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

Eric Rivas; Manisha Rao; Todd Castleberry; Vic Ben-Ezra

doi:10.1016/j.jtherbio.2017.10.001

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

J Therm Biol. 2017 Dec;70(Pt B):69-79. doi: 10.1016/j.jtherbio.2017.10.001. Epub 2017 Oct 6.

Authors

Eric Rivas¹, Manisha Rao², Todd Castleberry², Vic Ben-Ezra²

Affiliations

¹ Exercise and Thermal Integrative Physiology Laboratory, Department of Kinesiology and Sport Management, Texas Tech University, Lubbock, TX, USA. Electronic address: eric.rivas@ttu.edu.
² Exercise Physiology and Biochemistry Laboratory, Department of Kinesiology, Texas Woman's University, Denton, TX, USA.

PMID: 29108560
DOI: 10.1016/j.jtherbio.2017.10.001

Abstract

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 ± 6y, 77.9 ± 2kg), completed a 10-day HA (42°C, 28% RH) hyperthermia clamp (90min/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 30min at 118 ± 29W, last 60min at 11 ± 5W); and metabolic heat production (H_prod; day 1 and day 9, first 30min at 296 ± 26Wm^-2, last 60min 187 ± 33Wm-2).

Results: When Pre- and post- HA was matched for H_prod, there was no difference during the first 30 or last 60min of exercise for metabolic energy expenditure (MEE 363 ± 70/ 195 ± 32Wm^-2), H_prod (296 ± 67/ 187 ± 33Wm^-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 60min (303 ± 49/ 174 ± 35Wm^-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 100W 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 ± 32Wm^-2), maximal capacity of the climate for evaporative heat loss (E_max = 266 ± 56Wm^-2), evaporative heat loss from skin (E_sk = 207 ± 38Wm^-2) or skin wettedness (E_req/E_max = 0.88 ± 0.23Wm^-2) were not different among each condition during the last 60min.

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.

Keywords: Heat acclimation; Heat tolerance; Metabolism.

MeSH terms

Acclimatization / physiology*
Adult
Energy Metabolism
Exercise / physiology
Female
Humans
Male
Thermogenesis / physiology*
Young Adult