Whole-blood DNA-methylation patterns during portable-sauna use in firefighters: an exploratory, single-arm pilot study

Research in model organisms has consistently shown that mild, non-lethal heat stress can prolong lifespan. Although causal data are lacking in more complex animals, human observational research has linked sauna bathing to cardiovascular protection and improved age-related outcomes. To examine whether sauna use may influence aging-related biology, we retrospectively analyzed longitudinal whole-blood methylomic data from 18 firefighters who used a portable sauna. A separately processed cohort of 36 matched adults provided descriptive context; firefighters showed higher DNA methylation– estimated maximal oxygen uptake (DNAmVO2 max), lower AdaptAge, and higher DamAge. After 8–18 weeks, eight predictors met a within-family Benjamini–Hochberg false-discovery-rate (FDR) threshold of <0.05 underage/sex-adjusted epigenetic age acceleration: PCDNAmTL, DepressionBarbu, SystemsAge Blood, SystemsAge Lung, DNAmPulsePressure, DNAmHDL, DNAmCystatinC, and DNAmDHEAS. DNAmDHEAS increased, whereas the other predictors changed in directions consistent with short-term findings for high-intensity interval training, another hormetic stressor. None met the within-family FDR threshold after adjustment for 12-cell leukocyte composition, although effect estimates were not uniformly eliminated. At individual cytosine-phosphate-guanine (CpG) sites, BACON empirical-null calibration retained 126 acute and 329 long-term candidates at P<0.0001; none was FDR-significant. DMRcate identified five acute and 140 long-term differentially methylated regions using a harmonic mean of individual CpG FDR values <0.05. Long-term gometh analysis also identified ATP-binding cassette (ABC) transporter enrichment (FDR=0.026). These exploratory findings suggest that portable-sauna use may influence the blood methylome within 8–18 weeks while underscoring the importance of leukocyte-composition and technical sensitivity analyses.