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

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.

Keywords: acclimatization; estradiol; progesterone; skin blood flow; skin conductivity; thermoregulation.

FIGURE 1

7‐day heat acclimation schematic for the sustained and periodic groups.

FIGURE 2

Mean and individual surface (a) and rectal (b) temperatures while walking during minutes 20–25 of heat exposure on Days 1, 4, and 7 of two heat acclimation regimens. *p < 0.05 from day 1 within a group, ^† p < 0.05 from day 4 within a group, ^‡ p < 0.05 from males. Data presented as mean ± SD.

FIGURE 3

Mean and individual heart rate responses while walking during minutes 20–25 of heat exposure on days 1, 4, and 7 of two 7‐day heat acclimation regimens. *p < 0.05 from day 1, ^† p < 0.05 from day 4, ^‡ p < 0.05 from males. Data presented as mean ± SD.

FIGURE 4

Mean relative change (∆) and individual responses in skin blood flow upon entering the heat chamber (pre‐exercise 38°C exposure) and prior to exiting (post exercise 38°C exposure) on days 1, 4, and 7 of two 7‐day heat acclimation regimens. PF, periodic female; PM, periodic male; SF, sustained female; SM, sustained male. *p < 0.05 from sustained pre and post exercise 38°C exposure, ^† p < 0.05 from pre exercise 38°C exposure, ^‡ p < 0.05 from males post exercise 38°C exposure. Data presented as mean ± SD.