Heat Acclimation for Runners

Endurance Science logoIt seems a bit funny to be talking about heat acclimation in April given that many of us are still trying to decide if we should risk running in the snow/sleet/cold rain. However, the Marathon des Sables is currently in progress (temps over 120F/50C this year) and several adventure and stage races in Central and South America are about to get underway. (It’s also the season for the Libyan Challenge, but that event was canceled this year.) All of these races take place in hot environments. A question that often comes up for competitors, particularly if they live somewhere that is currently in the grip of winter/spring, is “how can I prepare for the effects of heat and humidity”?

In the following article I will explain how heat affects running performance, what happens during heat acclimation, and methods of acclimating to the heat, including my own experience with heat acclimation. We invite you to share your experiences with heat acclimation and hot weather events by leaving a comment.

How Heat Affects Running Performance
There is little doubt that exercise performance is impaired in hot environments. While the effect of heat on performance varies with the sport (for example, less effect on cycling than running), there is a great deal of empirical data showing a link between ambient temperature and performance. Various authors have suggested performance impairments of between 1.6 and 3% in marathon times for every 10 degrees above 55 degrees Fahrenheit. Below is an interesting table from a paper by Scot Montain and colleagues at the US Army Research Institute of Environmental Medicine illustrates the relationship between elite marathoner finishing times and course temperature in the New York City Marathon.

New York City Marathon Top Times Temperature

New York City Marathon: Top Times vs Temperature

The effect seems to be less dramatic for faster runners. The following table from a 2007 paper by Matthew Ely demonstrates this nicely:

Fast slow marathoners heat

Effect of heat on slower vs faster marathoners.

Why are we slower in hot conditions? There are a variety of proposed mechanisms, but the one that is most widely accepted is based on cardiac output limitations.

When we exercise, we produce a great deal of heat. One of the principle ways that we get rid of this excess heat is through sweating (evaporative heat loss), as well as conduction and radiation of heat from our skin. To achieve this, our bodies have to send a considerable amount of blood to the skin. This blood is therefore not available to perfuse working muscles and deliver oxygen to them. So a portion of our blood volume is essentially no longer able to participate in oxygen delivery and energy formation in our exercising muscles. The greater the amount of heat that we need to dissipate, the greater the proportion of blood that is diverted to the skin (up to a point – this can’t increase forever).

What is necessary for cooling isn’t the hemoglobin (the red blood cells in blood) but the plasma, which is essentially water with a number of different proteins and electrolytes in it. However, your body can’t separate the red cells (which are the oxygen carriers) from the plasma – they all go along for the ride to the skin.

How We Acclimate to Heat
If it is plasma that is the essential cooling component, is it possible to improve this problem by increasing our total plasma volume? Yes, and that is exactly what happens as we adapt to heat over time. Whether you acclimate naturally to higher temperatures over the course of a season, or in a heat chamber, the most significant change that occurs is an increase in plasma volume. Other things occur as well (such as changes in sweat sodium concentration, resting core temperature and heart rate), but plasma volume expansion is the key. After extensive acclimatization, plasma volume can have expanded by as much as 2 liters!

This may explain why the fittest athletes adapt to heat stress more quickly than the less fit. One of the by-products of endurance training (especially at high intensities) is an increase in plasma volume. So just by training hard, you can derive some amount of heat acclimation. What about specifically training in a hot environment to improve performance in a hot race? There is extensive evidence that it is possible to improve our performance in hot environments by training in similar conditions prior to competition. Several studies have demonstrated performance improvements in terms of maximum work rate, perceived exertion, time to failure at submaximal work rates, and time to complete a specific distance.

In the last few paragraphs we’ve explored a bit about the effect of heat on performance (bad) and the effects of acclimation on this (good). The most meaningful physiological adaptation that occurs is an increase in plasma volume (a lot like adding more radiator fluid to a car). However, there are some other adaptations that occur – changes in sweat rate, changes in sweat sodium concentration and changes in core resting temperature, to name a few. The various adaptations occur with different amounts of acclimatization. Here’s a graphical representation of the times over which an athlete can gain these benefits:

Heat Acclimatization Changes runners

A timeline of various heat acclimation adaptations.

