Resumen de Whole-Body Heat Exchange during Heat Acclimation and Its Decay.
Martin P. Poirier, Glenn P. Kenny, Daniel Gagnon, Brian J. Friesen, Stephen G. Hardcastle
AB Purpose: The purpose of this study was to quantify how much whole-body heat loss increases during heat acclimation and the decay in these improvements after heat acclimation. Methods: Ten males underwent a 14-d heat acclimation protocol that consisted of 90 min of cycling in the heat (40[degrees]C, 20% relative humidity) at approximately 50% of maximum oxygen consumption. Before (day 0), during (day 7), and at the end (day 14) of the heat acclimation protocol as well as 7 and 14 d after heat acclimation (days 21 and 28), whole-body heat exchange (evaporative and dry) was measured using direct calorimetry during three bouts of 30-min exercise at 300 (Ex1), 350 (Ex2), and 400 W[middle dot]m-2 (Ex3), each separated by 10 and 20 min of recovery, respectively, at 35[degrees]C and 16% relative humidity. Concurrent measurements of metabolic heat production (indirect calorimetry) allowed for the direct calculation of change in body heat content ([DELTA]Hb). Results: After accounting for an increase in net dry heat gain, increases in whole-body evaporative heat loss were evident for Ex2 and Ex3 on day 7 (Ex2, 4.9 +/- 5.6%; Ex3, 9.0 +/- 6.0%; both P <= 0.05) and all heat loads on day 14 (Ex1, 7.6 +/- 8.3%; Ex2, 7.7 +/- 5.5%; Ex3, 11.2 +/- 4.6%; all P <= 0.05) relative to day 0 (Ex1, 494 +/- 27 W; Ex2, 583 +/- 21 W; Ex3, 622 +/- 36 W). As a result, a lower cumulative [DELTA]Hb was measured on day 7 (-18 +/- 8%, P <= 0.001) and day 14 (-26 +/- 10%, P <= 0.001) compared with that measured on day 0 (1062 +/- 123 kJ). Most of these improvements were retained after 2 wk of nonexposure to the heat. Conclusions: This is the first study to quantify how much 14 d of heat acclimation can increase whole-body evaporative heat loss, which can improve by as much as approximately 11%.
