Exercise in hot or thermoneutral conditions? Comparative effectiveness on aerobic performance and physiological adaptations in the heat: A meta-analysis.

BACKGROUND

Exercise in hot conditions (ExH) is traditionally used to improve heat resilience. Emerging evidence suggests that exercise in thermoneutral conditions (ExN) may elicit comparable adaptations, challenging the benefits of heat exposure. However, synthesized comparative evidence is lacking. Accordingly, this study aimed to compare the effects of ExH and ExN on markers of heat adaptation and aerobic performance in the heat, and to conduct exploratory analyses of within-group changes.

METHODS

Five databases (PubMed, SPORTDiscus, Cochrane Library, Scopus, and Web of Science) were searched up to November 20, 2025. Data extracted included key physiological outcomes (e.g., core temperature (Tcore), heart rate (HR), skin temperature (Tsk), and sweat rate (SR)) and aerobic performance in the heat. Effect sizes were calculated as Hedges' g. Multilevel meta-analyses were employed to examine the within- and between-group effects. Subgroup analyses were conducted to examine potential moderators, such as training status and performance outcome type.

RESULTS

Twenty-three studies were included. Compared with ExN, ExH induced greater reductions in resting Tcore (g = 0.51), exercise Tcore (g = 0.55), and exercise HR (g = 0.69), and greater increases in SR (g = 0.56) and aerobic performance (g = 0.56), but it did not significantly affect exercise Tsk (g = 0.47). Relative to baseline, ExH elicited moderate-to-large improvements in all heat-related outcomes, while ExN showed significant improvements in exercise Tcore (g = 0.41) and exercise HR (g = 0.57). Subgroup analyses indicated the additional benefits of ExH were more pronounced in elite athletes (Tier 3) and longer intervention periods, with no other significant moderators (all p > 0.05).

CONCLUSION

Additional heat exposure during exercise confers greater physiological adaptations to heat and improvements in aerobic performance in the heat compared with ExN, although no additional benefit was observed for exercise Tsk. These effects were moderated by intervention duration and training status. Notably, ExN can also induce modest improvements in key thermoregulatory and cardiovascular outcomes.