The advice to «spend a week in the sauna before a hot race, or run in an extra layer» migrates from article to article. The number «five days» comes up especially often — it is convenient, it fits into a taper and it does not force you to rewrite the plan. The study we are about to discuss tested that timeline against a hard criterion, and five days did not pass it.

A note on the date up front: the work appeared in the Journal of Applied Physiology in May 2025. We had not covered it before, and its conclusion is practical and inconvenient, so I am catching up.

What exactly was measured

Most heat acclimation work looks at the familiar things: heart rate at a standard workload, plasma volume, the sweating threshold. Here they took a harsher metric — the critical WBGT temperature.

The WBGT index (wet-bulb globe temperature) folds air temperature, humidity and radiant load into a single number. The «critical» level is the point past which the body stops coping: heat loss no longer covers heat production, and body temperature starts to climb relentlessly. In essence it is the border between «hot but bearable» and «it only gets worse from here».

That border is precisely what they watched: would it move upward after acclimation.

How it was run

28 people with no prior acclimation: 15 in the adaptation group, 13 in the control.

The protocol was controlled hyperthermia in heat and humidity: 35 °C and 65% relative humidity. Each session took core temperature to about 38.5 °C and held it there for 60 minutes. The block ran from 5 to 9 days.

Tests with a progressive heat load were run four times: at baseline, after 5 days, after 9 days, and 4–8 days after the block finished.

That last time point is the most interesting one, and protocols like this usually do not have it.

Results

The critical WBGT temperature rose after nine days. After five it did not.

The statement is simple, the implication unpleasant: half the protocol did not deliver half the result. On this measure it delivered nothing.

The second result concerns the rate of rise of rectal temperature — how quickly the body heats up on subsequent heat exposure. It did not fall immediately after the block, but after the nine-day protocol was followed by a recovery period. The authors conclude that a break after acclimation ends is needed for this effect to show up.

Why five days still «work» in other articles

There is no contradiction here, and it is worth saying so plainly so it does not look as though the whole prior literature is being cancelled.

Heat adaptations unfold at different speeds. Plasma volume expansion is fast — a visible shift is there by day three to five. Lower heart rate at a standard workload and an earlier onset of sweating also appear early. Those are exactly the markers the «five days is enough» recommendations were built on.

Shifting the critical tolerance limit, though, is about the capacity to work in conditions where heat loss is fundamentally on the edge. That adaptation assembles more slowly. There is also a separate line of work showing that thermoregulatory adjustments with progressive acclimation are evident predominantly in uncompensable conditions rather than compensable ones — that is, exactly where it is genuinely hot.

Put simply: five days make moderate heat easier for you. Nine days move the line beyond which it becomes dangerous.

What to do about it

Plan a block of nine days or more if the race really is in heat and humidity, not merely «warm weather».
Leave a gap between the block and the start. On these data the reduction in heating rate appeared after a recovery period of 4–8 days. Finishing acclimation the day before the race is a bad idea for two reasons at once: the effect has no time to emerge, and you arrive tired.
Five days is not zero. If there is no more time, a short block still gives you plasma volume and heart rate shifts. Just do not count on it raising your tolerance ceiling.
The study protocol is brutal. An hour at 38.5 °C core temperature is a serious load on the body, and in the study it was supervised. Improvising here is inappropriate, especially with any heart condition.
Acclimation does not cancel the rest. Rehydration, pre-cooling and dousing along the course work on top of adaptation, not instead of it.

Key points

What was measured is the critical WBGT temperature — the border past which heat loss stops covering heat production.
28 participants, 15 of them in the acclimation group; controlled hyperthermia at 35 °C and 65% humidity, about 38.5 °C core temperature for 60 minutes per session.
The critical limit rose after 9 days and did not change after 5.
The rate of rise of rectal temperature fell only after the recovery period that followed the nine-day block.
Earlier five-day recommendations are not cancelled: the fast adaptations (plasma volume, heart rate, sweating) appear early. What assembles slowly is specifically the shift in the tolerance ceiling.
In practice: a block of nine days or more plus a 4–8 day gap before the start, not acclimation squeezed up against the race.

Sources: «The effect of heat acclimation on critical environmental limits and rate of rectal temperature change», Journal of Applied Physiology, May 2025. https://doi.org/10.1152/japplphysiol.01004.2024, https://pubmed.ncbi.nlm.nih.gov/40192265/. «Thermoregulatory adaptations with progressive heat acclimation are predominantly evident in uncompensable, but not compensable, conditions», Journal of Applied Physiology. https://journals.physiology.org/doi/full/10.1152/japplphysiol.00220.2019