What is Heat Acclimatization and How Can We Use It in Training?

If you've been on a tropical vacation and spotted someone wearing a three-piece suit, looking cool as a cucumber, while you sweat away in your shorts and flipflops, you’ve probably wondered about heat acclimatization without even knowing it.

Brock Armstrong
8-minute read
Episode #379

Photo of an athlete in the heat pouring water on his head.

In my recent article, Is It Ever Too Cold to Exercise Outdoors?, I mentioned some of the physiological adaptations that were possible to get by training in hot conditions. That idea seems to have peaked some of your curiosity. I received a number of questions asking exactly what heat adaptation means, what the benefits are, and how to go about getting them.

I’ll get to that in a minute, but first a story:

A few years ago, I ran a marathon in Chicago in hopes of qualifying for the Boston Marathon. I had trained hard and was prepared to give it my all. I was on pace to achieve my goal time but the sun was getting high in the sky, the temperature was rising, and so was my heart rate. I was only nearing the halfway mark, so there was no way I was going to drop my pace. I dug deep and pressed on. But then I started to get worried as runner after runner in front of me dropped their pace and dumped more and more water over their heads at every aid station. Finally, when a very formidable runner that I had been running near for about 20 kilometres suddenly careened wildly across the road and then proceeded to face-plant on the cement, I decided it was time to reconsider how hard I was willing to push for this BQ (Boston Qualifier).

I finished the race, and after a lengthy lounge under a shady tree in the park, I got myself back to the hotel and took a cold shower. I then found out that between the time that I arrived at the starting area and the time that I crossed the halfway point, the temperature had gone from 59°F  (15°C) to 80°F  (27°C).

Later that night I was sipping a cold beer, exhausted but no worse for wear, I couldn’t help but wonder how that obviously well-trained runner pushed herself to the point of face-planting.

Your Body and Heat

Normally, your skin, blood vessels and sweat levels adjust to the heat. But these cooling systems will eventually fail if you exercise in high temperatures (and high humidity) for too long.

When you exercise in the heat, your body circulates blood away from the core and closer to the skin (aka sweat), where it can be cooled more effectively by maximising the thermogenic effects of evaporation. But by shunting your blood to your skin, that draws oxygen away from your working muscles while also lowering the amount of blood that your heart can pump with each beat. Because you are losing fluid you are also losing blood volume and this places additional demand on your already hard-working heart. This gets even worse if it is humid outside. In a high humidity situation sweat won’t evaporate from your skin and that, in turn, pushes your body temperature even higher.

Normally, your skin, blood vessels, and sweat levels adjust to the heat. But these automatic cooling systems will eventually fail if you exercise in high temperatures (and high humidity) for too long.

As blood (and fluid in general) becomes a precious commodity and your heart is forced to work harder and faster, your heart rate gets higher and higher, your breathing becomes faster, your blood pressure drops, and your core temperature rises.

If you keep going past this point, eventually, your body will send a message to your brain to “stop this madness,” and that is when exercise goes from being fun to uncomfortable or difficult to straight-up impossible. That is clearly what happened to that poor face-planting marathoner.

Dangers of Exercising in the Heat

According to the Mayo Clinic, Heat illnesses include:

  • Heat cramps: sometimes called exercise-associated muscle cramps, which are painful muscle contractions that can occur with exercise.
  • Heat syncope: a feeling of lightheadedness occurring after standing for a long period, or standing quickly after sitting for a long time.
  • Exercise-associated collapse: feeling lightheaded or fainting immediately after exercising, which can occur especially if you immediately stop running and stand after a race or a long run.
  • Heat exhaustion: when your body temperature rises as high as 104 F (40 C), you may experience nausea, vomiting, weakness, headache, fainting, sweating and cold, clammy skin.
  • Heatstroke: a life-threatening emergency condition that occurs when your body temperature is higher than 104 F (40 C). At this point, your skin may actually be dry from lack of sweat.

Incidentally, later that night in Chicago, my friend and I were endlessly amused by the uncontrollable twitching and pulsing that my calf muscles were doing (a thing called fasciculations). My calves fully cramped (very painfully) a few times and the constant twitching kept me up much of the night and honestly started to freak me out a little. It really looked like a swarm of calf-snakes were trying to burst through my skin. This was likely just due to dehydration and a slight electrolyte deficiency.

Looking back now, had I known that this cold-weather-Canadian was going to be racing in what I would consider hot weather even for “Vacation Brock,” I would have absolutely added some heat adaptation protocols to my training program. In fact, two years later I did just that while training for an Ironman Triathlon I was racing in Thailand in December.

The Science Behind Heat Adaptation

Even the Mayo Clinic says that if you're used to exercising indoors or in cooler weather, it can take at least one to two weeks to adapt to the heat. In a 2010 research study called A heat acclimation protocol for team sports, they saw a noticeable difference in just ten days. After only four 30-45 min sessions of intermittent exercise in 30°C, with 27% relative humidity, the 17 well-trained female athletes showed a lower core temperature and an accompanying rise in thermal comfort which resulted in an increase in their overall exercise capacity.

In a more recent study on the physiological responses to overdressing in 13 well-trained runners, they saw similar advantages from simply wearing an uncomfortable number of layers while training. In this study, they tested the heart rate, core temperature, mean skin temperature, sweat rate, and extracellular heat shock protein in 13 (seven male, six female) well-trained runners. They had the runners run on treadmills for about 60 minutes and compared physiological and cellular responses to exercise in 40°C, with minimal clothing and in 15°C while wearing five bulky layers, including a wool hat, winter mittens, a rain jacket, and plastic pants.

The overdressed runners didn't entirely overheat the way they did in the actual hot conditions, but it still triggered some of the desired adaptations. Ten of the runners reached a core temperature of 38.5°C, which is the magic temperature where heat adaptation begins.

Another study compared heat acclimation to pre-cooling by applying icy towels to nine amateur 5k runners’ head and neck, dunking their hands and forearms in nine degree Celsius water, and strapping ice packs to their chests, backs, and upper legs for 20 minutes. This precooling protocol gave the runners a 3.7 percent improvement in performance. But only five days of 90-minute training sessions that kept their core temperature above 38.5°C, led to a 6.6 percent improvement in performance. And interestingly, doing both the heat adaptation and the pre-cooling led to a seven percent improvement.


All content here is for informational purposes only. This content does not replace the professional judgment of your own health provider. Please consult a licensed health professional for all individual questions and issues.

About the Author

Brock Armstrong Get-Fit Guy

Brock Armstrong was the host of the Get-Fit Guy podcast between 2017 and 2021. He is a certified AFLCA Group Fitness Leader with a designation in Portable Equipment, NCCP and CAC Triathlon Coach, and a TnT certified run coach. He is also on the board of advisors for the Primal Health Coach Institute and a guest faculty member of the Human Potential Institute.