Heat training sped up time trials but left VO2max untouched

Heat acclimation is a standard part of preparing for competition in hot conditions, and it is usually justified by a chain of physiological changes: expanded plasma volume, lower core and skin temperature, a lower heart rate at a given effort, and better performance as a result. Each of those links is testable, and they do not have to move together. If performance improves without the markers changing, the standard explanation for why heat training works needs revisiting. This review pooled the randomised evidence to see which parts of the chain actually hold up.

The authors searched Cochrane CENTRAL, EMBASE, CINAHL and PubMed, plus reference lists, for randomised controlled trials testing heat acclimation in athletes. After screening 508 titles and abstracts and 19 full-text articles, 11 randomised controlled trials with 215 participants were included; the mean age was 26.09 years and 91% were men. Outcomes extracted covered time-trial performance, VO2max, exercise heart rate, time-trial heart rate, maximal heart rate, core temperature, mean skin temperature, thermal comfort, plasma volume, blood lactate and rating of perceived exertion. Effects were pooled as standardised and raw mean differences between acclimated and non-acclimated groups.

Time-trial performance favoured heat acclimation, with a pooled standardised mean difference of 0.50, and heart rate during time trials also differed between groups. Maximal heart rate was 7 beats per minute lower in the acclimated groups. Against that, there were no significant differences for VO2max, exercise heart rate, core temperature, mean skin temperature, plasma volume or blood lactate concentration. The shifts in thermal comfort and perceived exertion were present but judged too small to carry practical meaning. The authors concluded heat acclimation may improve tolerance of the discomfort of heat without necessarily improving the physiological markers usually credited for performance gains.

This complicates the tidy textbook account in which plasma volume expansion and a lower core temperature are the engine of heat adaptation. Here the performance outcome moved while those mechanisms did not reach significance, which could mean the mechanisms are real but too variable across small trials to detect, or that tolerance and pacing matter more than the classic markers. The pooled sample is small and overwhelmingly male and young, and the protocols behind heat acclimation vary widely in length, temperature and exercise dose. Whether longer or hotter protocols would shift the physiology remains open, as does whether the same applies to women and to older or recreational athletes.

For anyone racing in the heat, this supports the idea that acclimating is worth doing, while suggesting the benefit is modest and may not be visible in the numbers a lab or wearable would show you. A lower maximal heart rate and a better time trial are meaningful outcomes; an unchanged VO2max after heat work is not a sign the preparation failed. It is also a reminder that the mechanism a training method is sold on is not always the mechanism that delivers. Judge heat preparation by how you cope and how you perform, not by a single physiological marker.