[ddeck]

The study was in Finland, where the sauna to population ratio is around 1:2. Also, the study claims to have adjusted for resting heart rate, which is a reasonable proxy for cardiovascular fitness:

In analysis adjusted for age, alcohol consumption, body mass index, systolic blood pressure, smoking status, Type 2 diabetes, previous myocardial infarction, resting heart rate and serum low-density lipoprotein cholesterol, compared with men with only 1 sauna bathing session per week, the HR for dementia was 0.78 (95% CI: 0.57–1.06) for 2–3 sauna bathing sessions per week and 0.34 (95% CI: 0.16–0.71) for 4–7 sauna bathing sessions per week. The corresponding HRs for Alzheimer's disease were 0.80 (95% CI: 0.53–1.20) and 0.35 (95% CI: 0.14–0.90).

http://m.ageing.oxfordjournals.org/content/early/2016/12/07/...