The change in metabolic heat production is a primary mediator of heat acclimation in adults

Purpose This study examined whether heat acclimation (HA) results in either predominate improvements in heat dissipation or reduced endogenous heat production via individual components of the human heat balance equation. Methods Twelve healthy inactive subjects (5 females, mean ± SD): age 28 ± 6 y, 77.9 ± 2 kg), completed a 10-day HA (42 °C, 28% RH) hyperthermia clamp (90 min/day exercise, ∆1.5 °C in rectal temperature (T_re)) and control workload matched (CON: 23 °C, 42% RH) protocols in a counterbalanced design separated by at least 2 mo. Pre-and post-HA were matched for external work rate (EX_WR; day 1 and day 10 first 30 min at 118 ± 29 W, last 60 min at 11 ± 5 W); and metabolic heat production (H_prod; day 1 and day 9, first 30 min at 296 ± 26 W m^-2, last 60 min 187 ± 33 W m-2). Results When Pre- and post- HA was matched for H_prod, there was no difference during the first 30 or last 60 min of exercise for metabolic energy expenditure (MEE 363 ± 70/ 195 ± 32 W m^-2), H_prod (296 ± 67/ 187 ± 33 W m^-2) or T_re (∆2.1 ± 0.5 °C). When pre- and post-HA was EX_WR equivalent, HA significantly attenuated MEE during the first 30 and last 60 min (303 ± 49/ 174 ± 35 W m^-2), H_prod (241 ± 44/ 168 ± 33, W·m^-2), and ∆T_re (∆1.3 ± 0.4 °C) (each P < 0.0001). When ∆T_re, ∆T_sk, ∆T_b were each normalized per 100 W H_prod, no differences were found for any pre-to post-HA comparison. Heat loss required (E_req) to maintain steady state internal temperature (E_req = 220 ± 32 W m^-2), maximal capacity of the climate for evaporative heat loss (E_max = 266 ± 56 W m^-2), evaporative heat loss from skin (E_sk = 207 ± 38 W m^-2) or skin wettedness (E_req/E_max = 0.88 ± 0.23 W m^-2) were not different among each condition during the last 60 min. Conclusion The mechanisms that underlie heat acclimation are not wholly attributed to heat dissipation enhancements per se, but are significantly influenced by metabolic heat production alterations under uncompensable heat stress environments.