Does Heat Training Actually Improve Endurance Performance?

Key Points:

• Heat training can increase performance significantly in hot conditions and a lesser amount in cool conditions.
• Heat training gains vary greatly between studies. Dependent on protocol design and management of overall training load.
• The rate of re-acclimatization and decay of heat adaptations is important to consider when tapering for events.

Heat training has gained huge popularity in recent years. This could be linked to the Tokyo Olympics in 2021. Due to coronavirus travel restrictions, many athletes could not travel to hot climates to undergo training camps. Instead, at-home heat training protocols were implemented to prepare themselves for the high temperatures of Tokyo [1]. This article will explore current scientific research to identify what performance gains can be expected from heat training. We also explore the reasons why some studies show no performance increase and what we can learn from them. Finally, we will explore if re-acclimatization is more rapid than initial acclimatization.

Does it work? - Research Overview

Due to most heat training studies being done with a relatively small sample size (20 participants or less), we're going to look at a metanalysis. This combines multiple different studies, creating a much larger effective sample size, therefore increasing statistical power.

A metanalysis on the effects of heat adaptation pooled 135 studies and found that the most common length of heat training protocol was 7-14 days [2]. Amongst the studies, they found that there is considerable heterogeneity within the data for most variables. Meaning that the improvements measured in different variables (such as performance, sweat rate, and plasma volume) vary quite significantly between studies. However, in our opinion, this is likely due to differences in the protocol (frequency, duration, intensity) and study populations (untrained or trained individuals). Suggesting that the specific protocol used is important in achieving the biggest performance increase. Additionally, it may be due to not accounting for participants that have previously been acclimatized to the heat (either by specific heat training or living in a warm climate). This is because re-acclimatization to heat is much more rapid than initial acclimatization, which we will discuss in more detail below. The authors concluded that heat training improves exercise performance in the heat, with longer protocols likely being more effective than shorter ones.

How much gain can be expected? - Individual Studies

Now that we have a solid backing of 135 studies that show heat training can improve exercise performance, we can investigate individual studies to see the actual quantitative gains you can expect.

A study that took 12 well-trained cyclists (average VO2 Max = 67) through a 10-day heat training protocol found significant performance increases in both hot (38°C) and cool (13°C) conditions [3]. For one-hour power, participants gained +6% in cool conditions, and 8% in hot conditions. No changes were seen in the control group.

The heat acclimatization protocol was over 10 days. Each day (consecutive days/no rest days), participants cycled in 40°C, performing 2x45mins @50% of VO₂max with 10 minutes rest between intervals. If you haven't had VO₂max lab testing, this intensity is roughly equivalent to 50% of your six-minute power. This is an easy intensity (under normal conditions), usually in Zone 2 of a 5-Zone model, and below LT1 (Lactate Threshold 1). This intensity was chosen as it would be enough to induce heat adaptations but not training adaptations in these athletes; therefore, the results are more likely to be from heat training alone. Our opinion is that 2x45mins is unnecessary and excessive. The studies are designed to guarantee strong heat stimulus for research purposes, not to be optimal for athletes. The risk of dehydration and heat injury vastly outweighs any potential benefit (if any) that would be gained from riding this long in each session. However, specifics aside, it is reassuring that other studies (with crossover design and/or control group) show very similar levels of performance increase [4]. Our estimations for performance gains with a well-designed heat training protocol can be found in Figure 1.

Does it always work? - Studies showing no gain

Some studies show no improvement in cool conditions from heat training. As mentioned above, the performance increases in cool conditions are always lower than the increases in hot conditions. This is obvious and to be expected because the adaptations mostly help cool the body. Why can some studies show no improvement in performance even whilst proving physiological adaptations?

A five-week long heat training study that included 21 well-trained cyclists (average VO2 Max = 58) showed physiological adaptations such as plasma volume expansion, increased sweat rate, and lower heart rate in hot conditions [5]. Time trial performance in cool conditions (14°C) increased by 5-6% in the control group (no heat training) and the heat training group. This led the authors to conclude that heat training was not superior compared to normal training for improving aerobic power in cool conditions. They also concluded that the increases in blood plasma volume and thermoregulatory benefits did not outweigh the potential drawbacks (potential hemodilution). In our view, it's possible that the five-week protocol didn't improve TT performance in cool conditions because plasma volume expansion slightly diluted haemoglobin. Basically, the plasma volume expansion slightly diluted hemoglobin concentration, so oxygen delivery per unit of blood was unchanged (See Figure 2). A shorter protocol (≈14 days) might have increased cardiovascular adaptations without this dilution, potentially producing small gains even in cool conditions. Although, our opinion is that it is unlikely that blood dilution is the real issue here, as similar heat training protocols (five-weeks) have shown increases in haemoglobin mass [6]. This would negate any dilation of the blood that is occurring and preserve/increase oxygen carrying capacity. Additionally, even without an increase in haemoglobin mass, the higher blood plasma volume usually maintains or improves oxygen carrying capacity due to an increase in cardiac output. Therefore, the lack of difference in outcomes between the heat group and the control group can more likely be explained by poor study design.

It is possible that the five-week protocol did not improve cool-condition time-trial performance because athletes were told to maintain most of their normal training alongside heat sessions (including high intensity sessions). The additional physiological stress from daily heat exposure may have limited recovery and blunted performance gains, even though thermoregulatory adaptations occurred. Therefore, they would have been more fatigued during their normal training sessions. Although participants were instructed to replenish 150% of the total fluid loss per session in the hours after cycling, there was no way of checking compliance. If diet and rehydration were suboptimal, in some individuals this may have resulted in chronic fatigue, dehydration, and electrolyte imbalances. Therefore, a lot of the heat group subjects were likely more fatigued on the day of the testing than the control group subjects.

