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Heat Acclimatization

Summer running. You either love it or hate it, probably depending on where you live. In areas where temperatures and humidity levels begin to soar, runners may notice a drop off in pace and enjoyment as their bodies adapt to this new stressor. This period of adaptation is called heat acclimatization. It’s not your imagination. Running at the beginning of summer may feel more difficult. Your usual all-day pace might get slower despite feeling like the same amount of effort. The good news is, the worst of the misery is temporary. Your body is going through some interesting changes to help you become more resistant to higher temperatures. These adaptations are important for your safety, as heat-related illnesses such as heat exhaustion and heat stroke are actually some of the more dangerous aspects of running. 

Aside from just turning your body into a more heat-tolerant machine, heat acclimatization has a big added bonus. Much like training at altitude and racing at a lower elevation can lend a performance boost to your running, training in heat and racing at a cooler temperature can potentially lead to some PR-worthy performances. 

What exactly happens to your body during heat acclimatization and how long can you expect to suffer through it? Grab a cold drink and let’s see what science has to say about it.

How long does heat acclimatization take? 

Full heat acclimatization takes about 14 days, with the majority of the changes happening in the first week. Not everyone will adjust to the heat at the same rate. Age and gender, surprisingly, have been found to not be the biggest factors in how long it will take you to acclimate. The exact length of time is determined mainly by your fitness level. People who are more fit tend to adapt more quickly. If you’ve built up a high VO2 max in cooler months, it will typically translate into a quicker acclimatization.

 How should you approach your workouts while you are adapting to heat?  You may want to ease into hot weather running. Cut back a little on mileage and definitely cut yourself some slack on pace for a couple of weeks. This will reduce your risk of heat-related muscle cramps or heat exhaustion. As a bonus it’ll probably make you hate your runs less. Passive heat exposure (ie hanging out in your backyard) will also give you a heat acclimation boost, but some solid workouts will definitely speed up the process. 

What happens to your body during heat acclimatization?

Decreased heart rate and increased plasma volume – Your body will begin to conserve more sodium as it adapts to the heat. This allows you to retain more fluid and increase your blood plasma. Your heart rate also begins to slow down and become more efficient at moving the blood into both muscles and capillaries in your skin for more effective performance and cooling. 

Decreased core body temperature- During heat acclimatization your body starts to get more efficient at maintaining a lower body temperature. This will help your perceived exertion feel less extreme and make you a little more comfortable (if you are the type of person who can ever describe running as “comfortable”). 

Earlier onset of sweating-   Obviously the hotter it is during your workout, the more your average person is going to sweat. As you adapt to the heat, you also begin to sweat sooner in your run. Humidity levels will play a big part in just how much you sweat and how effective that sweat is at cooling your body. In humid heat you will likely sweat more.

How long will you keep your heat adaptation? 

You lose about 2.5% of your heat adaptation benefits per day of no heat exposure. For this reason you will want to make sure that you are still getting in some hot workouts while tapering for a race if you are trying to use heat acclimatization as a performance tool.  Fortunately, spending time in air conditioning will not ruin your acclimatization. Just as fitter individuals gain their heat adaption more quickly, they also are better at retaining it. So keep up your workout routine and you’ll enjoy these performance benefits longer. 

Love it or hate it, we have some hot months to run through. Keep at it and acclimate like a champ. Fall will be here before you know it and you can look forward to some cooler runs with some great performance enhancements thanks to heat acclimatization. 

Kara Sasser is a USAT Level 1 Triathlon Coach with TriCoach Georgia, a Boston Qualifier and top 10 AG Ironman finisher. She is part-owner of sports nutrition company SlayRx.com and enjoys writing about all things health, fitness and outdoors. 



Périard JD, Travers GJS, Racinais S, Sawka MN. Cardiovascular adaptations supporting human exercise-heat acclimation. Auton Neurosci. 2016;196:52-62. doi:10.1016/j.autneu.2016.02.002


Armstrong, L.E. (1998). Heat acclimatization. In: Encyclopedia of Sports Medicine and Science, T.D.Fahey (Editor). Internet Society for Sport Science: http://sportsci.org. 10 March