Imagine this: you have been training regularly, you are feeling fit, and getting ready for the Ironman world championships. You fly into Kona, get off the plane, and you just lost 25% efficiency - you have lost your race and not going to make your target time before you even start. Why? Because you are not properly acclimated to the new hot climate!
By not being properly acclimated, you can lose up to 25% efficiency, as shown in the graph below (1). Everyone is different - how long it takes to get used to a hot climate, how much efficiency is lost without proper acclimation, and how fast acclimated effects are lost always depend on every specific individual. However, there are common issues that affect everybody and which you simply cannot avoid.
We can make an analogy about altitude acclimation, although sports scientists don't like this comparison as the physiological adaptations are very different. When you go higher and higher, less oxygen is available in the blood. Your brain then prioritizes the blood flow towards the vital organs, diverting it away from the muscles, meaning less power can be produced than at lower altitudes. After spending a couple of days at higher altitudes, the body begins to acclimate and performs more efficiently in those conditions.
Hotter conditions can be analogous to altitude. When you get hotter, blood is diverted for cooling, so less oxygen reaches the muscles for power generation. The main performance blocker is simple to understand: VO2max decreases if the body is exposed to heat stress.
The solution to this problem is simple: you need to spend more time in hotter conditions, to limit the effects of overheating and help the body adapt and become more efficient.
What's the best way to get good heat acclimatization?
The best way to properly acclimate heat is to spend time and train (up to a month) at the location in those conditions. Therefore, you need to just live in Kona for a period of 2-4 weeks before the event takes place, doing a daily training of 100 minutes in a hot environment (artificial or natural). However, as effective as this may be, it is not an option for most of us.
There are many other techniques used to heat acclimate before an event including training indoors without ventilation, training in a sauna, or hot baths. For instance, heat shock triggers the emission of HSP (heat shock proteins), whose production helps to prevent heat stress. If the body is frequently exposed to such heat stress (e.g. sauna), it increases the tolerance level, which increases your time to exhaustion. Many other different techniques and strategies are documented, such as a paper describing the most relevant ones is referenced at the bottom of this article.
What is the best way for you to acclimate to heat? That will be up to you and your coach to come up with the best strategy given the resources at your disposal.
How long does it take to acclimate?
Following the research mentioned above, it takes around 14 days to fully acclimate to hot conditions. However, most effects start after 2 days. Core body temperature is an early indicator of proper heat acclimatization (from day 2 to 4).
As shown in the graphic above (1), acclimatization helps to increase the VO2max level again, as well as improve the sweat rate, plasma volume, core body temperature, skin temperature, heart rate, and stroke volume.
Should you perform heat acclimation prior to an important event? Well, you just said it was an important event, so the answer should be pretty clear.
(1) Sébastien Racinais, Michael Sawka, Hein Daanen, and Julien D. Périard (2019): Heat Acclimation, in Heat Stress in Sport and Exercise (Julien D. Périard • Sébastien Racinais Editors), Springer, p. 159-179.