Lorenzo et al. 2010, Heat acclimation improves exercise performance

J Appl Physiol 109:1140-1147. 10-day heat acclimation in 12 trained cyclists (8 acclimated, 4 control) raised VO2max ~5% cool / ~8% hot, improved time-trial 6-8% in both conditions, expanded plasma volume ~6.5%. The single positive result behind the cool-condition transfer claim; better-controlled crossover and RCT designs (Keiser 2015, Karlsen 2015, Mikkelsen 2019) have not replicated the cool-condition gain. Contested for cool-condition performance; the plasma-volume expansion itself is robust.