> Vitamin D naturally occurs when people go outside and get exposed to the sun, and going outside is correlated with all sorts of healthy traits. There is a di

> Vitamin D naturally occurs when people go outside and get exposed to the sun, and going outside is correlated with all sorts of healthy traits. There is a difference between studying the effects of vitamin D and the effects of vitamin D supplements though. I agree with you on the "general health" variable, because I think it's likely there is some correlation between being somewhat health-conscious and taking vitamin D supplements (unprompted). However, there is no correlation between going out in the sun and taking supplements. (There might actually be a slightly negative one as some people might start taking them in winter when they don't get a lot of sunlight). So if you see a strong correlation between taking vitamin D supplements and reduced prevalence of dementia, I don't think this could be caused by sunlight exposure.