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Med Sci Sports Exerc. 2015 Feb;47(2):390-400. doi: 10.1249/MSS.0000000000000401.

Whole-body heat exchange during heat acclimation and its decay.

Author information

1
1School of Human Kinetics, Human and Environmental Physiology Research Unit, School of Human Kinetics, University of Ottawa, Ottawa, Ontario, CANADA; and 2CanmetMINING, Natural Resources Canada, Sudbury, Ontario, CANADA.

Abstract

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°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·m (Ex3), each separated by 10 and 20 min of recovery, respectively, at 35°C and 16% relative humidity. Concurrent measurements of metabolic heat production (indirect calorimetry) allowed for the direct calculation of change in body heat content (ΔHb).

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 ΔHb 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%.

PMID:
24870585
DOI:
10.1249/MSS.0000000000000401
[Indexed for MEDLINE]

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