Reader response: Sauna bathing reduces the risk of stroke in Finnish men and women: A prospective cohort study

We read with interest the article by Kunutsor et al.,1 which describes the association of sauna bathing with the risk of stroke in a large cohort of Finnish men and women. Cohort studies comparing lifestyle factors in relation to cardiovascular risk can suffer from a specific form of selection bias, for which different terms are used throughout the literature: healthy volunteer bias or healthy user bias.2 Individuals who volunteer for a study may possess different characteristics than those in the target population. This implies that unobserved factors may explain the difference in stroke risk and not the exposure to, in this case, sauna bathing. The authors adjusted for baseline risk factors, but during the mean 14 years of follow-up, large differences in risk factors and exposure may have occurred, which could account for the observed effect. Moreover, the lack of an adequate explanation of the biological mechanism and the similar effects on conditions such as Alzheimer disease and respiratory conditions suggest healthy subject bias rather than a specific effect of sauna bathing.3,4 In view of the healthy volunteer bias and an inadequate reference group, any inferences from this study cannot be generalized to the general population.