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 ± 6y, 77.9 ± 2kg), completed a 10-day HA (42°C, 28% RH) hyperthermia clamp (90min/day exercise, ∆1.5°C in rectal temperature (Tre )) 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 (EXWR ; day 1 and day 10 first 30min at 118 ± 29W, last 60min at 11 ± 5W); and metabolic heat production (Hprod ; 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 Hprod , there was no difference during the first 30 or last 60min of exercise for metabolic energy expenditure (MEE 363 ± 70/ 195 ± 32Wm-2 ), Hprod (296 ± 67/ 187 ± 33Wm-2 ) or Tre (∆2.1 ± 0.5°C). When pre- and post-HA was EXWR equivalent, HA significantly attenuated MEE during the first 30 and last 60min (303 ± 49/ 174 ± 35Wm-2 ), Hprod (241 ± 44/ 168 ± 33, W·m-2 ), and ∆Tre (∆1.3 ± 0.4°C) (each P < 0.0001). When ∆Tre , ∆Tsk, ∆Tb were each normalized per 100W Hprod , no differences were found for any pre-to post-HA comparison. Heat loss required (Ereq ) to maintain steady state internal temperature (Ereq = 220 ± 32Wm-2 ), maximal capacity of the climate for evaporative heat loss (Emax = 266 ± 56Wm-2 ), evaporative heat loss from skin (Esk = 207 ± 38Wm-2 ) or skin wettedness (Ereq /Emax = 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.