Training in the heat beats a cool gym for racing in the heat

Athletes and coaches have long used training in the heat to build heat resilience before competing in hot conditions, on the logic that the body only adapts to a stress it actually meets. More recently, some evidence has suggested that ordinary training in comfortable, thermoneutral conditions produces much the same adaptations, which would make the extra suffering of hot sessions unnecessary. Until now the comparative evidence had not been drawn together in one place. The practical stake is straightforward: whether the heat itself does anything the training does not.

The authors searched five databases, PubMed, SPORTDiscus, Cochrane Library, Scopus and Web of Science, up to 20 November 2025, for studies comparing exercise in hot conditions with exercise in thermoneutral conditions. They extracted physiological markers of heat adaptation including core temperature, heart rate, skin temperature and sweat rate, along with aerobic performance measured in the heat. Effects were expressed as Hedges' g and combined using multilevel meta-analysis, examining both differences between the two training conditions and changes from baseline within each. Subgroup analyses tested whether training status and the type of performance outcome changed the picture. Twenty-three studies were included.

Compared with thermoneutral training, hot training produced greater reductions in resting core temperature, exercising core temperature and exercising heart rate, and greater increases in sweat rate and aerobic performance in the heat, with effect sizes ranging from moderate to large. Skin temperature during exercise was the exception and did not differ significantly between conditions. Measured against their own baselines, hot training improved every heat-related outcome by a moderate to large margin, while thermoneutral training produced significant improvements in exercising core temperature and exercising heart rate. Subgroup analysis found the extra advantage of heat was more pronounced in elite athletes and in longer interventions, with no other significant moderators.

This pushes back on the suggestion that comfortable training is just as good for heat resilience: on these pooled results, the heat adds something real to thermoregulatory and performance measures. At the same time it partly vindicates the sceptics, because thermoneutral training did move core temperature and heart rate during exercise in the right direction. Since the extra gains concentrated in elite athletes and longer programmes, the size of the benefit for everyone else is still unclear. The lack of any additional effect on skin temperature also leaves a mechanistic loose end.

This is a reason to treat heat training as a specific tool for a specific goal rather than a general fitness upgrade. If your target event or work environment is hot, the pooled evidence supports the idea that training in heat prepares you better than training in comfort. If it is not, your usual sessions already appear to shift some of the same markers, just less. None of this speaks to how much heat, for how long, or how safely, and heat carries risks that a pooled comparison of trained participants does not capture.