In line with previous reports,9, 28 both SBP and FSB were independently associated with CVD mortality in this cohort of middle-aged and older Finnish men. The associations persisted on mutual adjustment for each exposure. Our current findings based on the prospective associations of the SBP-FSB phenotype with the risk of CVD death showed that CVD mortality was increased in men with elevated SBP and low FSB, but the risk was attenuated to null in men with elevated SBP combined with high FSB. In interaction analysis, the association between the combined exposures (i.e., high SBP and low FSB) and CVD mortality risk exceeded the sum of their associations considered separately.
Though high blood pressure or hypertension is a major risk factor for CVD globally,7 systolic hypertension is considered a more important determinant of cardiovascular outcomes than diastolic hypertension.8 The relationship between SBP and CVD has been described as strong, graded and causal.6, 34 Sauna bathing (a passive heat therapy) has been reported to produce physiological responses and adaptations that are similar to those produced by moderate or high intensity physical activity.35 Pathways proposed to underlie the associations between passive heat exposure and decreased risk of CVD include beneficial modulation of cardiovascular risk factors such as blood pressure, lipids, and natriuretic peptides; reduction in oxidative stress and low-grade systemic inflammation; improvement in endothelial function; beneficial modulation of the cardiac autonomic nervous system; improved arterial stiffness, arterial compliance, and intima media thickness; and overall improvement in cardiovascular function.10 In our previous prospective study, we showed that regular sauna bathing was associated with reduced risk of incident hypertension.18 Based on the current findings and previous evidence of the ability of frequent sauna baths to mitigate the adverse effects of other risk factors,25, 26 one may conclude that the protective effects exerted by frequent sauna baths are large enough to offset the adverse effects of high SBP. Further investigations are required in the form of mechanistic studies.
These findings add to the emerging evidence on the ability of frequent sauna exposure to prevent some adverse health outcomes and also mitigate the adverse effects of other risk factors.25, 26 Regular physical activity plays a pivotable role in the management of high blood pressure,30 the major risk factor for CVD; regular aerobic exercise results in mean reductions in blood pressure of 5–7 mmHg among individuals with hypertension and these reductions translate to a reduced risk of CVD of 20–30%.36 Given that recent evidence suggests that regular heat therapy is able to lower blood pressure to a degree comparable to that of physical activity,37 this suggests that adding frequent sauna bathing to regular physical activity may yield substantial benefits on blood pressure and cardiovascular risk. Indeed, in a recent randomized controlled trial, we showed that sauna bathing had a substantial supplementary effect on levels of cardiorespiratory fitness, SBP and total cholesterol when combined with exercise;17 eight weeks of regular sauna bathing sessions combined with exercise produced a mean reduction in SBP of 8 mmHg as compared to exercise training alone in individuals with at least one traditional cardiovascular risk factor.17
Some may argue that given that sauna bathing is more commonly used in Nordic countries, the potential beneficial implications may not be applicable in other populations. However, several definitive epidemiological and interventional investigations have reported robust evidence on the health benefits of sauna bathing over the last decade; furthermore, sauna bathing is now becoming a common lifestyle activity on a global scale.38, 39 Sauna use has a good safety profile, and most people in generally good health can tolerate it without significant risks.10 Individuals at risk of orthostatic hypotension should exercise caution during sauna sessions because of the pronounced blood pressure lowering effect, which may also occur during the recovery period after a sauna session. Contraindications to sauna use have included unstable angina pectoris, recent myocardial infarction, uncontrolled hypertension, decompensated heart failure or severe aortic stenosis.10 Consistent with physical activity and exercise recommendations, there is enough evidence to justify the promotion and wider use of sauna among the population.
The current study is novel, being the first evaluation of the clinically significant interplay between SBP, sauna bathing and CVD mortality. Other strengths include formal investigation of the interactions between SBP and FSB in relation to CVD mortality, the use of a population-based prospective cohort design comprising a relatively large sample homogeneous sample of men, the long-term follow-up duration of the cohort, and availability of a comprehensive panel of potential confounders for adjustment. The limitations are mostly inherent to the study design and included the lack of generalisability of the results to women, misclassification bias due to self-reported sauna habits, lack of data on possible changes in the use of medication during the long follow-up and potential biases of observational cohort designs such as residual confounding, reverse causation, and regression dilution bias.