Heat acclimation
Evidence: moderate
One to two weeks of exercise in the heat reliably expands plasma volume and improves performance in hot conditions (strong). Whether it also lifts performance in the cool is contested and probably small or absent in trained athletes. Distinct from sauna-as-recovery.
Heat acclimation reliably prepares the body for racing in the heat. Around one to two weeks of repeated exercise in the heat lowers core temperature and heart rate at a given effort, brings sweating on earlier and harder, and expands plasma volume by roughly 4 to 7%, with a clear performance benefit when the race itself is hot (Tyler et al. 2016). These adaptations are well replicated and are the secure case for the method.
The contested claim is that heat training also lifts performance in cool conditions, as if it worked like altitude. That idea rests largely on one small study in which 10 days of heat acclimation raised VO₂max by about 5% and time-trial performance by 6 to 8% even when the test was cool (Lorenzo et al. 2010). Better-controlled work has not reproduced it. A crossover study found the gains in the heat but none in temperate conditions, and infusing albumin to expand plasma volume did not restore the cool-condition effect (Keiser et al. 2015). The longest controlled trial, 5.5 weeks with a matched control group, found cool-condition time-trial power rose by the same amount in the heat group and the control, so the apparent benefit was the training, not the heat (Mikkelsen et al. 2019). For a runner preparing a cool-weather race, heat training is not a reliable ergogenic aid; for a hot race it clearly is.
The adaptation needs roughly one to two weeks of repeated exercise-heat exposure; a single sauna sitting does not do it (Racinais et al. 2015). This is the key distinction from passive sauna use as recovery, which is poorly evidenced. Heat acclimation is an active training stress with measurable adaptations; sauna-as-recovery is a comfort measure. The grade is moderate overall: strong for the heat-condition benefit and the physiology, weak for the cool-condition transfer.
Protocol
How to acclimate
Around 5 to 14 consecutive days of exercise in the heat, roughly 60 to 90 minutes per session at an effort that raises core temperature and provokes good sweating. Adaptations (plasma-volume expansion, earlier and greater sweating, lower heart rate at a given effort) appear within about a week and decay over a similar period once exposure stops, so the block is timed to finish close to the target race.
In practice, the sessions are a genuine training stress, so they replace rather than add to hard running, and they must be paired with sensible hydration and cooling to be done safely. The adaptation also decays once exposure stops, at roughly the rate it was gained, so the block is timed to finish close to a hot target race rather than weeks before it (Tyler et al. 2016).