News reports and heat-related tragedies have been casting this summer as particularly hot and dangerous. How dangerous and deadly is heat exercise?
Some backstory: In the 1984 summer Olympics, marathoner Alberto Salazar set a new world record.
Not in marathon running, mind you—he placed 15th, despite going into the Olympics as a heavy favorite. He set the world record for sweating. Doctors calculated that Salazar was sweating at a rate of over three liters per hour, compared to one liter per hour for an average human, or just over two liters per hour for the average marathon runner.
Salazar’s high sweat rate was the result of heat training. In preparation for the games, which were expected to take place in extremely hot conditions, Salazar had put himself through a brutal regimen of high-temperature running sessions. His body adapted to those sessions in part by adapting to sweat more. And despite losing an alarming amount of fluids, Salazar’s vital signs were perfectly normal post-race.
While Salazar didn’t do very well in the 1984 games, other athletes have used heat training to tremendous effect—Haile Gebrselassie credits heat training for his victory in the 2009 Dubai marathon as well as his world record–breaking performances in Berlin in 2007 and 2008. Notably, he sweated at a rate of 3.6 liters per hour in each of those races.
Heat acclimation can be extremely effective if done correctly. Done wrong, it can lead to overtraining or even heatstroke. Here’s a look at how heat training works, and how to do it safely and effectively.
Heat Exercise & Training: The Science
Heat training has been demonstrated in numerous studies to improve aerobic exercise performance. In one famous 2010 study, heat acclimation improved power output in trained cyclists by 5 percent and increased blood plasma volume by 6.5 percent. Interestingly, cardiac output improved by 9 percent in cool temperatures and 4.5 percent in hot temperatures, whereas VO2max improved by 5 percent in cool temperatures and 8 percent in hot temperatures.
In another study from 2012, a five-day period of high-temperature training and dehydration was shown to improve rowing performance. This study was notable in that it deliberately dehydrated the subjects during their heat training sessions. The researchers found that athletes who didn’t drink water during training sessions improved their endurance and thermoregulation more than the control subjects.
There’s also significant anecdotal evidence for heat training. It’s notable, for instance, that most champion marathon runners live and train in hot climates, while the records themselves are almost always set in much colder climates, like Berlin, London, or Chicago in October.
Benefits of Heat Acclimation
Heat exercise and heat acclimation training help the body adapt to a wider range of temperatures, and prepares it to withstand the intense heat generated by outdoor endurance activities like marathons, cycling events, and obstacle course races. It appears to work via several distinct mechanisms.
1. Heat acclimation causes the body to lower its resting body temperature.
The body normally produces a large amount of heat just to keep warm; heat training causes it to adapt by producing less heat.
2. Heat acclimation causes the body to increase its sweat rate.
Heat-trained athletes may sweat 50 percent more than other athletes, or three times as much as amateur trainees. Heat-trained athletes also start sweating earlier, so that the body preemptively cools itself rather than waiting until it starts to become too hot.
3. Heat exercise increases blood plasma volume.
This helps the blood transport nutrients through the body, and may also help the body cool itself. It also protects the body against the dangers of dehydration—more plasma protects the blood from getting excessively thick as the body dehydrates.
4. The left ventricle of the heart strengthens, improving its output.
In other words, your heart pumps more blood, faster.
5. The body adapts on a cellular level.
Individual cells improve their heat tolerance, adapting to better withstand heat stress.
Is Heat Exercise Dangerous?
It certainly can be. I don’t need to tell you that athletes can and do die of heatstroke. And while dehydration is necessarily a part of heat training, it can be taken too far. Dehydration of over 7 percent of your body weight can put you at acute risk of death.
Not only can you get dehydrated during training, but the body can over-adapt, developing an excessive sweat response. A case study on Alberto Salazar states “his increased sweat rate was an unnecessary physiological adaptation to training in the heat.” In other words, his body cooled itself more than necessary, and shed more water than necessary.
That said, Salazar didn’t hurt himself, and endurance athletes often obsess over hydration status more than they need to. It turns out that if you just drink as much water as you want without monitoring your intake or following a set schedule, you’ll be fine, at least during a half-marathon. And although training adaptations cause you to sweat more, they also cause you to get thirstier during training—your body still regulates its hydration status quite well.
Excessive hydration without consuming electrolytes will cause hyponatremia, a potentially dangerous lack of sodium. You should favor sports drinks, like Gatorade, over water, or else take salt tablets during events lasting more than four hours.
Most studies have looked at marathons and half-marathons, or cycling, swimming, or rowing events of similar length. Spartan races can be much longer—as much as 24 hours—so you can get more dehydrated, and stay dehydrated for longer than marathon runners. That means you need to drink fluids whenever you’re thirsty, and take extra care to replenish both your water and electrolyte levels.
How to Incorporate Heat Acclimation into Your Training
The other downside of heat acclimation training is that the extra stress of training in the heat can lower your work volume. As such, I don’t recommend training all-out in extreme heat. Instead, I recommend the following two strategies.
1. Start spending about 20 minutes in the sauna after your workouts
Allow yourself to get a little dehydrated—don’t drink much, if any water during workouts, and only start rehydrating after your post-workout sauna session. Unless you feel faint of course, in which case hydrate away. By exposing yourself to heat after your workouts, you make heat acclimation safer while also keeping it from getting in the way of your training.
2. If you do want to try heat exercise, don’t do it with every workout
Instead, separate your workouts into high-intensity workouts in cool temperatures, and lower-intensity workouts in hotter temperatures.
Remember Alberto Salazar? He took this too far, running 180 miles a week, often in extreme temperatures. And while he didn’t die, he did suffer from chronic overtraining for many years, losing his chance at Olympic gold. Haile Gebrselassie, on the other hand, employed heat training intelligently and in moderation, and it made him a champion.
Ultimately, heat training is a useful tool for improving endurance, but it’s just another dial you can use to adjust training stress. As always, need to find your sweet spot—enough training stress to force your body to adapt, but not so much as to become overtrained.