Heat Acclimation Methods and Considerations
The work needed to achieve the benefits heat acclimation is reasonable. Most laboratory based heat acclimation protocols have athletes spend about 1 hour a day in a heat chamber for 7-10 days. Importantly, this needs to occur as close to the time of the competition as possible, as the adaptations conferred by acclimation decay rapidly without ongoing exposure. So there’s no point in spending 2 weeks in a heat chamber a month before the race – the effects will decay in 1-3 weeks.

As previously mentioned, the benefits of heat acclimation decay rapidly if you do not maintain heat exposure. Estimates vary, but it’s possible that you could lose half of the benefit in 10 days without ongoing heat exposure. This raises some logistical problems for athletes living in cold environments who are attempting to acclimate for a hot weather event. To benefit maximally from acclimatization, the heat training sessions should occur as close as possible to the event. That seems pretty straight forward. The problem is that acclimation is quite physically demanding, and most athletes attempt to taper in the week(s) prior to a big race. So, if you want to acclimatize optimally, it needs to occur during your taper – which may cause overtraining, or at least minimize the benefits of tapering.

As with all training, the more specific, the better. When it comes to heat acclimatization this means that your training climate should reflect the competition environment as closely as possible – the same temperatures as well as humidity. Why is humidity important? As anyone who has survived an East Coast summer knows, humidity makes it harder to lose heat via sweating. Training in a humid environment does confer some benefit it you are training for a dry, hot race, but not as much as training in a dry, hot chamber. Interestingly, there is better “transfer” of acclimation if you train in a dry, hot climate and then race in a humid, hot race than the other way round. So, as much as possible, match humidity and temperature of your acclimation phase to your race environment.

What about passive acclimation? That is, will sitting in a sauna at the YMCA get us ready for running in Death Valley? Essentially – a bit, but not much. Acclimatization is vastly greater (and more rapid) is you exercise during the heat exposure. Whether this is again the principle of specificity, or whether it is simply that core temperature rises faster with active acclimation (increased core temperature is probably the stimulus for the adaptions that occur) is not clear.

My Experiences
To address these problems, I worked with the exercise physiologists at Simon Fraser University’s Environmental Physiology Unit. They put together a low intensity heat acclimation program specifically tailored to my needs (the Marathon des Sables in Morocco). In brief, I ran on a treadmill at 50-60% VO2max for 45-75 minutes over 9 sessions. The first 3 sessions were at 95F (35C) and the next 5 were at 113F (45C). In the final (9th) session the temperature was reduced back to 95F (35C). This allowed us to compare my physiological data from the first and the last sessions. Here are some of the data we recorded. I think that you will agree that some real benefits were seen.

Borg perceived exertion

Change in perceived exertion after 9 days of heat acclimation.

The Borg Scale above is used to assess subjective overall effort – I clearly felt that the same speed was easier in the heat after 9 days. The “thermal comfort score” below shows that I was bothered less by the heat after acclimation.

Thermal comfort rating heat acclimation.

Change in thermal comfort rating after 9 days of heat acclimation.

One of the most important ways to prevent heat injuries and to maintain performance in extremely hot environments is to drink adequate amounts of fluid. This seems obvious, but it is actually surprisingly hard to do this if you are focusing on running. I think that one of the most useful benefits (for me – not living somewhere hot) was learning to drink earlier and more frequently. That I learned this lesson is well demonstrated in the graph of fluid balances on Days 1 and 9 below.

Fluid balance heat acclimation

Change in fluid balance after 9 days of heat acclimation.

Finally, what happened to heart rate? As you can see below, my heart rate at the same speed and temperature decreased by about 9% – a very significant improvement. Now, it didn’t get as low as what I would expect it to be in “normal temps,” but the improvement was significant, and could be expected to translate into a tangible benefit in competition.

heart rate heat acclimation

Change in exercise heart rate after 9 days of heat acclimation.

So, in my experience, heat acclimation based on well documented scientific principles can give athletes a significant performance enhancement in hot environments. However, it is important to recognize the effect of acclimation on the tapering period and to plan accordingly.

