Purpose Heat adaptation (HA) is a popular strategy to combat the negative effects of thermal stress. The HA literature has expanded since a 2016 meta-analysis, and we provide an updated meta-analysis, incorporating 39 additional studies and advanced analysis. Methods Following Pubmed searches, full-text original articles using human participants were reviewed using the four-stage PRISMA process. Data were extracted by at least two of the authors. Hedges` g effect sizes, 95% confidence intervals, and prediction intervals were calculated. Correlations were run where appropriate. Results One hundred and thirty-five total articles (96 previous, 39 new) were reviewed. Medium-term (8-14 days), active, constant work HA regimens remain the most common despite a recent focus on isothermal, passive, and short-term (= 7 days) alternatives. HA still improves subsequent exercise performance and capacity in the heat (g = 0.7), reduces resting core temperature (g = - 0.6) and heart rate (g = - 0.5), and increases sweat rate (g = 0.4) but the effect sizes are lower than previously reported. HA has a moderate or larger effect (g > 0.5) on lowering sweat onset temperature, mean heart rate, sweat sodium and chloride concentrations, resting thermal sensation, and thirst sensation, and increasing resting plasma volume. There is considerable heterogeneity within the data for most variables. Conclusion HA regimens can reduce physiological and perceptual strain and improve subsequent exercise performance and capacity in the heat. Longer regimens may be more effect than shorter ones, but the data are lacking. Passive HA is a practical, effective alternative to active HA.
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