of 80 golds at the 1980 Summer Olympics.

How Heat Hurts

Any sort of active movement generates heat. A car’s engine is cold when not in use, but as soon as the key is turned in the ignition, gasoline ignites and explodes, and the engine begins to warm. Drive it down the street and it gets warmer still. Faster driving generates more heat, as does driving up a twisty mountain switchback. If a car gets too hot, the engine shuts down. To prevent this, a system of fans and coolant fluid circulates around the engine, drawing heat away and keeping the car going.

The human body does the same thing. When the body moves, its temperature goes up; when the body moves more, the temperature increases in kind. A human, at rest, generates about 100 watts of energy. (For comparison, a standard incandescent lightbulb generates around 60 watts.) Studies have found the body’s heat production when exercising can exceed 1000 watts as it burns through its fuel–adenosine triphosphate, or ATP, created by the breakdown of the food we eat. The harder we work, the hotter we get. To limit how much the core body temperature goes up, blood vessels dilate to bring thermal energy to the surface for dissipation. Sweat, a common byproduct of both exercise and warm ambient temperatures, serves to pull heat away from the skin as it evaporates. This keeps the body from overheating and shutting down. That necessary evaporation is easy when the air is cool and dry, but where things get tricky is when the air is hot, adding a layer of heat to the already-hot body. If it’s humid, it becomes even more complicated, as sweat sticks to the skin instead of evaporating, trapping the heat in like a wool blanket. This forces our body to work even harder to cool itself, creating a cycle of increasingly serious health consequences.

When a body is affected by heat, it throws up red flags. First, it slows down (on average, runners slow between 1 and 4.5 seconds per mile for every 1.8 degrees Fahrenheit increase in the temperature above 59 degrees). Then nausea, dizziness, or headache may set in. A rapid heart rate will be present as it tries to pump more blood to cool the body. As a body’s temperature continues to rise, it paradoxically stops producing sweat, and goosebumps and chills may set in. Confusion and disorientation are next. At the most extreme stage, the brain and other vital organs begin to shut down.

A slow and gradual acclimation process allows the body to gradually adjust to the heat and build up adaptations, so that extremes don’t feel so extreme. When exposed to heat, the body adapts by increasing blood volume, giving it more capacity for the shift in distribution of the output from the heart. The capacity for making sweat increases as well, to aid in the removal of excess heat. In one study, heat-adapted elite cyclists could briefly sustain a core temperature of 104 degrees without ill effects—for someone not adapted to the heat, that same core temperature could be lethal. However, even after a heat acclimation protocol, some still flounder in the heat. Some people don’t acclimate well (if at all), and some temperatures are simply too hot, even for athletes who are heat-adapted.

Understanding the mechanisms of the body’s responses to heat is the life’s work of Dr. Nisha Charkoudian, Research Physiologist at the USARIEM. Heat is a major issue within the armed services: In 2018, 2,792 active-duty service members were diagnosed with heatstroke or heat exhaustion. Like the coal miners of South Africa, the physical consequences of being in the heat affect the bottom line, costing the military as much as $10 billion per decade.

“The military is really interested in heat physiology and research, because we have to be,” Charkoudian said. “The Army will come to us and say, ‘We have to deploy our soldiers to Afghanistan, Iraq,’ where it’s over 100 degrees in the desert in the middle of the day. They still have to do stuff—you can’t just sit around and not do anything. We have to find ways to improve their performance, or at the very least prevent them from feeling terrible.”

For every deployment, an 8- to 14-day heat acclimation protocol is recommended to help soldiers become more efficient in their new environment. But in recent years, heat acclimation protocols have begun even earlier, starting with the first day a new soldier sets foot on base.

“We don’t just fight in the heat, we train in the heat, too,” Charkoudian said. “A lot of our training bases are in the south of the United States. We think about where the major training bases are, like Fort Benning, Fort Bragg, Fort Jackson, those are all in the south. We have these 18 year-olds who have played video games more than they’ve exercised, and we ship them to Fort Benning, where they’re exposed to heat and this intense exercise. There’s a real risk there, and there have been rare instances where people have died by heat, not by fighting the enemy.”