* * * * *

William Henderson, MD, FRCPC is a critical care physician and exercise physiologist who provides coaching and exercise physiology testing services. Articles like this and many more can be found at EnduranceScience.com.


  • S Montain, M Ely and S Cheuvront. Marathon Performance in Thermally Stressing Conditions. Sports Med 2007; 37 (4-5): 320-323.
  • Ely MR, Cheuvront SN, Roberts WO, et al. Impact of weather on marathon-running performance. Med Sci Sports Exerc 2007; 39 (3): 487-93.
  • Y Kobayashi et al. Effects of Heat Acclimation of Distance Runners in a Moderately Hot Environment. Eur J Appl Physiol 1980; 45, 189-198.
  • Armstrong LE, Maresh CM.The induction and decay of heat acclimatisation in trained athletes. SportsMedicine 1991;12(5):302-12.
  • ME Clegg, S Ghaffari, ML Walsh, W Henderson and MD White. A case study: Acclimation for the Marathon des Sables. Canadian Society for Exercise Physiology, Vancouver 2009.

There are 20 comments

  1. Andrew

    Awesome! Clearly explained and quite helpful! If you'd be willing, in the future, to share with us any further studies [that have increased sample sizes, perhaps?] that'd be excellent too :)

    Thanks for all the great info, specific recommendations, and providing the figures/reasons behind your conclusions :)

  2. Josh

    So if you heat acclimatize for a spring/early summer race does that also cause you to loose your cold "tolerance" that you've built up over winter. The reason I ask is that we don't always know what race day will bring. So often spring races can be unseasonably hot (heat acclimation would be good) but what if its unseasonably cool and you've just spent the last weeks heat acclimating are we now at a performance disadvantage?. I know it easier to just put on more clothes to stay warm but I wonder if you can have the best of both worlds. Maybe heat acclimate like you propose (while exercising) but also go without that extra layer of clothing during non exercise periods to keep the cold tolerance. Thoughts?

  3. William

    Thanks Andrew for the kind words – I'll post more data in the future. I'm trying to get a sense of how much science people want…

    Josh, fortuantely cold and heat acclimation are independent (you can acclimate to both at the same time). They don't "cancel out". To be honest if the race is so cold that you can't warm up by running, it's pretty darn cold. Cold acclimation isn't very well studied, and effects on performance aren't as well understood as heat.

  4. Stu

    Started serious running here in Malaysia a couple years ago. Ran my first marathon last November (Penang Bridge Intl Marathon) with a start time of 1:30am. When I made a trip to the US last month I noticed significantly better pace and set some fastest times for 15 mile long runs in 55 to 60 degree weather. Now I'm back in Malaysia training for my second marathon, Borneo Intl Marathon on May 2nd. I start my long runs by 5am and go through 4 22oz bottles of water and rehydration mix (stop in at a 7-11 along my route to replenish). By 7am (sunrise)it's close to a very humid 90F.

    The warm humid climate affects shoe fit as well. I have to go a half size bigger over here.

  5. Chris Barber

    This is a great article with a lot of good, scientific information. I never knew that it was better to train in a hot, dry climate and race in a humid climate than the other way around. Maybe it's because I have always trained in hot, humid climate and have found this more difficult than running dry climates. I have found that when dealing with heat and sweat the best defense is to have a lot of sodium in your system. I learned this the hard way though. Read about it here: http://www.seriousrunning.com/blog/running/salt-i

  6. george aber

    Some pretty good information here, but you make no mention of one vital ingredient in the acclimitazation recipe: the change in metabolism whereby the body, after the third week of adaptation, shifts substrate use from CHO to FFA (carbohydrate to free fatty acid). This is important to note since it represents a major characteristic of the acclimitazation process.


  7. Humidity. . . ugh | Run With Me

    […] The greater the amount of heat that we need to dissipate, the greater the proportion of blood that is diverted to the skin and therefore not available to supply working muscles and deliver oxygen to them. (Dr. Bill Henderson in his article, “Heat Acclimation for Runners”) […]

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