“Frequent sauna use cuts your stroke risk by 61%.”

The research found this

The figure comes from a 2018 paper in Neurology. Researchers in Kuopio asked 1,628 men and women aged 53 to 74, none of whom had had a stroke, about their sauna habits at an examination between 1998 and 2001. Over a median of 14.9 years, 155 of them had one.1Source 1 · Cohort studySauna bathing reduces the risk of stroke in Finnish men and womenKunutsor SK, Khan H, Zaccardi F, et al.. Neurology, 2018.Read the paper ↗PubMed ↗

Against people who went once a week, those who went four to seven times had a hazard ratio of 0.39 once age and sex were taken into account. Adding established heart risk factors barely moved it, and with physical activity and socioeconomic status included it was 0.38. The link didn't differ between men and women or by age.1Source 1 · Cohort studySauna bathing reduces the risk of stroke in Finnish men and womenKunutsor SK, Khan H, Zaccardi F, et al.. Neurology, 2018.Read the paper ↗PubMed ↗

The full paper sits behind Neurology's paywall, and the abstract leaves out the size of each group, the strokes in each, the estimate for two or three sessions a week and the full list of adjustments. The authors' later review gives subtype results: a hazard ratio of 0.42 for ischemic stroke and 0.33 for hemorrhagic stroke, and the hemorrhagic interval runs from 0.07 to 1.51, which includes no effect.2Source 2 · ReviewCardiovascular and Other Health Benefits of Sauna Bathing: A Review of the EvidenceLaukkanen JA, Laukkanen T, Kunutsor SK. Mayo Clinic Proceedings, 2018.Read the paper ↗PubMed ↗ Its subtype counts of 129 and 34 add up to 163, which is more than the 155 strokes in the paper.

Japan offers a larger comparison with a different kind of heat. Among 30,076 adults followed from 1990 to 2009, people who soaked in a hot tub almost every day had a 26% lower hazard of stroke than people who bathed twice a week or less.3Source 3 · Cohort studyHabitual tub bathing and risks of incident coronary heart disease and strokeUkai T, Iso H, Yamagishi K, et al.. Heart, 2020.Read the paper ↗PubMed ↗ The hazard ratio for bleeding inside the brain was 0.54, and bleeding around the brain showed no link. When strokes in the first ten years were excluded, the trend was no longer significant.

Two letters to Neurology questioned the result. Dippel and colleagues argued that healthier people are the ones who keep bathing often, and that similar links with Alzheimer's disease and respiratory illness point to that bias.4Source 4 · ReviewReader response: Sauna bathing reduces the risk of stroke in Finnish men and women: A prospective cohort studyDippel DW, Fransen PS, Ikram MK. Neurology, 2019.Read the paper ↗PubMed ↗ A second letter worried about people in their sixties, many of whom have undetected narrowing in the arteries to the brain, where heat and fluid loss could cut blood flow.5Source 5 · ReviewReader response: Sauna bathing reduces the risk of stroke in Finnish men and women: A prospective cohort studyYu Y, Guo WT, Yan XL, et al.. Neurology, 2019.Read the paper ↗PubMed ↗ The authors replied that the result held when they excluded the first two years of follow-up and people with existing disease, and they agreed that observational studies can't prove cause.6Source 6 · ReviewAuthor response: Sauna bathing reduces the risk of stroke in Finnish men and women: A prospective cohort studyKunutsor SK, Laukkanen T, Laukkanen JA. Neurology, 2019.Read the paper ↗PubMed ↗