The work stands somewhat apart: nearby in the archive are studies on cell aging, measuring heat inside cells, and DNA copying. All of them deal with molecular mechanisms, but none looks at the whole organism under heat stress. Here, for the first time, two lines come together: epidemiological data on the cardiovascular benefits of sauna and laboratory evidence that heat stress extends lifespan in worms and fruit flies. The authors take a step from correlations toward mechanisms, testing whether sauna leaves a mark on the human epigenome.
The pilot study involved 18 firefighters (15 men, 3 women, mean age 42.6 years). They used a portable sauna for about 20 minutes at least four times a week. Blood samples were taken three times: before the start, one hour after the first session, and after 8-18 weeks of regular use. DNA from leukocytes was analyzed on Illumina MethylationEPIC v2.0 arrays, which cover about 919,000 methylation sites. Then dozens of epigenetic clocks and biomarkers were calculated, and the results were compared with a control group of 36 healthy people of the same age and sex, processed separately.
After a single sauna session, there were no significant changes: none of the 613 predictors passed the multiple-comparison correction. After 8-18 weeks, eight measures changed when adjusting only for age and sex: the telomere length estimate (PCDNAmTL) decreased, the methylation level associated with HDL cholesterol dropped, while the depression risk score, SystemsAge blood and lung age, pulse pressure, cystatin C, and DHEAS increased. However, all these effects vanished when blood immune cell composition was taken into account. At the level of individual CpG sites, none passed genome-wide correction, but after BACON calibration, 126 acute and 329 long-term candidates were identified. Analysis of methylation regions revealed 5 acute and 140 long-term differentially methylated regions, with 134 of the 140 long-term regions being hypermethylated. Among the candidate genes were FOXO3, linked to heat shock response, and HPDL, involved in protection against oxidative stress.
Unlike neighboring studies that examine cellular processes in vitro or in model organisms, this work is the first to test the effect of heat stress on epigenetic clocks in living people. It is not just a correlation but a longitudinal observation with three time points. The main conclusion: methylation changes after sauna are closely tied to immune cell composition, which casts doubt on interpreting such changes as tissue 'rejuvenation' or 'aging.' The work sets a new standard of caution for interpreting epigenetic clocks in intervention studies.
The field could move toward controlled trials with a thermoneutral group, longer follow-up, and direct measurement of body temperature. Of particular interest is comparison with other hormetic stressors, such as cold or high-intensity exercise, which also cause temporary shifts in epigenetic clocks. Perhaps the shared mechanism involves immune system remodeling, and then epigenetic clocks could become a tool for monitoring adaptation to stress.
The results will affect how epigenetic clocks are interpreted in clinical and anti-aging research, as well as the design of future studies on sauna and heat stress.
A larger randomized study with a control group is needed, with direct measurement of body temperature and immune cell composition, as well as tracking the dynamics of changes over several months after the sauna course ends.
The work addresses a fundamental problem in the epigenetics of aging: to what extent methylation changes reflect tissue aging rather than shifts in cell population composition. This is directly related to validating epigenetic clocks as biomarkers of age and to understanding how external stressors affect the epigenome. The results highlight the need to account for cellular heterogeneity when interpreting any interventions aimed at slowing aging.
At the start of the study, the firefighters had a higher methylation-predicted maximal oxygen uptake (VO2max) than the control group, which makes sense for a profession requiring high physical fitness. At the same time, they also had a higher epigenetic damage score, DamAge, and a lower adaptation score, AdaptAge, which may reflect accumulated occupational stress.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.