How Elite Athletes Respond to Extreme Heat

At the 2016 world championships in Qatar, cyclists swallowed thermometer pills before competing. Here’s what scientists learned.

The first event of the 2016 UCI Road World Championships in Qatar was the women’s team time trial, a 40-kilometer race under the mid-afternoon sun in temperatures averaging 98.4 degrees Fahrenheit (36.9 C). That morning, three cyclists from one of the teams had swallowed ingestible core-temperature-sensing thermometer pills with their breakfast, as part of a study to investigate the effects of hot-weather exercise in real-world competitive conditions. When researchers later downloaded the data, the three women had peak temperatures during the race ranging from 105.4 to 106.7 degrees (40.8 to 41.5 C)—all higher than the 105 degree (40.5 C) threshold that’s considered a key diagnostic sign of exertional heat stroke. Yet they hadn’t collapsed. In fact, the team won a medal.

This data comes from a newly published study in the British Journal of Sports Medicine from researchers at the Aspetar Orthopaedic and Sports Medicine Hospital in Qatar. A total of 40 cyclists at the 2016 world championships in the individual and team time trials and the road race competed after swallowing thermometer pills. The results show that elite athletes in the wild push themselves in ways that are rarely seen in the laboratory.

Back in the 1990s, a series of studies suggested that we have a sort of thermal circuit-breaker that kicks in when we get too hot. If you ride a bike to exhaustion in a hot room, you’ll throw in the towel when your core temperature gets a little above 104 degrees (40 C). Given the potentially lethal consequences of heat stroke, most heat studies since then have enforced strict cut-offs at 39.5 or 40.0 C. Get that hot, and you’re pulled out of the study for safety reasons.

There are no such limitations in elite sports. And research since the 1990s has suggested that the thermal circuit-breaker isn’t as cut-and-dried as initially thought. Trained athletes, for example, seem to be able to push their core temperatures higher than sedentary people, indicating that it may be possible to reset your thermostat over time. And as for professional athletes, there simply hasn’t been much data on how their bodies respond.

The new Doha data confirms that elite athletes play by a different set of rules. Ten of the 40 races in the study produced peak temperatures above 104 degrees (40 C); none of the athletes displayed any negative symptoms or were treated for heat illness. Importantly, that doesn’t mean these temperatures are benign for everyone. Training at an elite level may help prepare your body to handle heat stress. And the two women’s time-trial athletes who recorded the highest temperatures of all in the study, both above 41 C, happened to have completed a nine-day heat acclimatization protocol in Qatar prior to the championships. They were uniquely prepared for the heat, in other words.

Interestingly, none of the men in the study got their temperatures above 41 C. But that may be linked to another anecdotal wrinkle: the two men’s time trial teams that participated in the study both consumed an “ice slurry beverage”—basically a sports-drink slushee—before their races. That may have helped keep their core temperatures lower; or alternately, it may have led to artificially lower temperature readings from the thermometer pills, which were presumably sloshing around in the stomach with the slushee. That’s unfortunate, but you can understand why the researchers didn’t try asking for volunteers to race the world championships with a rectal thermometer inserted. Sometimes you have to take the data you can get.

If you want to get a sense of the individual variations in temperature response, the graph below is informative. It shows the highest temperature reached for each performance in the study. TTT is team time trial; ITT is individual time trial; RR is road race. M and W are men and women; the little medal icons indicate medal-winners.

There’s one final pattern that’s worth pointing out. The time trials lasted roughly 45 minutes; the road races lasted a little over three hours for the women and almost six hours for the men. All the races took place during the heat of the day, under similar weather conditions. Which event would you expect to produce the highest core temperatures?

Here’s some representative data from a woman who competed in both the time trial and the road race:

It’s a pretty dramatic difference. The shorter, more intense race produces far higher core temperatures than the prolonged slog. That’s the opposite of what most people expect, given that we tend to associate overheating with dehydration. But the dominant factor in how hot you get is how intensely you’re exercising. The human body is like an internal combustion engine, in that it’s about 20 to 25 percent efficient. If you’re cycling at 250 watts, that means you’re also generating another 1,000 watts or so in excess heat. The shorter the race, the higher the power, and the more heat you’re able to generate. And if it’s a hot day, you won’t be able to get rid of that heat fast enough, which will cause your core temperature to rise rapidly.

That same pattern was one of the surprises I encountered in researching my book, Endure. Alberto Salazar is famous for having run himself nearly to death on several occasions. Once was at the 1978 Falmouth Road Race, a 7-mile race that took barely more than half an hour. Salazar ran himself into heat stroke and was read his last rites. Another time was the 1982 Boston Marathon—the famous Duel in the Sun with Dick Beardsley—where he was so dehydrated that he was given six liters of IV fluids at the finish. In the latter case, though, he didn’t have heat stroke. In fact, the finish line doctors thought he had hypothermia because his temperature was below normal. The point: heat stroke isn’t about dehydration, it’s (at least partly) about exercise intensity.

After Scottish marathoner Callum Hawkins collapsed during the Commonwealth Games earlier this year, I wrote about some research that suggests our perceptions of heat are blunted by the presence of competition. In a sense, the new cycling data corroborates that finding, showing that motivated athletes can reach “dangerous” levels of overheating in important competitions. The tricky part is determining when an athlete is actually in danger (like Hawkins) and when they aren’t (like the medal-winning cycling team). Core temperature alone doesn’t seem to be a reliable guide, so watch instead for other warning signs like dizziness, disorientation, and nausea. And perhaps the best advice of all is to emulate the cycling team’s acclimatization strategy: if you plan to push your limits in the heat, prepare your body in advance.

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My new book, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance, with a foreword by Malcolm Gladwell, is now available. For more, join me on Twitter and Facebook, and sign up for the Sweat Science email newsletter.