Running in the heat can slow you down if you’re not used to it. However, if you run in hot, humid weather regularly, your body learns to adapt. When running in the heat, make sure to hydrate properly and adjust your pace accordingly to stay healthy and perform your best.

However many bad-weather-will-make-you-tougher quotes we collect, there’s still one aspect of weather that most of us do our best to dodge: heat. In fact, many of us do everything we can to avoid it: running at dawn or in the late evening, or even seeking shelter on treadmills in air-conditioned gyms.

It is, however, totally possible to run in heat. And considering most gyms are closed for the time being because of coronavirus restrictions, running outdoors might be your only option.



In the 1996 Atlanta Olympics, Portuguese distance star Maria Fernanda Moreira Ribeiro set an Olympic 10,000-meter record of 31:01.63 under hot, humid conditions (82 degrees with 60 percent humidity, according to historical data from Weather Underground). And last year, at the world championship marathon in Doha, Qatar, American Roberta Groner finished sixth overall and first for Americans in a time of 2:38:44 behind winner Ruth Chepngetich of Kenya after battling 92-degree heat and 73 percent humidity the whole way.

The bottom line is that the human body is remarkably adaptable to heat. Its ability to adjust to high temperatures is faster and more dramatic than its ability to react to any other environmental stress that nature can throw at us, such as altitude or cold, says Lawrence Armstrong, a heat researcher at the University of Connecticut.

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In a paper in the January 2015 issue of Comprehensive Physiology, Daniel Lieberman suggests that our heat tolerance probably results from the fact that our ancestors evolved to hunt and forage at midday on the African savannah—a time of day at which, if you are heat tolerant enough to handle it, you are relatively safe from less heat-tolerant predators. Since then, humans have dispersed all across the globe, but wherever we live, he suggests, we still largely retain these ancient heat-tolerant genes.

“Heat is a paradox,” Lieberman says. “On the one hand, we evolved to run in heat. But on the other, if one is not well-adapted, it can be mighty dangerous.”

Physiologically, running in heat produces a cascade of reactions that begin with the fact that our muscles aren’t all that efficient. In fact, says Yannick Molgat-Seon, who spent more than two years at the University of Ottawa’s Thermal Ergonomics Laboratory, 80 percent of the energy generated by our muscles winds up as heat. In cold weather, that inefficiency is what keeps us warm.

But during exercise, the body has to get rid of it. One way is by sweating. “Less obvious,” Molgat-Seon says, “is promoting blood flow to the skin.” That’s important because it’s what carries excess heat from your muscles to the skin, where it can be lost to the environment.

But the body only has so much blood. “You have a competition between blood going to the skin and blood going to the active musculature,” Molgat-Seon says. “In that battle, the muscles always lose.” Even when you’re barely sweating, your muscles are getting less oxygen and therefore are less efficient.

Even if you are well acclimated, heat will slow you down—and can even be harmful. Last year, 141 runners at the Hamburg Half Marathon in Hamburg, Germany, needed to be treated for a heat-related illness, and 57 were hospitalized. The temperature was reported to be 35 degrees Celsius, or 95 degrees Fahrenheit.

Here’s a guide for what to expect and how to prepare as the thermometer rises.

Running in 50°F to 59°F Heat

Most runners don’t think of these temperatures as hot—but the longer the race, the more the heat will affect you.

In a study published in Medicine & Science in Sports & Exercise, a team led by exercise physiologist Matthew Ely studied decades’ worth of elite and sub-elite marathon performances at temperatures ranging from 41 to 77 degrees Fahrenheit.

What they found was surprising: Even at temperatures of 50 to 59 degrees, there was a slight drop in performance, comparable to about a one- to two- minute slower time for a 2:10 elite, depending on whether the race was at the top or bottom end of this temperature range. For 3:00 marathoners, the time difference amounted to four to eight minutes. (Runners who were slower weren’t included in the study.) A recent review of nearly 2 million marathon finishers by the French National Institute of Sport and Physical Education found that the optimal temperature for male pros was below 40 degrees.

There are several reasons for this. For one, studies like these tend to use temperatures at the start of the race, which means a race that started at 59 degrees could have finished at 70 degrees.

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Running in 60°F to 69°F Heat

These are the temperatures at which most start to view conditions as less than optimal. The Run SMART Project calculator by Jack Daniels, author of Daniels’ Running Formula, calculates that at 69 degrees, a 40-minute 10K runner can expect to be slowed down by 1.7 percent (a bit more than six seconds per mile).

For marathoners, Ely’s study finds another one- to four-minute slower time for top-level elites and 3:00 men, respectively. However, some people are simply genetically better in heat, meaning that in any given study there will be considerable variability.

The moment you start running in heat, rather than avoiding it, your body quickly begins to adapt in multiple ways. Within a week, Armstrong says, your blood plasma volume starts expanding. That may increase your weight by a pound or two, but it gives you that much more fluid to sweat away without dehydrating. It also makes it easier for the body to supply blood to the skin without excessively reducing the flow to the muscles.

Another adaptation is that you start to sweat earlier in your workout (or race), as your body learns to anticipate the upcoming buildup in core temperature and takes preemptive measures to stave it off. You also begin to sweat more profusely. Your sweat becomes less salty, as the body works to conserve sodium. And your heart rate slows down slightly at any given effort level, allowing the heart to fill more completely between beats, thus having more blood to pump out.

“That’s called stroke volume,” Armstrong says. “With each beat, you’re pushing more blood, not only to exercising muscle [but] to cool the body.”

Even your perception of hot-weather running effort changes, Armstrong says. Amazingly, he adds, all that’s needed is about eight to 14 days of heat training. Brett Ely, Matthew’s wife, also spent several years as a heat researcher at the Army research station in Natick, Massachusetts, looking for optimal protocols to train soldiers for deployment to Iraq. Ultimately, she concluded that you can adjust to heat in as little as 10 days.

