Andrew T. Garrett

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 38.5°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 T_a 35°C, 60% RH). Results: 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). 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.

A rethinking of current heat-acclimation strategies is required as most research and advice for improving physiological strain in the heat includes maintaining hydration using long-term acclimation protocols (>10 days). Furthermore, these strategies have tended to use untrained and moderately trained participants. Therefore, the aims of this review were to (i) investigate the effectiveness of short-term heat acclimation (STHA) with moderately and highly trained athletes; (ii) determine the importance of fluid regulatory strain, which has a thermally independent role in heat adaptation; (iii) assess the impact of STHA on a marker of thermotolerance (inducible heat-shock protein 70 [HSP70]); and (iv) provide further information on the decay of acclimation to heat. The review suggests that 5-day STHA is effective, and adaptations may be more pronounced after fluid regulatory strain from a dehydration-acclimation regimen. Furthermore, highly trained athletes may have similar physiological gains to those who are less trained using STHA. However, research has tended to focus on untrained or moderately trained participants and more information is required for highly trained populations. HSP70 response is upregulated across STHA. This indicates increased thermotolerance and protective adaptive change that may indicate HSP70 response as a useful marker of heat acclimation. Physiological adaptations after heat acclimation are relatively short term and may vanish only a few days or weeks after removal from heat exposure. From a practical perspective 5-day STHA may be the preferred acclimation regimen for moderately and highly trained athletes as it has been shown to be effective, less expensive and less likely to disrupt the tapering for competition in elite performers. Furthermore, updated information on the time course of acclimation decay may allow a reliable estimate of how long individuals can be free from heat exposure before reacclimation is required. This is particularly pertinent in present times as many athletes, civilians andmilitary personnel increasingly have to relocate to different climates of the world, often within a short period of time.