Short read
Kunutsor, Jae and Laukkanen analysed middle-aged men from the Kuopio Ischaemic Heart Disease Risk Factor Study (KIHD) for pneumonia, published in the European Journal of Clinical Investigation in 2021. Two exposures were combined: cardiorespiratory fitness, measured by respiratory gas analysis, and self-reported sauna frequency. Over a median of 26.6 years, 529 cases of pneumonia were recorded.
Taken separately, high fitness had a hazard ratio of 0.75 (0.61 to 0.91) compared with low fitness, and frequent sauna use 0.81 (0.68 to 0.97) compared with infrequent use. In the four-way analysis, the reference group was men with low fitness and infrequent sauna use. Against that group, high fitness alone came to 0.88 (0.65 to 1.20) and frequent sauna use alone to 0.89 (0.71 to 1.13). Both intervals include one, so neither reached statistical significance. Men with both had a hazard ratio of 0.62 (0.48 to 0.80), a 38 % lower relative risk, and that interval stays below one.
The data are observational and cover men only, from the same cohort as most Finnish sauna findings. Men who do both probably also differ in characteristics that were not recorded. The paper appeared during the debate on respiratory infections and COVID-19, which belongs in any reading of it.
The message lies in the individual groups: neither habit alone produced a robust signal, the combination did. That does not show that sauna use prevents pneumonia. It shows an association that, in these men, appeared only where training and sauna use came together.