REF-1246 · Benjamin et al., 2022 · Journal of Science and Medicine in Sport · Paper · PubMed

How often must I heat-train to keep acclimation?

Twice a week, in one small RCT of 24 trained men: after 8 weeks, heart rate in the heat was 151 vs 174 bpm with no heat training. Once a week: no clear difference from none.

B-limited, mixed evidenceMeasured on trained athletes

Benjamin and colleagues at the Korey Stringer Institute tested how often trained athletes need to train in the heat to keep the benefit of heat acclimatisation (HAz; by the usual terms, natural heat) followed by heat acclimation (HA; artificial heat) for 8 weeks.

Twenty-four aerobically trained men completed five heat tests: at baseline, after HAz, after HAz + HA, and 4 and 8 weeks into the maintenance phase, each 60 min of steady-state treadmill exercise at 59.1 ± 1.8% of the speed at VO2peak in WBGT 29.6 ± 1.4 °C, with heart rate and rectal temperature recorded.

After HAz + HA they were randomised to no heat training (HT0), heat training once a week (HT1) or twice a week (HT2); each heat-training session was heated exercise in WBGT 33.3 ± 1.3 °C to achieve hyperthermia (38.5-39.75 °C) for 60 min (the abstract names no temperature site for these sessions).

HAz + HA lowered heart rate and rectal temperature in the heat test (both p < 0.001). At week 8 heart rate was 174 ± 22 bpm in HT0, 151 ± 17 bpm in HT2 (different from HT0, p < 0.023) and 159 ± 17 bpm in HT1 (no evidence of a difference from HT0, p = 0.112).

In HT0, rectal temperature, expressed as % change from post-acclimation, was significantly higher at week 8 (1.8 ± 1.4%, p = 0.009); at week 4 the change was 0.6 ± 1.3% (p = 0.218), in a sentence that does not name the group. The authors conclude that "bi-weekly" heat training (their HT2, twice a week) maintains the adaptations for 8 weeks.

Our reading: the first randomised maintenance-dose data in the base, supporting the house dose of two sessions a week for exercise-heat sessions; once a week shows no evidence of a difference from none, which with about 8 per group is not evidence that it fails. The abstract gives no acclimated values, no rectal-temperature data for the training groups and no group sizes.

Protocol

Design: randomised, between-group (abstract); three arms after a common acclimatisation and acclimation phase.
Participants: n = 24 men, age 34 ± 12 y, body mass 72.6 ± 8.8 kg, VO2peak 57.7 ± 6.8 mL/kg/min ("aerobically trained athletes"); group sizes, training background and season are not given in the abstract.
Acclimatisation and acclimation: HAz then HA (by the usual terms, natural then artificial heat exposure); how each was done (duration, sessions, conditions, season) is not given in the abstract.
Heat tests (five): baseline, after HAz, after HAz + HA, week 4 and week 8 of heat training; 60 min of steady-state treadmill exercise at 59.1 ± 1.8% vVO2peak in WBGT 29.6 ± 1.4 °C in an environmental laboratory; heart rate and rectal temperature recorded throughout.
Maintenance arms: HT0 no heat training; HT1 once a week; HT2 twice a week. Each session: heated exercise in WBGT 33.3 ± 1.3 °C to achieve hyperthermia (38.5-39.75 °C; site not given in the abstract) for 60 min. Other training during the 8 weeks is not described in the abstract.
Analysis: repeated-measures ANOVAs.
Funding and interests: not given in the abstract.

Key Results

Heat test at week 8 (60 min, WBGT 29.6 °C)No heat training (HT0)Once a week (HT1)Twice a week (HT2)
Heart rate, bpm174 ± 22159 ± 17 (vs HT0 p = 0.112)151 ± 17 (vs HT0 p < 0.023)
Rectal temperature, % change from post-HAz + HAweek 8: 1.8 ± 1.4% (p = 0.009); week 4: 0.6 ± 1.3% (p = 0.218; group not named, most plausibly HT0)not given in the abstractnot given in the abstract
Acclimatisation plus acclimation: heart rate and rectal temperature both improved significantly (p < 0.001); the size of the improvement is not given in the abstract.
What the abstract does not report: heart rate at baseline or after acclimation (so how much of the gain each group kept cannot be judged), week-4 heart rate, rectal temperature in HT1 and HT2, whether HT1 and HT2 differed, sweat rate, plasma volume, perceived exertion or any performance measure.
Abstract issues:
"Bi-weekly" means twice a week here (HT2, "twice per week heat training"), not every two weeks; a reader of the title alone could take it the other way.
The week-4 rectal-temperature figure (0.6 ± 1.3%) is not attributed to a group; the sentence goes on to name HT0 for week 8.
"p < 0.023" gives a threshold rather than an exact p value.

