Heat training can help athletes -- and the rest of us -- adapt to hotter weather

Athletes are kicking, running, biking, jumping, flipping and otherwise pushing themselves to their physical limits during the Olympic Games this summer. For some athletes, those events could push the limits of their heat tolerance.

“[The] combination of heat and humidity can really overwhelm an athlete fairly quickly,” says Dr. Francis O’Connor, a professor of military emergency medicine and medical director for the Consortium for Health and Military Performance at Uniformed Services University in Bethesda, Md. He’s co-author of a recent piece in the medical journal JAMA on heat-related illness in athletes.

O’Connor says exertional heat stroke is most likely to happen in competition. “In Paris, you’re going for a medal. Or if you’re a soldier trying to finish your course, or you’re trying to get a [personal record] at a marathon – you will push, and you may push yourself into a problem,” he says.

Heat stroke is a very serious condition where a person’s core temperature is generally 104 degrees or higher and they’re in an altered mental state; it can lead to organ failure and death. It can happen even if you’re not doing competitive sports, but stay out in the heat too long without proper hydration.

One of the best approaches to staving off trouble, O’Connor says, is heat acclimatization, also known as heat training. The idea involves controlled exposure to heat stress, which causes changes to the body that help it deal with hot, humid conditions. In fact, the Olympics medical department published a guide recommending heat training in advance for athletes coming to Paris.

Heat training is not just for competitive athletes. It’s recommended for people in the military and those who work outdoors in hot weather. It could even be useful for generally healthy members of the public, O’Connor says. “People should not be afraid of the heat,” he says. “We can develop and add an adaptive response to help us succeed in the heat. But it's got to be controlled.”

Done right, heat training could help people stay a bit more comfortable in the long, intense stretches of heat marking the extraordinarily hot summer of 2024 and future heat waves expected more frequently due to climate change.

A solution with caveats

There are two important points to note before exploring the topic of heat training.

One is that it is not for everyone. Heat-related illness is serious and can be deadly. And it can come on quickly.

“Everybody needs to be aware and self-evaluate and work with their health care provider, trainers and others to take in any personal considerations,” says Eugene Livar, an epidemiologist and chief heat officer for the notoriously hot state of Arizona. And, when people are working long hours in the sun, or otherwise exerting themselves a lot in the heat — that can lead to heat stroke, which is considered a severe medical emergency.

Two is that the ultimate goal for our planet, say climate scientists, is not to say, oh well, it’s getting hotter and hotter so it’s up to individuals to figure out how to deal with it. The scientists emphasize that as climate change continues to drive up temperatures, the ultimate global strategy to rein in heat is to cut the pollution – largely from burning fossil fuels – that’s responsible.

But in the meantime, it’s pretty hot out there. In recent weeks, temperatures in Las Vegas, Nev., and Delhi in India have topped 120 degrees.

Healthy adaptations to heat

Experts on heat and health say that careful, gradual exposure to heat can induce healthy adaptations that help the body better tolerate hot days. “There are data to suggest that it can improve overall circulation, enhance your immune system and cardiovascular health,” says Jason Kai Wei Lee, director of the Heat Resilience and Performance Center at the National University of Singapore Medical School. 

Research on heat acclimatization has focused on athletes, members of the military and outdoor workers, says Lee. Most people may not be up for as long of stints in hot weather as those groups, he says. But the same principles apply. Gradually increasing the dose of summer heat, with cooling breaks in-between, can aid acclimatization.

It’s hard to come up with a one-size-fits all regimen, since a person’s heat tolerance depends on factors such as age, baseline aerobic fitness, and whether their immune system is healthy, Lee says.

Sweating a lot is key

A person’s susceptibility to overheating can vary day-to-day, based on factors such as poor sleep, fatigue and sickness, Uniformed Services University’s O’Connor says.

“As I evaluate war fighters who have had heat strokes, anecdotally, the great majority of them had an underlying infection,” he says. “Colds, gastroenteritis, sinus infections, periodontal disease, blisters – these things automatically elevate your body temperature,” he says, so adding hot ambient temperature, or heat stress, on top of that is more likely to tip a person into the realm of heat illness.

Your risk for overheating also depends on what you’re trying to do in the heat – go for a walk with a friend or a long, fast exertional run – along with external factors like what the temperature is and whether they can reliably go somewhere to cool down.

And since the body relies on evaporating sweat to cool itself off, it matters how much someone has prehydrated, what they’re wearing and how humid it is.

But the bottom line is that you can’t improve your tolerance without breaking a sweat. “You've got to build up, but at a minimum, you’ve got to get out there and sweat,” O’Connor says. “If you're not inducing any sweating, that's not enough stress to induce the physiologic adaptations.”

Take it slow

The advice from National Singapore University’s Lee is to take it slow and easy, with long rest breaks in between. “The broad, five strategies we advocate would be aerobic conditioning, heat acclimatization, work-rest cycle, cooling and then hydration – in that order,” he says.

Lee says it’s preferable to acclimatize before the hottest days. One suggested protocol, he says: “Exercise every other day progressively in a hot environment. On Monday, go for a 15 minute walk, rest on Tuesday, go for 30 minute walk” on Wednesday, rest on Thursday – you get the idea.

Over several weeks of regular heat exposure, the body starts to adapt to cool down more efficiently. “The very clear one is an increase in your ability to sweat,” Lee says. As the sweat evaporates, it cools the body down.

Circulation improves and blood volume increases, so the heart doesn’t have to work as hard to pump blood through the body.

That gradual heat exposure will also help people learn how their bodies respond to the heat, and to drink water when they’re thirsty and slow down when they’re hot, Lee says – all things that help keep the core temperatures down.

Always watch for signs of overheating

Research shows it takes a few weeks of exposure to get used to exercising in the heat — and maybe just a week or so out of the heat to lose it.

So while heat training might help some, it won’t be a cure-all for extreme, sudden spikes in heat, which are getting more common with climate change.

Instead, it’s a piece of a larger strategy for how to approach the hot days safely. “When it comes to heat, stay cool, stay informed, stay hydrated and wise, and stay connected by checking on your friends, family and neighbors throughout heat season,” Arizona’s Livar says.

And if you’re out in the heat – keep an eye out for signs of heat illness in yourself or those around you. “If you’re having an excessive headache, excessive fatigue – this is really a time to tell yourself you’ve got to step back, cool down, slow your pace,” O’Connor says.

And watch closely for signs of cognitive changes that could indicate heatstroke.

“If you’re out with a friend and you notice that friend is less communicative, maybe has a staggered gait, appears to be confused, has a change in personality – that’s concerning,” he says.