It seems a bit funny to be talking about heat acclimation in March, given that most us are still trying to decide if we should risk running in the snow/sleet/rain. However, over the next several weeks the Marathon des Sables, the Libyan Challenge (cancelled this year) and several adventure and stage races in Central and South America will get underway. 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”?

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 5C above 12C. 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.

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

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 haemoglobin (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. 

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 litres!

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.

The work needed to achieve this benefit is reasonable. Most laboratory based heat acclimation protocols have athletes sped 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.

In the next post, I’ll talk a bit more about the specific protocols that can be used, and outline what I did to get ready for the Marathon des Sables – a multiday footrace through the Sahara.

• 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 , Y Ando , S Takeuchi , K Takemura , N Okuda , Y Isobe , S Takaba,  K  Ohara . Effects of Heat Acclimation of Distance Runners in a Moderately Hot Environment Eur J Appl Physiol 45, 189-198 (1980)

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