Despite Living in a Cold Region, the Months of Testing Matter for Thermoregulatory Research

Purpose: To test the hypothesis that the month at which participants complete their first heat exposure will impact the magnitude of adaptations following a 5-day heat acclimation (HA) intervention. Methods: Physically active individuals (n=42) first completed the Israeli Defense Force heat tolerance test (HTT1; 5.0 km·h -1 , 2% grade, 120min, 40°C, 40% relative humidity) and then a modified heat tolerance test (HTT2; 65% vVO 2 max, 2% grade treadmill run, 40°C, 40% relative humidity, until rectal temperature reached 39.5°C) before (pre-HA) and after (post-HA) a 5-day HA protocol. Absolute changes (Δ) for variables associated with the HA response across both HTTs were calculated between post-HA and pre-HA. HTT1 variables include pre-exercise (T0) heart rate (HR), peak HR, average HR, baseline rectal temperature (Trec), T0 Trec, peak Trec, average Trec, Trec rate of rise (RoR), peak mean skin temperature, whole body sweat loss (WBSL), post-exercise rating of perceived exertion (RPE), post-exercise thermal sensation (TS), and post-exercise overall fatigue (FAT). HTT2 variables include total exercise time, T0 HR, peak HR, average HR, baseline Trec, T0 Trec, TrecRoR, peak mean skin temperature, WBSL, RPE, TS, and FAT. All participants completed the study between November and March and were grouped into three groups based on the month of their first heat exposure: November/December (ND), January (J), and February/March (FM) (n=13, 14, & 15, respectively). Comparisons between groups for participant characteristics and Δ of the variables were conducted using one-way ANOVAs and post hoc Tukey HSD (data are mean±SD; significance p≤0.05). Results: There were no significant differences between groups for age, height, weight, VO 2 max, lean mass or fat mass percentage (p>0.05). Compared to ND and J, FM had greater reductions in Δpeak Trec(FM: -0.57±0.33°C, ND: -0.16±0.37°C, J: -0.18±0.37°C, p<0.05) and ΔTrec RoR (FM: -0.004±0.003°C·min -1 , ND: 0.000±0.003°C·min -1 , J: -0.001±0.001°C·min -1 , p<0.05) during HTT1 as well as significant reductions in Δaverage HR (FM: -19±10BPM, ND: -10±9BPM, p<0.05) and an increase in ΔWBSL (FM: 0.13±0.39L, ND: -0.33±0.42L, p<0.05) compared to ND. During HTT2, only ΔWBSL was significantly different between FM and J (FM: 0.37±0.33L, J: 0.17±0.23L, p<0.05). Conclusion: Although all participants were living in the New England region and began testing in the late fall through the winter months (i.e., November to March), greater magnitudes were identified in participants who were tested later into fall/winter than participants who were tested earlier. The statistically and clinically distinct differences highlight the importance of how the time of year can influence adaptations associated with heat acclimation and thermoregulatory research. Funding: 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.