Induction and decay of heat acclimation

Most advice for improving physiological strain in the heat includes maintaining hydration long-term acclimation protocols (>10 days). Therefore, the major aims of this thesis were to examine; (i) effectiveness of short term (5-day) heat acclimation (STHA) with moderately and highly trained athletes; (ii) fluid regulatory strain has a thermally-independent role in heat adaptation; and, (iii) impact of STHA on a marker of thermotolerance (inducible heat shock protein 70; HSP70). Ten moderately trained males completed heat acclimation (Acc) under 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 cross-over design. One Acc was undertaken with euhydration (fluid replenishment; EUH) and one with dehydration (no fluid intake; DEH) during daily Acc bouts. Participants completed an exercising heat stress test (HST) one week before, then on the 2nd day after Acc for both regimes. HST involved cycling at 40% PPO for 90 min (Ta = 35°C, 60% RH), 10 min rest and a ramp protocol (2% PPO each 30 s) to volitional fatigue. HSTs were further completed 1, 2, and 3 wks after Acc to track the acclimation decay. On a later occasion eight highly trained male rowers were heat acclimated under the same protocol but with DEH acclimation only and a rowing-specific HST (2000 m rowing performance test). Plasma volume (PV) at rest and cardiac output () during HSTs were measured using CO and CO2 rebreathing, respectively. A number of plasma constituents were measured: PV, AVP, aldosterone, HSP70, total protein, albumin, Na+, K+, Cl- and osmolality Short term heat acclimation resulted in physiological adaptation and enhanced exercise capacity for moderately trained participants. Compared to EUH permissive DEH during Acc bouts conferred larger acclimation-induced increases in resting PV by 4.1% (95%CI: -1.5 to 9.8%; p=0.06), (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), end-exercise (45.9: 3.6 to 84.4 mL; p=0.02) and decreased end-exercise by 17% (19: -29 to 9 b·min-1; p=0.08). Cardiovascular adaptations except PV persisted for one wk, but not two wks after Acc indicating that cardiovascular-related benefits from STHA may not be mediated by hypervolaemic responses per se. The highly trained athletes had functional heat adaptations of similar magnitude to lesser fitness-adapted participants across DEH acclimation, including resting PV expansion (4.5: 0.7 to 8.3%) and increased performance (-4.0: -6.3 to 0.6 s; p=0.02). Plasma total protein-corrected HSP70 concentration increased from rest to end-exercise across acclimation (p=0.001). There was a greater change from rest to end-exercise on day one versus day five Acc (p=0.05), indicating a reduced stress-induced increase and a protective adaptive change. There were weak to moderate relationships between hydration indices in dynamic circumstances indicating that there is no single measure to accurately assess hydration status. In conclusion, short-term (5-day) heat acclimation was effective with adaptations more pronounced after fluid regulatory strain from a dehydration acclimation regime. Similar findings were found using highly trained and lesser-fitness adapted participants. Thermotolerance was increased by dehydration acclimation