Abstract
Most heat acclimation data are from regimes longer than 1 week, and acclimation advice is to prevent dehydration. 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. Nine aerobically fit males heat acclimated using controlled-hyperthermia (rectal temperature 38.5°C) 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 35°C, 60% RH). During acclimation bouts, [aldosterone]plasma rose more across DEH than EUH (95%CI for difference between regimes: 40-411 pg ml(-1); P = 0.03; n = 5) and was positively related to plasma volume expansion (r = 0.65; P = 0.05), which tended to be larger in DEH (CI: -1 to 10%; P = 0.06; n = 9). In HSTs, resting forearm perfusion increased more in DEH (by 5.9 ml 100 tissue ml(-1) min(-1): -11.5 to -1.0; P = 0.04) and end-exercise cardiac frequency fell to a greater extent (by 11 b min(-1): -1 to 22; P = 0.05). Hydration-related effects on other endocrine, cardiovascular, and psychophysical responses to HSTs were unclear. Rectal temperature was unchanged at rest but was 0.3°C lower at end exercise (P < 0.01; interaction: P = 0.52). 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.
Highlights
Heat acclimation and acclimatisation confer several thermoregulatory, cardiovascular and neuroendocrine adaptations
The relationship between variables was calculated using the Pearson Product Moment Correlation. Nine participants completed both acclimation regimes and the heat stress tests (HSTs) before and after each acclimation, except that one participant became exhausted in one heat stress test at 67 min, so his end-exercise data were taken as the 60-min measurement in all HSTs
All blood measures were obtained from all participants on HST- and Blood volume (BV)-measurement days, but a full cross-over of measures on acclimation days was obtainable from only five participants due to difficulty with cannulation or maintaining cannula patency on one of these additional days in the remaining four participants
Summary
Heat acclimation and acclimatisation confer several thermoregulatory, cardiovascular and neuroendocrine adaptations These collectively reduce physiological and perceived strain during exertion, and improve functional capacities in both benign (Lorenzo and others 2010; Scoon and others 2007) and heat stressful (Gisolfi and Wenger 1984; Sawka and others 1996) environments. One main feature of adaptation to heat stress and exercise is an expansion of plasma volume (PV), which is mediated through sodium retention and increased intra-vascular albumin concentration (Patterson and others, 2004). This expansion in plasma volume appears to underlie other beneficial physiological and functional outcomes (Lorenzo and others 2010; Racinais and others 2012; Scoon and others 2007). We sought to examine if permissive dehydration during exercise and heat acclimation might enhance adaptation independently of effects on body temperature, by PV expansion
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