On the other end of the spectrum there are also heat training protocols that are likely too short to elicit meaningful performance increases. A five-day long study found no increase in time-trial performance in hot conditions, showing that five days is likely too short for heat training [9]. It did, however, find that neuromuscular function under heat stress was preserved under heat stress. This was tested by measuring the maximum contractions of the knee extensor muscles directly after the time-trial testing. Short-term heat training likely reduced central and peripheral fatigue mechanisms, preserving force production, but the adaptations were too small or too short-lived to translate into improved whole-body endurance performance.

The rate of loss of adaptations & Re-acclimatization

How fast you lose heat adaptations and how fast you regain them after a period of no heat stimulus is crucial for designing a protocol for a target event, whilst having the least impact on the taper period.

A metanalysis that grouped 12 studies looking at how fast you lose adaptations found that adaptations were sustained for longer when daily heat exposure duration was increased (i.e. 30mins/day Vs. 60mins/day), and when heat exposure intensity was decreased [7]. This shows that session duration is much more important for sustained adaptations than the temperature, and likely the relative power output. Additionally, studies with longer total protocol length produced more robust adaptations, meaning for everyday of no heat stimulus, less adaptation is lost. Roughly 2.5% of the key adaptations are lost per day of no heat exposure, meaning it takes around 2-3 weeks of no heat stimulus to return to your baseline state. The metanalysis also shows that five days is the minimum duration to elicit adaptations. However, as we discussed earlier, proof of adaptation does not guarantee meaningful performance increases. For many adaptations, regaining is 8-12 times faster than the initial acclimatization. However, for the sweat rate adaptation there is no difference. As for maintaining adaptations with the minimum effective dose, even once every 5th day after the initial period can reduce loss of adaptations by a significant amount (except sweat rate) [8].

Summary

Current evidence indicates that heat training can improve endurance performance, particularly in hot conditions, but its effectiveness is highly dependent on protocol design. Meta-analytical and experimental studies suggest that short-to-moderate interventions (approximately 7–14 days) are sufficient to induce meaningful performance gains. Studies reporting no performance benefit often involve excessive training stress, inadequate recovery, or protocols that are either too short or poorly integrated with normal training. Importantly, heat adaptations decay gradually and can be rapidly re-established, allowing heat training to be strategically timed with minimal impact on tapering. Future blog posts will look at the physiological adaptations in more detail, designing the best protocol for you as an individual, and whether you should undergo heat training.

References

1. Carr, A. J., Vallance, B. S., Rothwell, J., Rea, A. E., Burke, L. M., & Guy, J. H. (2022). Competing in Hot Conditions at the Tokyo Olympic Games: Preparation Strategies Used by Australian Race Walkers. Frontiers in physiology, 13, 836858. https://doi.org/10.3389/fphys.2022.836858
2. Tyler, C.J., Reeve, T., Sieh, N. et al. (2024) Effects of Heat Adaptation on Physiology, Perception, and Exercise Performance in the Heat: An Updated Meta-Analysis. J. of SCI. IN SPORT AND EXERCISE 6, 195–217. https://doi.org/10.1007/s42978-023-00263-8
3. Lorenzo, S., Halliwill, J. R., Sawka, M. N., & Minson, C. T. (2010). Heat acclimation improves exercise performance. Journal of applied physiology (Bethesda, Md. : 1985), 109(4), 1140–1147. https://doi.org/10.1152/japplphysiol.00495.2010
4. Périard, J., Nichols, D., Travers, G. et al. (2024) Impact of Exercise Heat Acclimation on Performance in Hot, Cool and Hypoxic Conditions. J. of SCI. IN SPORT AND EXERCISE 6, 275–287. https://doi.org/10.1007/s42978-024-00300-0
5. Mikkelsen, C.J., Junge, N., Piil, J.F., Morris, N.B., Oberholzer, L., Siebenmann, C., Lundby, C. and Nybo, L. (2019). Prolonged Heat Acclimation and Aerobic Performance in Endurance Trained Athletes. Frontiers in Physiology, [online] 10. https://doi.org/10.3389/fphys.2019.01372
6. Rønnestad, B. R., Hamarsland, H., Hansen, J., Holen, E., Montero, D., Whist, J. E., & Lundby, C. (2021). Five weeks of heat training increases haemoglobin mass in elite cyclists. Experimental physiology, 106(1), 316–327. https://doi.org/10.1113/EP088544
7. Daanen, H. A. M., Racinais, S., & Périard, J. D. (2018). Heat Acclimation Decay and Re-Induction: A Systematic Review and Meta-Analysis. Sports medicine (Auckland, N.Z.), 48(2), 409–430. https://doi.org/10.1007/s40279-017-0808-x
8. Pryor, J. L., Pryor, R. R., Vandermark, L. W., Adams, E. L., VanScoy, R. M., Casa, D. J., Armstrong, L. E., Lee, E. C., DiStefano, L. J., Anderson, J. M., & Maresh, C. M. (2019). Intermittent exercise-heat exposures and intense physical activity sustain heat acclimation adaptations. Journal of science and medicine in sport, 22(1), 117–122. https://doi.org/10.1016/j.jsams.2018.06.009
9. Osborne, J.O., Stewart, I.B., Borg, D.N., Beagley, K.W., Buhmann, R.L. and Minett, G.M. (2021). Short-term heat acclimation preserves knee extensor torque but does not improve 20 km self-paced cycling performance in the heat. European Journal of Applied Physiology. https://doi.org/10.1007/s00421-021-04744-y