Head-out immersion in hot water increases serum BDNF in healthy males.
Level 2 - randomized trial
Randomized crossover trial
PubMed 29157042 · doi:10.1080/02656736.2017.1394502
What was done
Eight healthy men underwent two 20-minute head-out water immersion (HOI) sessions in a randomized crossover design separated by at least 7 days: hot water immersion at 42°C and neutral water immersion at 35°C. Venous blood was sampled at rest, immediately after HOI, and at 15 and 30 minutes post-immersion. Investigators measured core body temperature, serum brain-derived neurotrophic factor (BDNF), serum S100β, plasma cortisol, and platelet and monocyte counts.
What was found
Core body temperature was significantly higher at the end of hot-HOI and at 15 minutes post-immersion (p < 0.01), returning to pre-immersion levels at 30 minutes. Serum BDNF was significantly elevated immediately after hot-HOI and at 15 minutes post-immersion (p < 0.05), returning to baseline at 30 minutes. Cortisol was lower at the end of hot-HOI and recovered during follow-up. S100β, platelet count, and monocyte count remained unchanged. Neutral-HOI produced no significant changes in core body temperature, BDNF, S100β, cortisol, or cell counts. Exact numerical concentrations were not reported in the abstract.
Why it matters
This study provides proof-of-concept in humans that passive hyperthermia transiently elevates circulating BDNF, suggesting thermal stress directly stimulates BDNF production rather than altering blood-brain barrier permeability or platelet dynamics.
Limits
The sample size was very small (n = 8) and comprised exclusively healthy young adult males. The study evaluated only short-term acute responses up to 30 minutes post-immersion without functional or cognitive endpoints, and absolute numerical values were omitted from the abstract.
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