November 29, 2018

expert reaction to study looking at sauna use and cardiovascular death risk

Research published in BMC Medicine demonstrates that regular sauna bathing is associated with reduced risk of fatal cardiovascular disease in middle aged men and women.

Prof Kausik Ray, Professor of Public Heath, Imperial College London, said:

“This is a 15 year observational study (so it cannot prove causation) suggesting that among sauna users both frequency and duration of sauna use were associated with survival benefits.  This study did not include non sauna users, so we do not know whether sauna use is associated with better outcomes versus non-users.  People who adopt health behaviours e.g. sauna use may have other favourable characteristics, diet or lifestyle adherence to therapies not captured in the study, so we cannot be certain unmeasured confounders did not account for the findings.”

Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:

“This is an interesting and competent piece of research, continuing work that the same team have done previously on possible relationships between sauna bathing and health.  However, it’s difficult to see what its implications might be for other countries where the lifestyle is a lot different from the part of Finland where the research was done.  Of the people studied, only 43 – that’s about 2 per cent –  said that they did not use saunas at all.  It would be much easier to find a lot of people who never use saunas in the UK, for instance.  The number of non-users is so small that it wasn’t possible for the researchers to compare the risk of death from cardiovascular disease (CVD – mainly strokes and heart disease) in people who didn’t use saunas with those who did.  Their main comparisons are with people who used a sauna only once a week – and three-quarters of their subjects used a sauna more than that.  That isn’t the only difference between this part of Finland and countries like the UK – for example, nearly half the people studied had their own summer cottage.

“Such differences also make it quite difficult to interpret the research results even for the region where the research was done.  The only difference in the risk of death from CVD for which there was clear statistical evidence was in comparing people who used the sauna quite a lot even for Finns (4-7 times a week) with those who used it much less (once a week).  But the research report makes it clear that very frequent users differ from the once-a-week people in many ways, on average – they drink considerably more alcohol, are more active physically, earn a bit more, are much less likely to have a degree, and are much more likely to live in a house rather than a flat.  The researchers made statistical adjustments for most of these differences in their comparisons of CVD deaths, and although this made the difference in death rates smaller, it was still there.  However, it remains possible that there are other differences between frequent and less frequent sauna users that are the real cause of the difference in death rates, rather than the actual sauna use, and that adjustments were not made for these differences.  Maybe frequent users tend to work in different types of job, or live in different types of area (rural or city, for instance).  Other research has shown that such things can, under some circumstances, lead to differences in CVD death rates.  That’s why the press release rightly says that the study doesn’t allow conclusions to be drawn about what causes what.  The research report does suggest various ways in which having a sauna might cause changes in the body that might protect against heart disease and strokes, but this study doesn’t provide evidence on whether any of these ways were actually operating in these people.  Maybe they were, or maybe they weren’t.

“We also can’t tell from this study whether it’s necessary to use a sauna frequently over a very long period of time to obtain health benefits.  That’s because, as the research report describes, in Finland people’s sauna habits seem not to change much over time.  The people studied were asked only once about how often they used a sauna, at the start of the research, but perhaps they had been using the sauna at the same kind of frequency for many decades before that.  That’s just one reason why, as the researchers themselves say, more studies would be needed to investigate possible effects of frequent saunas in “populations who are not accustomed to regular sauna bathing”.  And perhaps even in Finland, there are other aspects that need to be looked at again.  The researchers found quite a large difference between people with a history of diabetes and those without.  There was no real evidence of a difference in CVD death rates, between frequent and less frequent sauna users, for those who had diabetes, while there was quit strong evidence of a difference for those who had not had diabetes.  As the researchers point out, this comparison does have some statistical problems, particularly because most of the people studied had not had diabetes, but surely it would be important to look further at the role of diabetes before recommending frequent sauna use more widely.”

‘Sauna bathing is associated with reduced cardiovascular mortality and improves risk prediction in men and women: a prospective cohort study’ by Tanjaniina Laukkanen et al. was published in BMC Medicine at 01:00 UK time on Thursday 29 November 2018.

Declared interests

Prof Kausik Ray: “No conflicts.”

Prof Kevin McConway: “Kevin McConway is a trustee of the SMC.”