The No-Secret Secret of Heat Acclimation

To develop best practices for extreme efforts in extreme temperatures, researchers must first create a controlled environment to study such conditions. At Charkoudian’s laboratory in Natick, Mass., this means using small pods known as climate chambers, which allow researchers to plug in precise temperatures, humidity, and altitude. Today, a man runs on a treadmill in 104 degrees Fahrenheit and 40 degrees relative humidity in order to test out the effectiveness of using iced sheets to cool the body. Tomorrow, the dials will turn the opposite direction for a different experiment, testing consumption of a cocoa bean extract to help the fingers stay warm in cold conditions. By monitoring core temperature, skin temperature, heart rate, and blood pressure, researchers can get a better understanding of what happens to the body and when.

Runners and cyclists are quick to volunteer for these studies. Endurance athletes are often eager to maximize their performance, so participating has the benefit of getting training advice through proximity to the experts. But there’s also a bit of masochism for both athletes and highly trained soldiers—extremes are appealing, be it distance, intensity, or environmental conditions. A treadmill test in 104 degrees is not necessarily daunting. If anything, it’s a dare.

“We have a group of soldiers that are volunteering for our studies who just happen to be in the Army infantry [the most physically-demanding role in the Army],” Charkoudian said. “They’re very proud of just being strong and fast in general; that’s how they get into the infantry in the first place. And all of a sudden, in the climate chambers, their two-mile run test drops from 12 minutes to 20. I’ve seen it happen so often, where people beat themselves up because of things beyond their control.”

Despite a robust body of evidence showing that heat exposure causes shifts in a body’s breathing rate, muscle function, and VO₂ max—things that, objectively, make a person slow down—there’s still a prevailing belief that struggling in the heat is a personal failing. Whether a sergeant at a boot camp or a coach at cross-country practice, “dig deeper” and “tough it out” are predominant themes. Yet, while some amount of exhaustion can be overcome by grit, the only way right now to train your body to withstand the physical experience of heat is through gradual acclimation.

“If I went to Phoenix on a 100-degree day, and on the first day I went on a three-hour run, I’d probably get heatstroke,” Charkoudian said. “I’d be sick for several days, and I wouldn’t be able to do much at all while recovering. But if I started slowly, with shorter runs on the first few days, and then gradually increasing my time spent running each day, I’d be able to handle the heat better.”

There’s something there, perhaps at the basic biological level, to suggest there’s a way to shorten the process. We just don’t know what it is yet.

On average, heat acclimation takes about 10 days. Some people adapt in only six days, while others take up to 14. This variation isn’t fully understood, though some research suggests the recovery period between acclimation workouts may play a role. “It’s like strength training,” Charkoudian said. “We don’t build muscle while we’re lifting the weights, we build muscle while we’re resting and recovering. The same is true for all adaptations, including heat.”

Is it possible to shrink the adaptation window from ten days to five, or maybe even a weekend? That’s the 10-billion-dollar question. The military has a vested interest in shortening the heat acclimation window, since it would put soldiers into action faster in training and when deployed. Endurance athletes, too, are eager to shorten the acclimation window—ask anyone who wants to race the Ironman World Championships in Kailua-Kona, Hawaii without the exorbitant cost of a two-week trip, or runners traveling from a cold Wisconsin winter to a sunny marathon in Arizona.

Over the years, researchers have tried just about everything to shorten the heat acclimation window: taking vitamin C supplements, soaking in a hot bath after regular workouts, sitting in a sauna for increasingly longer sessions, training twice a day in the heat instead of a singular session per day. Athletes have experimented, too, by taking hot yoga classes, running in extra layers of clothing, or setting up their bike trainer in the laundry room with the dryer running full blast. Still, the evidence shows the most consistently effective protocol is the old-school way: on-site heat acclimation over a period of ten to 14 days.

“When we look at the data [of certain studies], about half of the people are acclimated in a shorter period of time, like six days instead of 14, and half need the full acclimation period,” says Charkoudian, “So there’s something there, perhaps at the basic biological level, to suggest there’s a way to shorten the process. We just don’t know what it is yet.”

