TARGET-D is an in-progress study that supplements vitamin D based on blood levels (your idea).
I think that study is already concluded but the results haven't been published yet.
https://clinicaltrials.gov/study/NCT02996721?tab=study
alt Hacker News
I think that study is already concluded but the results haven't been published yet.
https://clinicaltrials.gov/study/NCT02996721?tab=study