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Laukkanen and colleagues analysed the Kuopio Ischaemic Heart Disease Risk Factor Study (KIHD) for two endpoints: dementia of any type and Alzheimer's disease. The prospective cohort included 2,315 men aged 42 to 60 who were healthy at baseline, followed for a median of 20.7 years. Sauna use was grouped into once, two to three times and four to seven times a week. The study appeared in Age and Ageing in 2017.
Men with four to seven sessions a week had a hazard ratio of 0.34 for dementia and 0.35 for Alzheimer's disease compared with men who went once a week, a relative risk 66 % and 65 % lower. Against the same reference group, two to three sessions a week were associated with a 21 % lower relative risk. The models were adjusted for age, alcohol consumption, BMI, systolic blood pressure, smoking status, type 2 diabetes, previous myocardial infarction, resting heart rate and LDL cholesterol.
The design is observational and covers men only, so it shows an association and not a cause. Dementia diagnoses collected over such long periods are prone to error. The lead author, Jari Laukkanen, frames the result cautiously: sauna use may relate to heart and memory through similar mechanisms that are not yet well understood, and relaxation and wellbeing may also play a part.
The authors suspect a shared vascular mechanism behind the cardiac and the cognitive findings but stress that causality remains open. For practice, the numbers describe men in one Finnish cohort, measured against weekly use rather than against no sauna. They do not show that sauna use prevents dementia in any individual.