At the Muscular Level

Inside a climate chamber at the exercise physiology lab at the University of Nebraska at Omaha, a cyclist on a spin bike pushes through a time trial. Sweat drips off his body, both from the exertion and from the hot and humid conditions created for the experiment of the day. Once the time trial is over, he moves to a sanitized surgical setup, where Dr. Dustin Slivka anesthetizes the skin of the thigh, makes an incision, and pulls out a little chunk of muscle.

Slivka has performed this procedure hundreds of times in the lab and in the field, with his longtime mentor and collaborator Dr. Brent Ruby, from the University of Montana. He has conducted research on athletes immediately behind the finish line at the Badwater ultramarathon and at Ironman triathlons, on wildland firefighters in an Airstream trailer just outside the fire zone, and at a high-altitude camp on Mount Rainier. To the untrained eye, the samples collected in these environments might all look the same. To Slivka, each sample is a tiny molecular journal of how the body is affected by heat.

Most heat exertion studies focus on large-scale data points, like how much slower a runner gets in the heat or how much harder a time trial feels in humidity. These performance metrics can be colored by a variety of factors though, from how much sleep you got the night before to how much emotional stress you’re feeling that day. That’s why Slivka likes studying the molecules of the body instead—they simply hum along. The smallest details found in genetic material provide a straightforward way to explain what happens in the body when exposed to extreme temperatures.

“We know certain things happen when we exercise in the heat,” Slivka said. “We know that our core temperature goes up. We know there’s enhanced blood volume, because heat stress stimulates your body to produce more plasma. We know easy efforts can suddenly feel hard. But the question I’m looking at is why. When we drill down to it, what’s happening on a molecular level?”

By breaking down the muscle into the tiniest building blocks, Slivka can look at the function of mitochondria, or the powerhouse of ATP production in the cell that converts energy from food into energy for movement. A biopsy provides a very specific snapshot of what is happening in the cells at a very specific moment in time: what signals are being sent out to the body, and which genes and proteins are triggering which adaptation processes. Molecular analysis has led to some surprising discoveries about heat adaptation—specifically, that not all heat sources are created equal.

“Though we see certain markers are changed after a bout of exercise in the heat, it’s not the same across the board,” Slivka explained. “It’s different whether we’re in a hot environment or creating heat, like applying heat to the skin.” This may explain why it’s so hard to create an effective heat acclimation protocol through artificial means. Our cells may be simple, but they’re incredibly smart. Running or riding a bike on a hot day is a different stimulus than applying a heating pack to the skin or sitting in a hot bath. Different molecular pathways seem to be triggered, and different adaptations are created (which can be helpful or harmful) as a result. The same is true for cold conditions. Exercising outside in cold weather generates a positive chain reaction on the molecular level, while a post-run ice bath might not be as beneficial as so many hope.

“It’s always so interesting, seeing the differences between the science and the practice,” Slivka said. “So many people want to know if training in the heat is a good thing or a bad thing, is training in the cold a good thing or a bad thing? Well, maybe it’s a little bit of both, and it may depend on which real systems you’re looking at.”

Individual variations also come into play. Like Charkoudian, Slivka believes there might be something at the molecular level that could one day explain why certain people adapt to hard efforts in the heat with relative ease and why others struggle. Unlocking this information could help create a more personalized approach to heat adaptation.

“Everything we do is based on averages,” Slivka said. “When you read almost any research article, you’re looking at comparison of means. The typical measures don’t address the intricacies of individual physiology, which is something that is really difficult. We know that yes, some people are better suited for the heat. Now we need to find out what specific thing, or more likely, combination of things, are behind that. And if we can do that, we can finally answer the question of how can we exercise better in these challenging conditions?”

An Unfortunate (and Growing) Necessity

In 1984, applying the science of basic heat acclimation provided a performance advantage. By 2084, it will be all but a necessity. In some places, it already is.

According to an ongoing temperature analysis conducted by scientists at NASA’s Goddard Institute for Space Studies (GISS), the average global temperature on Earth has increased by 2 degrees Fahrenheit since 1880. Two-thirds of the warming has occurred since 1975. Record-breaking heat waves are becoming the norm, not the exception, each summer. In 2020, Phoenix notched 145 days over 100 degrees Fahrenheit and 50 days in excess of 110 degrees. On Sept. 6 of the same year, Los Angeles recorded its highest temperature ever at 121 degrees; in Chicago, days above 90 degrees jumped 182 percent.

