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

Short duration heat acclimation (HA) (≤5 daily heat exposures) elicits incomplete adaptation compared with 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: 1) “condensed” HA (CHA; n = 17 males) consisting of 4 ͓ 75 min·day^-1 heat exposures [target rectal temperature (T_rec) = 38.5 ^̥C] for two consecutive days, with 2) “traditional” HA (THA; n = 15 males) consisting of 1 ͓ 75 min·day^-1 heat exposure (target T_rec = 38.5^̥C) for eight consecutive days. Physiological responses to exercise heat stress, hypoxia, and normoxic exercise performance were evaluated pre- and postintervention. 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., D 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., D 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.