Similar heat acclimation outcomes despite sex-specific response patterns: Evidence for equivalent adaptation in males and females during uncompensable heat stress

Heat tolerance (HT) and adaptation differences between sexes remain poorly understood, particularly during uncompensable heat stress. Purpose: This study investigated sex-specific responses to a five-day heat acclimation (HA) protocol in 40 participants (22F/18M, 23 ± 4 years). Methods: Participants completed heat tolerance tests (HTT) before (PreHA) and after HA (PostHA), exercising at 65% vV˙O2max (5.6 ± 0.76 mph) until reaching T rec 39.5°C or volitional fatigue. Results: Despite lower V˙O2max (44.5 ± 5.1 vs. 51.4 ± 5.7 mL·min-1·kg-1, p<0.001) and higher body fat percentage (25.8 ± 4.8 vs. 15.7 ± 5.3%, p<0.001), females demonstrated similar initial HT as males, with comparable time to temperature (T2T, 31.90 ± 8.36 vs. 29.29 ± 6.23 min, p=0.267). HA improved HT in both sexes, with females showing significant increases in T2T (31.90 ± 8.36 vs. 38.96 ± 13.13 min, p=0.015) and males trending toward improvement (29.29 ± 6.23 vs. 32.52 ± 7.69 min, p=0.052). The magnitude of improvement was not statistically different between sexes (F: 7.06 ± 12.17 vs. M: 3.23 ± 6.79 min, p=0.223). End-exercise T rec increased PostHA (39.29 ± 0.38 vs. 39.39 ± 0.34°C, p=0.029), primarily driven by changes in females (p=0.025). T rec rate of rise decreased post-HA (0.074 ± 0.012 vs. 0.069 ± 0.015 °C·min-1, p=0.013), with significant reductions in females (0.068 ± 0.010 vs. 0.062 ± 0.013 °C·min-1, p=0.012) but not males (0.080 ± 0.012 vs. 0.077 ± 0.014 °C·min-1, p=0.298). Whole-body sweat loss (WBSL) increased PostHA in both sexes (1.307 ± 0.511 vs. 1.523 ± 0.481 L, p<0.001), though females consistently demonstrated lower WBSL than males (1.13 ± 0.37 vs. 1.74 ± 0.45 L, p<0.001). Females had consistently higher mean skin temperatures (36.62 ± 0.58 vs. 36.34 ± 0.51 °C, p=0.036) and peak skin temperatures (37.34 ± 0.67 vs. 37.02 ± 0.61 °C, p=0.041) than males. HA reduced end-exercise skin temperature (37.26 ± 0.68 vs. 36.96 ± 0.89 °C, p=0.028), particularly in females (37.51 ± 0.78 vs. 37.06 ± 0.90 °C, p=0.071). Peak heart rate decreased PostHA in males (186.47 ± 10.22 vs. 177.79 ± 11.16 bpm, p<0.001) but not in females (189.76 ± 11.42 vs. 186.19 ± 13.67 bpm, p=0.281). Both sexes reported reduced thermal sensation (7.06 ± 0.72 vs. 6.74 ± 0.91, p=0.016) and thirst (6.08 ± 2.15 vs. 5.23 ± 2.15, p=0.046) PostHA. Classification of HTT performance revealed that 67.5% passed pre-HA (71.4%F vs. 63.2%M), increasing to 82.5% post-HA (85.7%F vs. 78.9%M). Failed participants demonstrated lower baseline T rec (36.81 ± 0.57 vs. 37.11 ± 0.45 °C, p=0.109) and reached volitional fatigue earlier despite similar WBSL. Analysis of participants grouped by increased or decreased T2T showed minimal physiological differences, with the increased group showing higher total HR AUC during HA (4717.6 ± 961.2 vs. 4041.5 ± 954.2 bpm·min, p=0.042) and higher PostHA HR AUC (500.9 ± 125.5 vs. 364.6 ± 136.4 bpm·min, p=0.005). Conclusions: While females and males demonstrate different physiological pathways to heat adaptation, both sexes show similar capacity for improvement following short-term HA. T2T criteria may inadequately capture the complex nature of HT and adaptation, as evidenced by minimal physiological differences between those who increased versus decreased their T2T. DOD BA200299 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.