Heat Acclimatization and its Effect on the Body

Different environmental conditions can affect the body’s physiological response to activity and exercise. Heat and humidity have long been associated with a significant reduction in performance due to the combination of cognitive, perceptual, cardiovascular, and metabolic mechanisms. Often athletes turn to heat training also known as heat acclimatization as a means of “getting used to” the heat and its effects on the body. This review will look into the effects of heat acclimatization (HA) on exercise performance, maximum oxygen uptake (Vo2 max), temperature regulation, plasma volume (PV), and other key metrics to determine its positive and negative effects and if it should be used in an elite athletes training regiment.

To begin we will look at mechanisms the body uses to help maintain thermal homeostasis. The two predominant systems for maintaining thermal homeostasis are vasodilation and increased sweat rate (Pethick et al., 2019). These two systems also allow the body to adapt to heat and elicit favorable physiological adaptations that should help increase or maintain performance through varying conditions. An article by Pokora et al., looked into the effect of medium-term sauna-based heat acclimation (MPHA) on thermophysiological and plasma volume responses to exercise in elite cross country skiers. They took 14 highly trained athletes and subjected them to Finnish sauna baths, for a total of 45 minutes after training sessions. Athletes were instructed to follow a 10-14 day acclimation period and follow the normal training sessions that occur during the transition phase of the annual training program (Pokora et al., 2021).

The sessions were designed to mimic the normal conditions of training in terms of both volume and intensity. The researchers were looking for differences in plasma volume, exercising HR, hematocrit, body, and localized skin temperature pre and post-sauna exposure. The independent variable was sauna exposure with two levels, pre, and post. The dependent variables were plasma volume, HR, body and skin temperature, physiological strain, heat tolerance, and maximal oxygen uptake. The aims of this study were twofold: the first objective was to investigate the effects of repeated sauna bathing on heat acclimation adaptations. And the second was to examine if repeated sauna bathing can affect physiological responses to exercise and the plasma water shift during exercise and recovery in elite cross-country skiers. They were able to conclude that the series of sauna sessions did not significantly affect either internal or external skin temperatures but did play a significant role by “lower heart rate at rest (by ~8 bs/min), lower SDP at rest, increased PV (by 7.42%), decreased TP in plasma (by 5.8%), and increased MCV (by 4.11%), but without a significant decrease in the core temperature or body and skin temperature” (Pokora et al., 2021).

An article by Pethick et al., in 2018 looked into the effects of short-term specific HA protocols on Plasma Volume, in international female soccer players. Eighteen non-HA trained subjects trained for 5 days in the heat and researchers documented their heart rate (HR), core temperature (Tc), sweat rate (SR), and perceptual ratings were recorded during all sessions. The researchers once again found significant differences in the Plasma volume and Hr values after just 3 days of training. This is consistent with a review done by Périard et al., in 2016, which stated “Within a week of acclimation plasma volume expansion occurs and heart rate is reduced during exercise at a given work rate. Core and skin temperatures are also reduced when exercising at a given work rate, whereas the sweating rate increases. Perceptually, the rating of thermal comfort has improved. As a result, aerobic exercise capacity is increased,” (Périard et al., 2016). This solidifies the notion that heat training can considerably improve the body’s response to heat within a short period. While many protocols differ in the length and intensity of HA, most physiological adaptations develop relatively quickly with 75–80% of the acclimation process occurring in the first 4–7 days (Périard et al., 2016).

Other studies, such as one done by Mee et al., in 2018 used similar testing protocols but attempted to find the effects of sauna on performance and other relevant internal and external metrics. They used a 5-day intervention twice throughout a heat acclimatization program to see the effects on a group of females. They also referenced data from Tyler et al., who in 2016 did a meta-analysis of a group of almost 20 studies and claimed sauna bathing to be the most effective passive method of incorporating heat training into an athletic training program (Tyler et al., 2016). This is due to the more intense uptick in internal body temperature, because of the surrounding water temperature. Results from the study by Mee et al., indicate that sauna exposure immediately before controlled HA training helped reduce thermoregulatory, cardiovascular, and perceptual strain during exercise in hot/humid environments. There was a 9% plasma volume expansion when participants had a sauna exposure immediately before HA. This plasma volume expansion is similar to that observed by others such as Neal et al., in 2016 and Garrett et al. back in 2009. And while this study in particular wasn’t looking at elite athletes it is still useful in identifying HA training procedures that elicit measurable differences in subjects.

Elite athletes generally do not have as drastic changes in Hr or Plasma volume through heat training because of the way their bodies are trained to adapt quickly to stimuli. Research from ‘real world’ testing suggests that if heat acclimatization is applied at the correct time, like a preseason training camp or during a low volume phase (Casadio et al., 2017), endurance gains in cool conditions are possible. There is no doubt, that heat acclimatization will enhance performance in hot conditions, but the effects when performing in cooler conditions are still rather lackluster. For athletes who train in cool climates and have to perform in warm climates, a heat acclimatization protocol for the days previous will lead to better performance.

Citations

Casadio, J. R., Kilding, A. E., Cotter, J. D., & Laursen, P. B. (2017). From Lab to Real World: Heat Acclimation Considerations for Elite Athletes. Sports Medicine, 47(8), 1467–1476. https://doi.org/10.1007/s40279-016-0668-9

Mee, J. A., Peters, S., Doust, J. H., & Maxwell, N. S. (2018). Sauna exposure immediately prior to short-term heat acclimation accelerates phenotypic adaptation in females. Journal of Science and Medicine in Sport, 21(2), 190–195. https://doi.org/10.1016/j.jsams.2017.06.024

Périard, J. D., Travers, G. J. S., Racinais, S., & Sawka, M. N. (2016). Cardiovascular adaptations supporting human exercise-heat acclimation. Autonomic Neuroscience, 196, 52–62. https://doi.org/10.1016/j.autneu.2016.02.002

Pethick, W. A., Stellingwerff, T., Lacroix, M. A., Bergstrom, C., & Meylan, C. M. (2018). The effect of a team sport-specific heat acclimation protocol on plasma volume in elite female soccer players. Science and Medicine in Football, 2(1), 16–22. https://doi.org/10.1080/24733938.2017.1384559

Pethick, W. A., Murray, H. J., McFadyen, P., Brodie, R., Gaul, C. A., & Stellingwerff, T. (2019). Effects of hydration status during heat acclimation on plasma volume and performance. Scandinavian Journal of Medicine & Science in Sports, 29(2), 189–199. https://doi.org/10.1111/sms.13319

Pokora, I., Sadowska-Krępa, E., Wolowski, Ł., Wyderka, P., Michnik, A., & Drzazga, Z. (2021). The Effect of Medium-Term Sauna-Based Heat Acclimation (MPHA) on Thermophysiological and Plasma Volume Responses to Exercise Performed under Temperate Conditions in Elite Cross-Country Skiers. International Journal of Environmental Research and Public Health, 18(13). https://doi.org/10.3390/ijerph18136906

Tyler, C. J., Reeve, T., Hodges, G. J., & Cheung, S. S. (2016). The Effects of Heat Adaptation on Physiology, Perception and Exercise Performance in the Heat: A Meta-Analysis. Sports Medicine, 46(11), 1699–1724. https://doi.org/10.1007/s40279-016-0538-5