Impact of cold exposure on acute heat adaptations.

This study will examine whether cold exposure immediately after sauna alters the acute heat shock protein (HSP) response to passive heating in humans. Heat shock proteins (particularly circulating HSP70) are cytoprotective signalling molecules that can increase following heat stress, and rapid cooling may plausibly attenuate (“blunt”) this response by reducing thermal strain and shortening the duration of elevated core temperature. Using a randomised, counterbalanced within-subject crossover design, participants will complete two laboratory sessions separated by ~1 week: (1) 30 minutes of sauna (~60°C, 10–20% RH) followed immediately by 10 minutes of cold-water immersion (~10°C), and (2) the same sauna exposure followed immediately by 10 minutes of thermoneutral immersion (~32°C). Core and skin temperature will be continuously monitored, and cardiovascular responses (heart rate, blood pressure) and perceptual responses (thermal comfort, RPE) will be recorded throughout. Venous blood samples will be collected at four timepoints per session (pre-sauna, post-sauna, post-immersion, and 60 minutes post-immersion) to quantify acute changes in HSP70 (primary outcome) and exploratory markers including BDNF, PGC-1α, and blood lactate. The primary expected outcome is that cold-water immersion will attenuate the post-sauna increase in circulating HSP70 compared with thermoneutral immersion, alongside a faster decline in core temperature. Results will provide mechanistic insight into whether common “recovery” practices (cold exposure after sauna) modulate heat-stress signalling, and will inform future studies of heat-based therapies and recovery strategies.