Short read
Kunutsor and colleagues took two exposures from the Kuopio Ischaemic Heart Disease Risk Factor Study (KIHD), a long-term prospective cohort of Finnish men, and analysed them jointly rather than one at a time: objectively measured cardiorespiratory fitness and self-reported sauna frequency. Sauna use was grouped as low, up to two sessions a week, or high, three to seven sessions a week. The paper appeared in Annals of Medicine in 2018.
Men with high fitness and 3 to 7 sessions a week had the lowest risk of cardiovascular and all-cause mortality, measured against men with low fitness and infrequent sauna use. The combination was also associated with a lower risk than either high fitness or frequent sauna use on its own.
The data are observational. Fitness and sauna frequency are both likely to be linked to a generally health-oriented lifestyle, and men who do both probably differ in ways the model cannot capture. The cohort includes men only and is the same Finnish cohort behind almost every other finding in this field.
The authors read the pattern as additive rather than competing. That is its relevance for recovery: in these data, sauna use sits next to training, not in place of it. The study does not show that sauna use can replace exercise, nor that adding sessions changes an individual's risk.