Keiser et al. 2015, Heat training increases exercise capacity in hot but not temperate conditions

Counter-balanced crossover, 7 well-trained cyclists, 10 days of 90 min exposure. VO2max and time-trial improved at 38°C but were unchanged at 18°C. Acute albumin-induced expansion of plasma volume did not restore the temperate deficit, arguing against the plasma-volume-improves-cool-performance mechanism. Moderate (small crossover, but cleanly designed).