A new Bayesian meta-regression of heat acclimation studies, published this month in Experimental Physiology and already the most-downloaded paper in the journal this year, has given coaches the clearest picture yet of how much heat exposure an endurance runner actually needs, how quickly adaptations appear, and how long they last once the training block ends. The headline conclusion — that most physiological adaptations associated with heat acclimation are induced within the first week and are generally considered complete after 10 to 14 days of exposure — is not itself new. What is new is the confidence interval around that claim, and the first credible quantification of the dose–response curve that coaches have been arguing about for twenty years.
The review pooled data from 78 studies and 1,412 participants, spanning trained distance runners, cyclists, military personnel, and recreational exercisers across laboratory and field protocols. Plasma volume expansion, the central early adaptation, plateaus at around day five with a mean increase of 6.9 per cent over baseline. Sweat-rate improvements are slower and more variable, with a meaningful effect appearing by day seven and the full adaptation by day fourteen. Resting core temperature at a standardised workload drops by an average of 0.32°C after ten days of exposure. All three effects exhibit diminishing returns: doubling the block from 14 to 28 days produces only a further 0.6 per cent plasma-volume gain, a finding that aligns with the anecdotal experience of British marathoners who have relocated to Kenya for four-week heat blocks.
For runners, the single most practical finding in the new review concerns active versus passive exposure. A head-to-head comparison of active protocols (running or cycling in the heat) and passive protocols (saunas, hot baths, or thermal chambers without exercise) shows active to be roughly twice as effective per hour of exposure for the adaptations most relevant to race performance — plasma volume expansion, sweat-rate adjustment and thermal tolerance. Passive protocols still produce benefits and are genuinely useful for runners who cannot get to a warm climate, but the effective dose is higher. The review estimates that 60 minutes of running at 70 per cent of maximum heart rate in a 35°C environment delivers equivalent adaptations to roughly 90 to 120 minutes of post-run sauna exposure.
The decay kinetics will matter more to readers planning summer marathons. Heat adaptations begin to decay roughly a week after the last exposure and are largely lost by day 28, though plasma volume is the slowest to regress. The practical implication is that the "pre-block" approach favoured by some coaches — completing a 14-day heat camp four weeks out from a hot race — is almost certainly suboptimal compared to a maintenance protocol of two or three heat sessions per week during the taper. The review's authors explicitly recommend a "bookending" structure: a concentrated block at 6 to 3 weeks out, followed by twice-weekly top-ups through race week, with the final session no later than 72 hours before the start.
None of this amounts to a revolution. Elite marathon and ultra camps have been running bookending protocols for a decade, and the findings largely validate the practice. What the review provides is a defensible evidence base for the recreational runner who has read conflicting things on the internet and wants to know whether a week of afternoon jogs in the park during a UK heatwave is worth anything. The answer, unusually, is yes: on the evidence assembled here, even relatively modest exposure delivers quantifiable gains for a summer race, provided the exposure is active and the block ends close to race day. Whether any given runner's A-race is worth the discomfort is, as ever, a separate question.