While several health impacts of climate change have been widely reported, there has been limited discussion of how human physical activity will be impacted. What will happen when conditions regularly exceed the human body’s ability to cool itself? Dr. Shane Maloney of the University of Western Australia says “dangerous days,” where heat will make outdoor activity hazardous, will increase exponentially each year. In one model developed by Maloney, outdoor activity will not be possible for unacclimated people an average 33 to 45 days per year by 2070, compared to four to six days per year at present. For those who are heat acclimated, the number of dangerous days will be between 15 and 26 days per year. Those days will likely be bracketed by more heat at varying levels below the threshold, making for months of near-unsustainable temperatures for exercise, even for those who are well-adapted.

RELATED: Hotter, Harder, and More Expensive: Why Triathletes Should Care About Climate Change

Currently, the American College of Sports Medicine recommends events be canceled or recommend voluntary withdrawal for athletes when the combination of heat, humidity, and ambient temperature (what is known as a Wet Bulb Globe Temperature measurement, or WBGT) exceeds 82 degrees Fahrenheit. But these recommendations are just that—recommendations. They’re not mandates, and not everyone agrees with them. Some critics point out that though the wet bulb temperature can be a reasonably good index in some environments, an accurate heat balance equation should contain up to 50 variables, far more than what is used in the WBGT. But mostly, the reason is that people really don’t want to call off their races.

“The recommendation is that once the WBGT gets above that threshold, you should close down everything. But I’ve had quite a few Olympic-level athletes, college and national-level athletes too, come through my lab,” Maloney said. “And every time, I ask them, ‘How often have you had events canceled because of the heat?’ And the answer?” Maloney holds up his right hand, his thumb and fingers curved into a circle. “Zero.”

Some sports are trying to outsmart climate change with technology. Maloney points out the increase in air-conditioned venues, like the 100,000-seat stadiums being built in Qatar ahead of the 2022 FIFA World Cup. But air-conditioning all 26.2 miles of a marathon or 140.6 miles of an Ironman is simply not feasible. What’s more, athletes who train indoors on the hottest days run the risk of losing previously-earned heat adaptations. Acclimation isn’t a one-off deal that sets the body up for a lifetime of sweltering comfortably, but a muscle that must constantly be flexed to maintain its strength. Some studies suggest that heat adaptations begin to decrease within 48 hours, so moving workouts to an air-conditioned gym for a week-long heat wave (or for a full season) will require a planned “booster shot” of heat exposure or re-acclimation period when outdoor training resumes.

Yet, because of climate change, heat acclimation may no longer be something athletes incorporate simply in the weeks leading up to an important hot race. Instead, it will likely be a consideration just to get out the door for a workout. Athletes and coaches may soon find themselves periodizing training plans with weather in mind, scheduling certain workouts to expose (or avoid) a certain heat stimulus. Advances in technology will aid in this process, said Maloney:

“We’ve got models in development now where people can not only look up the next few days of weather, but also track your acclimation status, and put in things you’ll be doing. How hard will you be running? What kinds of clothing will you be wearing? What color clothing? An index like wet bulb temperature is simple, and simple is easy to compute. Before, that was necessary. But now, with computer power, the full heat balance model is just as easy to compute.”

A better understanding of the physiology of heat, generated by researchers like Charkoudian and Slivka, is getting us closer to hacking the heat. Perhaps one day, with the right knowledge, runners will know how to perform at or near their best in any conditions, not just on 50-degree days. But until then, caution is the best thing to exercise in extreme conditions. “We have to recognize that certain circumstances are just not possible yet, no matter how badly we want that to be the case,” Charkoudian said. “The body can’t acclimate to the heat in just two days. Even if you’re fully acclimated, you’re still going to be moving a little slower than you would in cooler temperatures. We tend to beat ourselves up about things that are beyond our control, when our body is just doing exactly what it’s supposed to be doing.”

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