Mechanisms

Measured (abstract): acclimatisation plus acclimation lowered heart rate and rectal temperature in the heat; at week 8 the twice-weekly group's heart rate in the heat test was 23 bpm below the no-heat group's (our arithmetic); without heat training the rectal-temperature response was higher at week 8 than after acclimation.
Authors' interpretation: twice-weekly heat training is enough to maintain heat-acclimation adaptations for 8 weeks.
Inference: the decay in HT0 is consistent in direction with REF-1197 (about 2.5% of the heart-rate and core-temperature adaptation lost per day), but because the abstract gives no acclimated values, the share lost here cannot be computed. A non-significant change at week 4 with about 8 per group is no evidence of a difference, not evidence that nothing decayed.
Inference: if "hyperthermia (38.5-39.75 °C) for 60 min" means a held core temperature (the abstract names no site and does not say held), the heat-training sessions were close to the controlled-hyperthermia dose used to build acclimation in REF-3489 (38.5 °C for 60 min); the maintenance stimulus was a full session, not a short exposure.

Practical Application

How often must I heat-train to keep acclimation? Twice a week, in one small RCT of 24 trained men: after 8 weeks, heart rate in the heat was 151 vs 174 bpm with no heat training. Once a week: no clear difference from none.
The house maintenance dose now has a trial behind it, for exercise in the heat: two sessions a week of heated exercise to hyperthermia (38.5-39.75 °C) for 60 min (WBGT about 33 °C) left heart rate in the heat test 23 bpm below no heat training at 8 weeks in trained men (B-, from the abstract, about 8 per group); the authors read this as maintained acclimation, though the abstract gives no acclimated values to compare with.
Once a week is not shown to work, and not shown to fail: 159 vs 174 bpm, p = 0.112, with about 8 per group. Not from a study; the base's own practice: if only one heat slot a week is available, keep it and add the re-exposure days before a hot race suggested by REF-1197's authors (their suggestion, not tested).
Sauna or hot-water immersion as maintenance stays practice: this trial used exercise in the heat; whether two passive sessions a week hold the adaptation is untested (SYNTH-04).
Not tested in women, and no race or time-trial outcome was reported; the claim is about heart rate and core temperature in a heat test.

Limitations

Abstract only: group sizes, how acclimatisation and acclimation were done, acclimated heart rate and rectal temperature, the training groups' rectal-temperature data, the HT1 vs HT2 comparison, other training, funding and conflicts are not given; nothing beyond the abstract's statements could be checked.
Small: 24 men in three arms, about 8 per group; the once-a-week null and the week-4 rectal-temperature null are both underpowered.
Outcomes: heart rate and rectal temperature in a 60-min moderate heat test; no performance, sweat or plasma-volume outcome is reported.
Who: aerobically trained men only; women not studied.
Maintenance stimulus: 60-min exercise sessions to 38.5-39.75 °C in WBGT about 33 °C need a chamber or hot weather; sauna, hot-water immersion and overdressing were not tested.
Written from the abstract and record metadata (full text not read).
How this entry was checked and changed
Why this level

(from the abstract) — a parallel RCT of 24 would be B+ under the rubric, but three arms leave about 8 per group (group sizes are not given in the abstract), which is B design; held at B- because it is an abstract-only small study with key methods missing (group sizes, how acclimatisation and acclimation were done, the mode of the heat-training sessions) and its outcomes are markers (heart rate and rectal temperature in a heat test), not performance. Closed access; no open copy via Europe PMC or OpenAlex