Whole Body Sweat Rate and Loss Adaptations Are Not Different after Heat Acclimation Between a Two-Session Daily Protocol and a One-Session Daily Protocol

Purpose: To compare two heat acclimation (HA) protocols that differ in the frequency of heat exposures (1-a-day v. 2-a-day HA sessions). We hypothesized that there will be no differences in whole body sweat rate (WBSR) and whole body sweat loss (WBSL) adaptations between the HA protocols. Methods: Physically fit individuals (n=48; avgVO 2max : 47.8 ml·kg -1 ·min -1 ) underwent two heat tolerance tests before (pre-HA) and after (post-HA) HA. They completed the Israeli Defense Force heat tolerance test (HTT1; 5.0 km·h -1 , 2% grade, 120min, 40°C, 40% relative humidity (rH)) and a modified heat tolerance test (HTT2; 65% vVO 2max , 2% grade, 40°C, 40% rH, until rectal temperature (T rec ) ≥39.5°C) the following day. They also completed HA sessions (2% grade, 40°C, 40% rH, 70% vVO 2max until T rec >38.5°C then varied to maintain T rec >38.5°C for 60min). The traditional HA protocol group (TRAD; n=39) completed 5 days of single HA sessions, and the 2-a-day HA protocol group (2D; n=9) completed 3 days of 2-a-day HA sessions followed by repeating HTT1 and HTT2. Estimated WBSR was calculated for HTT1 and HA sessions, while estimated WBSL was calculated for HTT2. Both variables were normalized for body surface area. 2x2 ANOVAs with post hoc pairwise group comparisons assessed HTT1 and HTT2 pre/post-HA, and a 2x5 ANOVA with post hoc pairwise group comparisons assessed HA sessions 1-5. Data are presented as mean±SE, significance p<0.05. Results: For post-HA HTT1 and HTT2, no significant difference in WBSR (p=0.422) and WBSL (p=0.512) was found between groups. A slight decrease in post-HA WBSR compared to pre-HA in HTT1 was shown in both groups although not significant (TRAD pre-HA: 0.42±0.02 kg·m -2 ·h -1 , TRAD post-HA: 0.40±0.02 kg·m -2 ·h -1 , p=0.520; 2D pre-HA: 0.40±0.04 kg·m -2 ·h -1 , 2D post-HA: 0.37±0.04 kg·m -2 ·h -1 , p=0.524). Both groups demonstrated a significant increase in post-HA WBSL compared to pre-HA in HTT2 (TRAD pre-HA: 0.71±0.04 kg·m -2 , TRAD post-HA: 0.83±0.03 kg·m -2 , p<0.001; 2D pre-HA: 0.59±0.09 kg·m -2 , 2D post-HA: 0.89±0.08 kg·m -2 , p<0.001). No significant differences in WBSR were found between groups on any given HA session (HA1: p=0.371, HA2: p=0.240, HA3: p=0.746, HA4: p=0.308, HA5: p=0.195). Conclusion: WBSR and WBSL adaptations post-HA were similar between protocols suggesting that an HA protocol consisting of 5 exercise sessions is likely sufficient to elicit improvements in the sweating response. However, if HA must be completed within a shorter timeframe, the 2D protocol with 6 exercise sessions may produce the same sweat rate and loss adaptations. Funding: DoD BA200299 and DoD BA220010 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.