Like all studies into Vitamin D supplementation that aren’t randomized controlled trials, it’s very hard to tell apart cause and effect. 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. Also, publicity of the association of vitamin D with good health has led people who care about their health or who have access to good medical care to supplement vitamin D. Studies that look at the health of people who choose to supplement vitamin D are examining the overall health of those people and can’t specifically measure the effects of the vitamin D supplementation. That’s one of the problems randomized controlled trials are designed to solve.
The study mentions these limitations:
> Considering that sun exposure is the most important natural source of vitamin D, the lack of information on participant-level exposure to sunlight can be considered another limitation of the present study.
> While we included education in our models, the NACC dataset has a dearth of information related to SES and therefore, SES differences could not be accounted for in our study. Future studies using cohorts with more comprehensive data on SES and other social factors would certainly provide valuable information on the associations of SES with exposure to supplementation and risk of dementia.
For what it’s worth, controlling for a variable like education can only partially remove related bias from a result (you don’t have a perfect measure of how a person decided to take vitamin D).