MDPI follows an author-pays model, which incentivizes it to accept all submissions instead of publishing only good-quality work. In fact, this publisher is highly suspect, as it has allegedly pressured peer reviewers to accept low-quality work for publication in the recent past:
I would treat this paper with, at most, the same skepticism as a preprint. Vitamin D supplementation is so easy and inexpensive, that if this correlation is truly that strong, I'd expect to see it in The Lancet or Nature or any one of the other mainstream journals.
http://www.sciencemag.org/news/2018/09/open-access-editors-r...
https://www.universityaffairs.ca/features/feature-article/be...
I would treat this paper with, at most, the same skepticism as a preprint. Vitamin D supplementation is so easy and inexpensive, that if this correlation is truly that strong, I'd expect to see it in The Lancet or Nature or any one of the other mainstream journals.