A.T. GARRETT,1,2* N.G. GOOSENS,1 N.J. REHRER,1 M.J. PATTERSON,3 J. HARRISON,4 I. SAMMUT,4 AND J.D. COTTER School of Physical Education, Sport and Exercise Sciences, University of Otago, Dunedin, New Zealand Department of Sport, Health and Exercise Science, University of Hull, United Kingdom Defense Science Technology Organization (DSTO), Melbourne, Australia School of Pharmacology and Toxicology, University of Otago, Dunedin, New Zealand
Most heat acclimation data are from regimes longer than 1 week, and acclimation advice is to prevent
dehydration. Objectives: We hypothesized that (i) short-term (5-day) heat acclimation would substantially improve physiological strain and exercise tolerance under heat stress, and (ii) dehydration would provide a thermally independent stimulus for adaptation. Methods: Nine aerobically fit males heat acclimated using controlled-hyperthermia (rectal temperature 38.5C) for 90 min on 5 days; once euhydrated (EUH) and once dehydrated (DEH) during acclimation bouts. Exercising heat stress tests (HSTs) were completed before and after acclimations (90-min cycling in Ta 35C, 60% RH). Results: During acclimation bouts, [aldosterone]plasma rose more across DEH than EUH (95%CI for difference between regimes: 40–411 pg ml21; P 5 0.03; n 5 5) and was positively related to plasma volume expansion (r 5 0.65; P 5 0.05), which tended to be larger in DEH (CI: 21 to 10%; P 5 0.06; n 5 9). In HSTs, resting forearm perfusion increased more in DEH (by 5.9 ml 100 tissue ml21 min21 : 211.5 to 21.0; P 5 0.04) and end-exercise cardiac frequency fell to a greater extent (by 11 b min21 : 21 to 22; P 5 0.05). Hydration-related effects on other endocrine, cardiovascular, and psychophysical responses to HSTs were unclear. Rectal temperature was unchanged at rest but was 0.3C lower at end exercise (P < 0.01; interaction: P 5 0.52). Conclusions: Short-term (5-day) heat acclimation induced effective adaptations, some of which were more pronounced after fluid-regulatory strain from permissive dehydration, and not attributable to dehydration effects on body temperature. Am. J. Hum. Biol. 26:311–320, 2014.
Heat acclimation and acclimatization confer several thermoregulatory, cardiovascular, and neuroendocrine adaptations. These collectively reduce physiological and perceived strain during exertion, and improve functional capacities in both benign (Lorenzo et al., 2010; Scoon et al., 2007) and heat stressful (Gisolfi and Wenger, 1984;
Sawka et al., 1996) environments. The adaptive effects of medium to long-term heat acclimation (>8–12 days) have
received much research attention (Garrett et al., 2011). However, many of the important adaptations to heat
stress are cardiovascular, and occur relatively rapidly (i.e., nearly complete within 7 days (Patterson et al., 2004b), and the period available for heat acclimation may often be less than 7 days for sport, occupational exposure
(e.g., wild land fire fighting), or domestic exposure (esp. heat waves). Therefore, the extent of adaptations from
short-term heat acclimation requires more investigation, and was one focus of this study.