How runners train their bodies to compete in brutal heat

How much heat can the human body actually handle?

That question is becoming harder to ignore. This past March, runners in the Los Angeles Marathon were allowed to stop eight miles early as temperatures approached 90 degrees. At the 2024 Paris Olympics, organizers implemented cooling protocols and closely monitored athletes’ core temperatures amid concerns about heat.

As extreme heat becomes harder to avoid, more athletes are turning to a once-niche strategy: heat acclimation, a method designed to train the body to perform in rising temperatures.

In 2024, Manika Gamble, an Atlanta-based trail runner, put that approach to the test when she signed up for a multi-day race across the dunes of Namibia, where temperatures could soar as high as 143 degrees Fahrenheit.  

Two months beforehand, she jumped rope or trained in a sauna three days a week, layering those sessions on top of her regular training. Equipped with sun protection and plenty of water, she persisted and completed all 155 miles of the race.

Most runners will never face conditions quite that extreme. But as recreational athletes and everyday people contend with hotter environments, researchers say the same adaptations may matter for anyone spending time in the heat. The question now isn’t just whether the body can adapt—but how far those adaptations can safely go.

How does heat acclimation work?

At its core, heat acclimation is about training the body to cool itself more efficiently. With repeated exposure to high temperatures, a series of physiological changes begins to take shape.

“Humans have a really incredible ability to adapt to heat. It’s our unique skin blood flow and sweating system that makes us—even in the mammalian world—uniquely able to respond to high temperatures,” says Chris Minson, co-director of the exercise and environmental physiology lab at the University of Oregon. Those adaptations begin with regular heat exposure—like adjusting to the first stretch of hot summer days—but intentional training makes them more pronounced and brings them on faster.

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“As we exercise in the heat, we need to lose heat to the environment and maintain a body temperature that's relatively low,” says Julien Périard, director of the environmental physiology research laboratory at the University of Canberra Research Institute for Sport and Exercise. “The way we lose heat to the environment is via the evaporation of sweat.”

With heat training, the body becomes better at that process. You start sweating earlier and more efficiently, helping dissipate heat before your core temperature climbs too high.

The cardiovascular system changes, too. Périard says that most people experience an improvement in total blood volume, especially plasma volume. That added volume makes circulation more efficient, reducing your heart rate, says Robert Kenefick, who researches thermoregulatory responses to exercise and environmental stress at UMass Lowell. With less cardiovascular strain, an effort that once felt hard feels more manageable, even in the heat.

What are heat acclimation strategies for athletes?

For athletes, heat-acclimation training often takes place in an environmental chamber, where temperature and humidity are tightly controlled. One method Kenefick uses in studies is steady, low-intensity exercise in the heat.

“Typically, we have people walk for a hundred minutes every day for 10 days,” he says. On the first day, many can’t complete the full session without overheating. But as the days go on, their core temperature rises more slowly, their heart rate stays lower, and they can get further with less effort.

Athletes can replicate these adaptations outside the lab, too—by training in hot weather (or in a heated room) or by adding layers, like a hoodie. Most experts recommend pairing heat exposure with easier workouts, while keeping harder efforts, like speed sessions, in cooler conditions.

Researchers are also finding that passive methods, such as warm-water immersion or sauna sessions after workouts, can be equally effective and less taxing on the body. Kenefick says most people should use a combination of active and passive heat training methods.

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The key is to be conservative. Heat is a major physiological stressor, and adaptation takes time. Experts recommend building exposure gradually, monitoring how your body responds, and prioritizing hydration. “If you get dehydrated, your sweating decreases, then you're really in trouble,” says Minson.

Even relatively short protocols, lasting about 7 to 10 days, can lead to noticeable improvements. But Minson recommends taking a longer view, building exposure gradually over several weeks rather than compressing it into an aggressive block. Some athletes now use wearable sensors, like CORE, to track heat strain in real time, though experts say those tools should supplement—not replace—how you actually feel.

Heat acclimation also doesn’t mean you can throw the rest of the precautions aside. If you’re exercising on a hot day, you need to temper your expectations. “If you start slower and run easier, then you’re gonna be not generating as much heat in your body, so there’s less need to dissipate that heat,” says Minson. “You can continue that pace longer.”

How much can the body actually adapt?

While you can train your body to respond better in the heat, it has limits. Race organizers often rely on guidelines based on the wet-bulb globe temperature (WBGT) index, which accounts for temperature, sun angle, cloud cover, wind speed, and humidity, to determine when conditions become unsafe. In fact, Périard and fellow researchers found that over one-quarter of all endurance running events were held in moderate, high, or extreme heat.

One of the biggest factors that influences humans’ ability to regulate is humidity.  As it rises, the risks of heat-related illnesses increase. When the air is already saturated with water vapor, sweat can’t evaporate as effectively, limiting the body’s ability to cool itself. That’s why a 90-degree day with high humidity can be more dangerous than a much hotter, drier environment. One ongoing study by researchers at Penn State found that the safety threshold for young, healthy adults (without heat acclimation) is around 87 degrees Fahrenheit at 100 percent humidity.

But not everyone reacts the same to the heat, either. Jessica Mee, an environmental physiology researcher at the University of Worcester, studies how men and women respond differently to heat training. Some of her research suggests women have a lower heat tolerance than men and may benefit from longer periods of acclimation, but the evidence is still evolving.

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Despite the limitations, the broader takeaway is clear: “Heat acclimation works,” Périard says. “And with climate change, it’s only getting warmer and athletes are competing more often in hotter conditions.” Because of the potential benefits of heat training, he is now researching the public health implications of heat therapy not only for athletes but also for older adults and vulnerable populations who may not be able to exercise.

“The best thing a human can do is to be heat acclimated,” Minson says. “Short of knowing when not to run, that is the best preventative there is.