title: Whole-body heat exchange during heat acclimation and its decay.
abstract: OBJECTIVE
  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%.
authors.affiliation: 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.
authors.forename: Martin P
authors.initials: MP
authors.lastname: Poirier
authors.forename: Daniel
authors.initials: D
authors.lastname: Gagnon
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Friesen
authors.forename: Stephen G
authors.initials: SG
authors.lastname: Hardcastle
authors.forename: Glen P
authors.initials: GP
authors.lastname: Kenny
mesh_terms: D000064:Acclimatization / Q000502:physiology*; D000328:Adult; D001833:Body Temperature Regulation / Q000502:physiology*; D002151:Calorimetry; D015444:Exercise / Q000502:physiology*; D005080:Exercise Test; D006339:Heart Rate; D006358:Hot Temperature; D006801:Humans; D008297:Male; D013997:Time Factors; D055815:Young Adult