PD Dr. med. Rainer Brenke: Is there a lower susceptibility to flu-like infections as a result of regular visits to the sauna?
Summary of a synopsis prepared for the International Sauna Association
The ongoing COVID 19 pandemic has once again brought to public attention the interest in increased immune defence and thus a lower susceptibility to infection through regular sauna visits. The fact that coronaviruses are very temperature-sensitive and are inactivated within a very short time at temperatures above 40°C allows us to conclude that there is hardly any risk of infection in sauna cabins. However, this is not true for other areas of a sauna facility such as changing rooms, rest areas, dining areas etc. and special hygiene policies would be required for them. At the same time, hardly any regular sauna user doubts that the tendency to catch a cold and thus probably even to get the coronavirus is reduced by sauna bathing. But is there any scientific evidence for this? To answer this question, literature published since the 1940s was subjected to an analysis under two aspects:
- How reliable is the statement that regular sauna visits lead to a lower susceptibility to infections in terms of increasing resistance?
- Which mechanisms play a role in this?
It is astonishing that there is relatively little proven evidence to answer the first question, if we apply the standards of evidence-based medicine required today. Summary reports of sauna users’ experiences were already available about 80 years ago, and more precise statistics have been available since the 1950s. However, the older studies did not fulfill criteria such as randomisation, i.e., random assignment to a sauna group or a comparison group. Statistical methods were not as advanced then. For example, during a flu epidemic in the 1950s, the incidence of illness among sauna users was compared with non-sauna users. The result was impressive and showed that sauna users were better off. From today’s perspective, however, the result can only be regarded as an indication, since the other parameters (playing sports, eating, smoking, etc.) in which the groups otherwise differed were not recorded and also because no random allocation was done. In 1990, a study by Ernst et al. was published that at least comes close to the standards required today and is therefore still often cited today. In each case, 25 test persons went to the sauna either once or twice a week; the other test persons served as a comparison group. Later, the study was extended to include 25 subjects who took alternate showers (hot/cold) at least 5 times a week. After 3 months, the frequency of flu-like infections was significantly lower in the subjects who had increased resistance thanks to the sauna and alternating showers, than in the comparison group. However, this study was not strictly randomised either.
In the following years, there were repeated studies that confirmed the decrease in the frequency of flu-like infections, but did not fulfill all the statistical requirements that are customary today. These include studies of kindergarten and school children. In recent years, Finnish studies have been impressive with their large number of test subjects, but in Finland you can hardly find a comparison group that does not go to the sauna. The lack of randomisation also proves to be a shortcoming here, because it is not possible, for example, to generalise the statement that the more often one goes to the sauna during the week, the lower the risk of a whole range of diseases. It may also be that healthier people go to the sauna more often. The Finnish findings are therefore extremely exciting, but they also require verification through sophisticated research methodology.
In medicine, a distinction is made today between 4 levels of evidence and 3 classes of recommendations for action. From today’s perspective, we would derive evidence level IIa from the small number of high-quality but non-randomised studies on the preventive effect of the sauna against flu infections and thus the recommendation for action IIa, which means an acceptable and reasonable recommendation for the use of sauna as part of prevention. However, it should be pointed out that it is always about prevention, not cure. The sauna is not indicated as a therapy for an already existing infection with fever.
Studies on the physiological effects of sauna are much more extensive. When it comes to the topic of “resistance” to infections this much can be said for sure:
- There is improvement of the blood flow regulation in the skin and mucous membranes and thus an increased defence against infections. A stimulation of the so-called endothelial cell function plays a role here, i.e., the cells that line the blood vessels and can at least widen the larger arterial vessels.
- Stimulation of non-specific defence processes, such as immunoglobulin A production in the mucous membranes, interferon in the blood, killer cell activity in the blood and others. Specific antibody production seems to be less affected.
- Change in the autonomic nervous system with a decrease in the tensile state of the sympathetic nervous system activated during stress.
- Biochemical adaptations with better management of free radicals, which play a crucial role in inflammatory processes.
- If viruses have already settled in the nasopharynx but have not yet led to an illness, they could ideally be inactivated by the heat stimulus.
In the immunological, physiological and biochemical fields, more and more specific and precise findings on the effects of warm and cold stimuli, and thus also of the sauna, are being investigated and published. From our point of view, however, one focus of sauna research in the next few years should also be on the field of epidemiology, i.e., on clarifying beyond doubt whether, for example, regular sauna use leads to a lower susceptibility to viral infections. The question of frequency and duration of sauna visits should also be included in the investigations. Growing knowledge of subtle mechanisms is of particular interest when there is also clear evidence of a lower susceptibility to disease or a milder course of disease.