ANDREW T. GARRETT, NIELS G. GOOSENS, NANCY G. REHRER, MARK J.PATTERSON AND JAMES D.COTTER
Present address: School of Life Science, University of Hertfordshire, College Lane, Hatfield, U.K; School of Physical Education, University of Otago, Dunedin, New Zealand and Defence Science Technology Organisation (DSTO), Melbourne, Australia.
Most advice for heat adaptation is to use long-term (>10 d) regimes, in which hydration status is maintained. We tested the hypothesis that short-term (5-day) heat acclimation would confer substantial improvements in physiological strain and exercise tolerance for exercise in the heat, and fluid regulatory strain provides a thermally-independent stimulus for such adaptations. Ten moderately-fit males were heat acclimated using controlled hyperthermia (rectal temperature 38.5°C) for 90 min on five consecutive days (Ta = 40°C, 60% RH), on two occasions separated by a five-week washout, in a randomly assigned, cross-over design; one with euhydration (EUH) and one with dehydration (DEH) during acclimation bouts. One week before, then on the 2nd day after each acclimation regime, a heat stress test (HST) was completed, comprising cycling at 40% peak power output for 90 min (Ta = 35°C, 60% RH), before incrementing to exhaustion. Plasma volume (PV) at rest was measured using CO rebreathing. Acclimation exercise-induced response of [aldo]p became more pronounced across DEH ( 178 pg.mL-1 ; 95%CI: 33 to 324) but not EUH ( -47 pg.mL-1: -209 to 115) and this difference was significant (P=0.02).
Compared to EUH, permissive DEH during acclimation bouts conferred larger acclimation-induced increases in resting PV (4.1%: -1.5 to 9.8%; P=0.06), QF (4.2: 0.7 to 7.8 ml.min-1.100 ml-1; P=0.009), FVC (0.06: 0.02 to 0.10 ml. 100ml Tissue-1.min-1.mmHg-1; P=0.006) and decreased end-exercise fc by 17% (19: -29 to 9 b·min-1; P=0.05). In conclusion, short-term (5-day) heat acclimation was effective with several adaptations more pronounced after fluid-regulatory strain from a dehydration acclimation regime.
The adaptive effects of medium to long-term heat acclimation (>8-12 d) have received much research attention (Nielsen et al. 1993; Regan et al. 1996; Nielsen et al. 1997; Cheung and McLellan 1998; Weller and Harrison 2001; Patterson et al. 2004).. However, many of the important adaptations to heat stress are cardiovascular, and occur relatively rapidly (<7 d). Therefore, the use of short-term heat acclimation regime (Weller and Harrison 2001; Patterson et al. 2004) has been adopted in this study. The use of permissive dehydration during acclimation contradicts the existing fluid replenishment guidelines for heat acclimation, which recommend the maintenance of good hydration status during exposure to heat stress conditions (Armstrong and Maresh 1991; Convertino et al. 1996; Cheung and McLellan 1998; Casa et al. 2000). However, the reality for many people undergoing acclimation bouts is that some level of dehydration is normal, if not frequently inevitable (Noakes et al. 1988; Greenleaf 1992).