We thank both Kivimäki et al and Epstein and Shoenfeld for their comments about the association of sauna bathing and fatal cardiovascular events. We agree that observational studies cannot establish causality of the relationship given that the potential for residual confounding or reverse causation cannot be totally eliminated, as discussed in our article.1 However, we did observe graded inverse associations with sudden cardiac death (SCD), fatal coronary heart disease (CHD), and cardiovascular disease (CVD) events, which are the characteristics of a true inverse association between sauna bathing and outcomes. Furthermore, results were carefully adjusted for socioeconomic status (SES), physical activity, and cardiorespiratory fitness, all 3 of which are potential surrogate markers of healthy lifestyle.
Even though SES has been shown to influence future risk of CVD and may be associated with access to sauna, our analyses showed that adjustment for SES did not materially alter the association of sauna bathing with CVD. After adjustment for a wide range of cardiovascular risk factors and possible confounders, the associations persisted significantly for SCD, fatal CHD, and CVD events, suggesting the additional health benefits of sauna bathing. In Finland, sauna bathing is a part of the culture and accessible for almost everyone.
A positive, protective effect of sauna bathing on outcomes was observed among a small subgroup of subjects with diabetes, hypertension, and the lowest level of cardiorespiratory fitness. These analyses, however, should considered as explorative subgroup findings. Moreover, we believe it unlikely that people with diabetes or hypertension have a greater fear of cardiac challenge of sauna bathing compared with their counterparts. Of note, the risk of SCD is increased in subjects with dysfunction of the cardiovascular autonomic system, and a typical Finnish sauna session is characterized by warming up in the sauna with subsequent cooling and relaxation, which can positively modulate the autonomic nervous system and may further explain the strong benefit in the reduction of SCD in subjects with diabetes and hypertension.2,3
Epstein et al suggested that sauna bathing may exert its cardiovascular benefits via multiple pathways, including reduction in blood pressure and improvement in metabolic and lipid parameters. However, primarily as a physiological response, sauna bathing increases heart rate, which may not be tolerated among patients with advanced symptomatic CVD. Therefore, we agree that those who are not able to exercise at a low-intensity level may be advised to seek sauna therapy cautiously. Additionally, in particular circumstances, exposure to a very hot sauna could be potentially harmful. However, most people can tolerate a typical warm, dry sauna, which is a pleasurable activity with additional health benefits. Overall, we recommend taking sauna baths as a part of a healthy lifestyle for the prevention of CVDs.