# Comparison of heat acclimation after once daily and thrice daily heat exposures in healthy adults

## TL;DR

This study compares how different heat exposure schedules affect the body's adaptation to heat in men and women.

## Contribution

The study reveals that once-daily heat exposure is more effective for heat acclimation than thrice-daily exposure.

## Key findings

- Once-daily heat exposure significantly reduced rectal and surface temperatures over time, while thrice-daily exposure did not.
- Females consistently had higher temperatures than males during acclimation.
- Thrice-daily heat exposure was less effective in inducing heat adaptations compared to once-daily exposure.

## Abstract

Determine how matched duration but varied exposure scheduling impacts heat acclimation in male and female adults. Thirty males and thirty females walked daily (7 days, 38°C, 60% RH, 6.1 METs) in one of four groups (sustained males/females [SM/SF], periodic males/females [PM/PF]). SM/SF performed 90‐min exposures; PM/PF completed three 30‐min exposures 3 h apart. Females had similar ovarian‐hormone fluctuation. Acclimation markers were assessed within the first 30‐min exposure on days 1, 4, and 7. SM/SF rectal temperature decreased from day 1 to days 4 and 7 (37.5 ± 0.3°C, 37.3 ± 0.3°C, 37.2 ± 0.3°C, p < 0.001) and further decreased from day 4 to 7 (p = 0.011). PM/PF rectal temperature was unchanged between days 1, 4, and 7 (37.4 ± 0.3°C, 37.4 ± 0.3°C, 37.3 ± 0.3°C, p > 0.05). SM/SF 3‐site surface temperature decreased from day 1 to days 4 and 7 (37.1 ± 0.5°C, 36.9 ± 0.4°C, 36.8 ± 0.4°C, p < 0.001) but was unchanged from day 4 to 7 (p = 0.090). PM/PF 3‐site surface temperature was unchanged from day 1 (37.0 ± 0.4°C) to days 4 (37.0 ± 0.4°C, p = 0.726) and 7 (36.9 ± 0.4°C, p = 0.109) but decreased from day 4 to 7 (p = 0.013). Females had higher rectal (p < 0.001) and 3‐site surface (p = 0.036) temperatures than males throughout acclimation. Thrice‐daily exposures are not as effective at inducing heat adaptations compared to once‐daily exposures. Sex differences persisted throughout acclimation without altering adaptations.