Once- and twice-daily heat acclimation confer similar heat adaptations, inflammatory responses and exercise tolerance improvements

This experiment aimed to investigate the efficacy of twice-daily, nonconsecutiveheat acclimation (TDHA) in comparison to once-daily heatacclimation (ODHA) and work matched once- or twice-daily temperateexercise (ODTEMP, TDTEMP) for inducing heat adaptations, improvedexercise tolerance, and cytokine (immune) responses. Forty males,matched biophysically and for aerobic capacity, were assigned to ODHA,TDHA, ODTEMP or TDTEMP. Participants completed a cycling gradedexercise test, heat acclimation state test and a time to task failure(TTTF) at 80% peak power output in temperate (TTTFTEMP: 22°C/40%RH) and hot conditions (TTTFHOT: 38°C/20% RH), before and after 10-sessions (60-min of cycling at ~2W.kg-1) in 45°C/20% RH (ODHA andTDHA) or 22°C/40% RH (ODTEMP or TDTEMP). Plasma IL-6, TNF-α andcortisol were measured pre- and post-sessions 1, 5 and 10. ODHA andTDHA induced equivalent heat adaptations (P<0.05) (resting rectaltemperature [-0.28±0.22, -0.28±0.19°C], heart rate [-10±3, -10±4b.min-1] and plasma volume expansion [+10.1±5.6, +8.5±3.1%]) andimproved heat acclimation state (sweat setpoint [-0.22±0.18, -0.22±0.14°C] and gain [+0.14±0.10, +0.15±0.07g.sec-1.°C-1]).TTTFHOT increased (P<0.001) following ODHA (+25±4%) and TDHA(+24±10%), but not ODTEMP (+5±14%) or TDTEMP (+5±17%).TTTFTEMP did not improve (P>0.05) following ODHA (+14±4%), TDHA(14±8%), ODTEMP (9±10%) or TDTEMP (8±13%). Acute (P<0.05) butno chronic (P>0.05) increases were observed in IL-6, TNF-α or cortisolduring ODHA and TDHA, or ODTEMP and TDTEMP. Once- and twice-dailyheat acclimation conferred similar magnitudes of heat adaptation andexercise tolerance improvements, without differentially altering immune function, thus non-consecutive TDHA provides an effective, logisticallyflexible method of HA, benefitting individuals preparing for exercise-heatstress.