Effect of condensed heat acclimation on thermophysiological adaptations, hypoxic cross-tolerance, exercise performance and de-acclimation.

Short duration heat acclimation (HA) (≤5 daily heat exposures) elicits incomplete adaptation compared to longer interventions, possibly due to the lower accumulated thermal 'dose'. It is unknown if matching thermal 'dose' over a shorter timescale elicits comparable adaptation to a longer intervention. Using a parallel-groups design, we compared: i) 'condensed' HA (CHA; n=17 males) consisting of 4×75 min∙day^-1 heat exposures (target rectal temperature (T_rec)=38.5°C) for 2 consecutive days, with; ii) 'traditional' HA (THA; n=15 males) consisting of 1×75 min∙day^-1 heat exposure (target T_rec=38.5°C) for 8 consecutive days. Physiological responses to exercise heat-stress, hypoxia, and normoxic exercise performance were evaluated pre- and post-intervention. Thermal (T_rec over final 45 min: CHA=38.45±0.17°C, THA=38.53±0.13°C, p=0.126) and cardiovascular strain were not different during interventions, indicating similar thermal 'dose', although CHA had lower sweating rate, higher starting T_rec, and greater inflammation, gastrointestinal permeability and renal stress (p<0.05). However, CHA elicited an array of thermophysiological adaptations that did not differ from THA (reduced indices of peak thermal [e.g., Δ peak T_rec CHA=- 0.28±0.26°C, THA=-0.36±0.17°C, p=0.303] and cardiovascular strain, inflammation and renal stress; blood and plasma volume expansion; improved perceptual indices), although improvements in resting thermal strain (e.g., Δ resting T_rec CHA=-0.14±0.21°C, THA=- 0.35±0.29°C, p=0.027) and sweating rate were less with CHA. Both interventions improved aspects of hypoxic tolerance, but effects on temperate normoxic exercise indices were limited. The diminished thermal strain was well-maintained over a 22-day decay period. In conclusion, CHA could represent a viable acclimation option for time-restricted young healthy-males preparing for a hot, and possibly high-altitude, environment.