“Start gradually, rather than overheating yourself,” she says. “If you were going to high altitude, you wouldn’t do a hard workout on the first day. You’d ease into it.”

Running in 70°F to 79°F Heat

At these temperatures, Matthew Ely’s elite marathoners slowed down by three minutes, with the sub-elites losing 20 minutes.

Brett Ely experienced this when she ran the 2010 Miami Marathon in humid 71-degree temps. She thought she was in 2:40 shape, but she knew better than to attempt it. She backed off and ran 2:45:36, winning the race and beating a woman who had a PR of 2:38. “I think the reason I was successful was that I didn’t try to run my goal pace,” she says.

Brett Ely’s triumph reflected something else shown by her husband’s data: Women fare better in heat than men. By Matthew Ely’s charts, the average 2:45 male marathoner would have lost more than 10 minutes to the heat. The most likely explanation is that women, being generally smaller than men, have a higher surface-to-mass ratio—something that allows them to shed heat more efficiently to the environment.

It’s not just women who can benefit from being petite. In the men’s marathon at the Atlanta Olympics (run at a starting temperature of 74 degrees with 90 percent relative humidity), the winner, South African Josia Thugwane, weighed only 99 pounds. The silver medalist, Lee Bong-ju of South Korea, was also small, weighing 123 pounds.

The correlation between heat and body size doesn’t only affect marathoners. In laboratory experiments conducted shortly before the 2004 Athens Olympics, exercise physiologist Tim Noakes, author of Lore of Running, had two groups of men run 8K time trials on treadmills. One group weighed 110 pounds or less, while the other averaged 130 pounds. In cool temperatures, the two groups performed equally, but when the room temperature was jacked up to 95 degrees, the smaller men were 45 seconds per mile faster than their larger counterparts.

The concept applies at less extreme temperatures: The larger you are, the more strongly you will be affected by heat—so adjust your pace appropriately.

Running in 80°F to 89°F Heat

There comes a point when additional sweating doesn’t do you any good. “When you’re dripping sweat, there’s lost water as opposed to lost heat,” Molgat-Seon says. In extremely hot and humid conditions, you also begin to run into the limits of not only your body, but of physics: no matter how efficiently you sweat, it won’t evaporate fast enough to keep pace with the rate at which you are generating heat. Your only alternative is to slow down.

“Basically, when the air temperature exceeds 80 degrees and the humidity exceeds 70 percent, you will find that performance drops markedly,” Armstrong says.

“When the air temperature exceeds 80 degrees and the humidity exceeds 70 percent, performance drops.”

Acclimating to do the best that’s possible under these conditions requires dedication. In 2013, when the USATF outdoor nationals were held in Des Moines, professional 5K runner Kim Conley spent her last two weeks of training in the Midwest, getting used to the combination of heat and humidity. Runners training for the 2019 world championships in Doha similarly simulated hot, humid conditions in the weeks leading up to the race.

But just as runners taper off their workout volume before important races, Conley also backed off her heat training in the day or two immediately prior to the race, trusting that the adaptations had already taken place and what she now needed to do was make sure she didn’t enter the race overstressed. It worked. On race day, which saw starting-time temperature of 80 degrees with 60 percent humidity, Conley ran 15:37.80—not her best, but good enough to get her onto the U.S. world championship team.

Running in 90°F + Heat

The hotter it is, the harder it is to excel. In 2007, Kara Goucher became the first American ever to medal in the 10,000 meters in a world championship. She knew that Japanese summers are notorious for hot, humid conditions, and race day in Osaka was no exception. “[It] was humid and stifling,” she says, remembering 88 degrees with high humidity.

But she’d also prepared for it, doing summer track workouts in tights and long sleeves, as well as easier runs in “sauna suits—jacket and pants that were basically like rubber.” She went to Japan two weeks before the race. “By race day, I knew I could handle it,” she says.

The key to racing in outrageous conditions, adds Greg Pressler, a Badwater 135 veteran, is thinking about everything that might affect your performance, whether it’s monitoring pace or your choice of clothing. Stints in a sauna can also help with acclimation. Meanwhile, it’s also useful to work on hydration, even for shorter races.

“I’m a big proponent of in an effort to top off the electrolytes, while also ensuring maximal hydration,” University of Iowa cross country coach Layne Anderson says. It will pay off with larger blood volume and greater resistance to dehydration. Or try a similar product with natural ingredients like Skratch Labs Wellness Drink Mix.

It’s also possible to train yourself to drink more liquids. (Take the right electrolyte supplements so you don’t create dangerous imbalances.) You won’t be able to exceed one liter per hour, but most people aren’t used to consuming even that much, Pressler says, which means that in training for hot conditions, it’s easy to dehydrate.

The keys to not only surviving a hot race, but doing well, boil down to “hydration, practicing the conditions, and getting there early,” Goucher says.

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The Basics of Heat Training

As the temps rise, remember:

Even when it’s cool enough that you’re barely sweating, your muscles are getting less oxygen and therefore are less efficient—an important factor in longer races.

Instead of avoiding heat during training, help your body adapt to it. Within a short time your system will become more efficient, as it learns to anticipate the rise in core temperature.

When heat and humidity start to creep higher, it’s best to slow down. Your performance relative to the competition will often be better (though slower) if you remain conservative.

If you are working on acclimating to warm temperatures expected on race day, remember to back off two days prior to the competition to make sure you’re not overstressing your body.

Hydrate wisely and don’t create dangerous imbalances by not using the right electrolyte supplements. Train to consume more liquids to build resistance to dehydration.

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