Dry and humid heat acclimation produce similar physiological gains in trained cyclists

A crossover trial with 22 trained male cyclists compared eight days of controlled heart-rate heat acclimation in hot-dry versus warm-humid conditions, testing whether each environment produced distinct adaptations and whether training in one transferred to the other. Both regimens yielded broadly similar physiological changes, and each conferred meaningful cross-acclimation benefits when participants were later tested in the opposite environment.

What the study found

Both acclimation regimens were associated with reductions in core temperature and heart rate during subsequent heat stress tests, regardless of whether those tests were conducted in dry or humid conditions. The magnitude of these thermoregulatory improvements was broadly comparable between the two groups, suggesting that neither environment held a clear advantage in producing the core adaptations.

Plasma volume expanded in both conditions, and haemoglobin mass increased similarly across groups — changes that are generally linked to improved cardiovascular efficiency during exercise in the heat. Sweat rate also rose in both regimens, though it was higher throughout the dry-heat sessions, likely reflecting the greater evaporative demand of that environment.

One notable difference was that cyclists training in dry heat were able to sustain a higher power output for the same target heart rate, meaning the dry environment may better preserve training quality during acclimation. This did not, however, translate into meaningfully superior physiological adaptation by the end of the eight-day period.

Cross-acclimation — the degree to which training in one environment prepares the body for the other — was also similar between groups. The authors interpret this as evidence that the overall intensity of the heat stimulus, rather than its specific character, is the primary driver of adaptation.

Limits worth knowing

The trial enrolled only 22 participants, all trained males, which restricts generalizability to women, recreational exercisers, and untrained individuals. The eight-day acclimation window is relatively short, and longer protocols might reveal diverging trajectories. No funding disclosures were identified in the available information, but this could not be confirmed. The study did not assess performance outcomes beyond the controlled heat stress tests, so real-world competitive implications remain speculative. Bayesian credible intervals for some outcomes — particularly plasma volume in the humid group — crossed zero, indicating uncertainty about those specific effects.

About this entry. The Heartwood Review summarizes published, peer-reviewed research in its own words and links to the original source. It is not medical advice; consult a physician before beginning or changing any thermal or recovery practice. Findings summarized here describe the study cited, not any product sold by Teak & Marble Atelier. Corrections.