The hot air of a sauna makes your heart beat faster and your blood vessels widen. But under the skin something less obvious happens: heat leaves marks on the packaging of DNA. These marks are chemical tags that do not change the text of the genes themselves, but they influence which pages get read. For the first time, scientists have checked how regular sauna use rewrites such marks in living people.
The study involved 18 firefighters who sat in a portable sauna for about 20 minutes at least four times a week. Blood was drawn three times: before the start, an hour after the first session, and after 8-18 weeks of regular use. DNA from immune cells was analyzed on chips covering about 919,000 methylation sites. Then the researchers calculated dozens of epigenetic clocks, formulas that use these marks to estimate the biological age of different tissues.
But when the researchers accounted for immune cell composition, all the effects disappeared. This is the key point. Epigenetic clocks are often calculated from a mixture of cells, and a sauna can change the proportions of white blood cells: for instance, more cells of one type, fewer of another. Then the clocks show "aging" not because the cells have aged, but because the test tube ended up with a different set of cells.
At the level of individual marks, no single site passed strict testing, but after statistical calibration the team identified 126 acute and 329 long-term candidates. Among the candidate genes was FOXO3, linked to the heat shock response, and HPDL, involved in protection against oxidative stress. These are hints that heat really does switch on defensive programs in cells.
The results will help interpret epigenetic clocks correctly in studies of lifestyle and stress. If someone claims a sauna rejuvenated their blood by five years, it is worth asking: did the white blood cell composition simply change? Future work needs trials with a control group, body temperature measurements, and direct immune cell counts. Then it will become clear whether heat is an editor that only temporarily changes the markup of the text, or whether it can rewrite the text itself.
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.