Heat acclimation
Evidence: moderate
One to two weeks of exercise in the heat reliably improves performance in hot conditions (strong), with associated physiological adaptations including lower core temperature, lower heart rate, and earlier sweating onset. Whether plasma volume expansion is a consistent or significant adaptation is contested across meta-analyses. 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 carries a clear performance benefit when the race itself is hot (Périard et al. 2021; Tyler et al. 2016). These adaptations are well replicated and are the secure case for the method. Plasma volume also expands acutely, often cited at roughly 4 to 7%, but whether this is a consistent or lasting adaptation is contested across meta-analyses (Tyler et al. 2016).
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). Individual controlled trials have been sceptical: a crossover study found the gains in the heat but none in temperate conditions (Keiser et al. 2015), and 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 (Mikkelsen et al. 2019). Pooled evidence nuances this: a meta-analysis of 28 studies found a small positive thermoneutral VO₂max effect versus exercise-alone controls (Hedges’ g = 0.30), suggesting the cool-condition benefit is modest rather than absent (Waldron et al. 2021). For a runner preparing a cool-weather race, heat training is not a reliable ergogenic aid of the magnitude seen in hot conditions; 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). A repeated block does, though: three weeks of post-run sauna bathing in trained runners expanded plasma volume by about 7% and improved endurance by close to 2%, so the sauna is a legitimate delivery route for the heat stress rather than only a comfort measure (Scoon et al. 2007). What it does not do is speed recovery; that separate claim, sauna as recovery, is poorly evidenced. Heat acclimation is an active training stress with measurable adaptations. 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 decays once exposure stops, so the block is timed to finish close to a hot target race rather than weeks before it.