Personally, I like sauna bathing. It makes me feel fine. But is it healthy? More specifically, is it good for the cardiovascular system?
Finnish researchers had already shown in a large cohort study with 20 years of follow-up that increased frequency of sauna bathing is associated with a reduced risk of sudden cardiac death (SCD), fatal coronary heart disease (CHD), fatal cardiovascular disease (CVD), and all-cause mortality. Now the same group of researchers report more encouraging news for sauna-fans.
The aim of their new study was to investigate the relationship between sauna habits and CVD mortality in men and women, and whether adding information on sauna habits to conventional cardiovascular risk factors is associated with improvement in prediction of CVD mortality risk.
Sauna bathing habits were assessed at baseline in a sample of 1688 participants (mean age 63; range 53-74 years), of whom 51.4% were women. Multivariable-adjusted hazard ratios (HRs) were calculated to investigate the relationships of frequency and duration of sauna use with CVD mortality.
A total of 181 fatal CVD events occurred during a median follow-up of 15.0 years (interquartile range, 14.1-15.9). The risk of CVD mortality decreased linearly with increasing sauna sessions per week with no threshold effect. In age- and sex-adjusted analysis, compared with participants who had one sauna bathing session per week, HRs (95% CIs) for CVD mortality were 0.71 (0.52 to 0.98) and 0.30 (0.14 to 0.64) for participants with two to three and four to seven sauna sessions per week, respectively. After adjustment for established CVD risk factors, potential confounders including physical activity, socioeconomic status, and incident coronary heart disease, the corresponding HRs (95% CIs) were 0.75 (0.52 to 1.08) and 0.23 (0.08 to 0.65), respectively. The duration of sauna use (minutes per week) was inversely associated with CVD mortality in a continuous manner. Addition of information on sauna bathing frequency to a CVD mortality risk prediction model containing established risk factors was associated with a C-index change (0.0091; P = 0.010), difference in - 2 log likelihood (P = 0.019), and categorical net reclassification improvement (4.14%; P = 0.004).
(Hazard ratios for cardiovascular mortality by quartiles of the duration of sauna bathing. a Adjusted for age and gender. b Adjusted for age, gender, body mass index, smoking, systolic blood pressure, serum low-density lipoprotein cholesterol, alcohol consumption, previous myocardial infarction, and type 2 diabetes. CI, confidence interval.)
The authors concluded that higher frequency and duration of sauna bathing are each strongly, inversely, and independently associated with fatal CVD events in middle-aged to elderly males and females. The frequency of sauna bathing improves the prediction of the long-term risk for CVD mortality.
These results are impressive. What could be the underlying mechanisms? The authors offer plenty of explanations: Dry and hot sauna baths have been shown to increase the demands of cardiovascular function. Sauna bathing causes an increase in heart rate which is a reaction to the body heat load. Heart rate may be elevated up to 120–150 beats per minute during sauna bathing, corresponding to low- to moderate-intensity physical exercise training for the circulatory system without active muscle work. Acute sauna exposure has been shown to produce blood pressure lowering effects, decrease peripheral vascular resistance and arterial stiffness, and improve arterial compliance. Short-term sauna exposure also activates the sympathetic nervous and the renin-angiotensin-aldosterone systems and the hypothalamus-pituitary-adrenal hormonal axis, and short-term increases in levels of their associated hormones have been reported. Repeated sauna exposure improves endothelial function, suggesting a beneficial role of thermal therapy on vascular function. Long-term sauna bathing habit may be beneficial in the reduction of high systemic blood pressure, which is in line with previous evidence showing that blood pressure may be lower among those who are living in warm conditions with higher ambient temperature. Regular sauna bathing is associated with a lowered risk of future hypertension. Typical hot and dry Finnish sauna increases body temperature which causes more efficient skin blood flow, leading to a higher cardiac output, whereas blood flow to internal organs decreases. Sweat is typically secreted at a rate which corresponds to an average total secretion of 0.5 kg during a sauna bathing session. Increased sweating is accompanied by a reduction in blood pressure and higher heart rate, while cardiac stroke volume is largely maintained, although a part of blood volume is diverted from the internal organs to body peripheral parts with decreasing venous return which is not facilitated by active skeletal muscle work. However, it has been proposed that muscle blood flow may increase to at least some extent in response to heat stress, although sauna therapy-induced myocardial metabolic adaptations are largely unexplored. There is also evidence that regular long-term sauna bathing (average of two sessions per week) increases left ventricular ejection fraction. Heat therapy may improve left ventricular function with decreased cardiac pre- and afterload, thereby maintaining appropriate stroke volume despite large reductions in ventricular filling pressures. Additionally, previous studies have demonstrated a positive alteration of the autonomic nervous system and reduced levels of natriuretic peptides, oxidative stress, inflammation, and norepinephrine due to regular sauna therapy.
It is possible that the results are influenced by confounding factors that the researchers were unable to account for. It is also possible that people who are already ill avoid sauna bathing and that this contributed to the findings. However, the authors did their best to explore such phenomena in sub-group analysis and found that a causal relationship between sauna and CVD risk is still very likely. As a sauna-fan, I am inclined to believe them and the sceptic in me tends to agree.