Seems flawed to me. When the individuals with genetic factors that make them Vit-D deficient go to the doctor, they will almost certainly be told to take Vit-D

Seems flawed to me. When the individuals with genetic factors that make them Vit-D deficient go to the doctor, they will almost certainly be told to take Vit-D supplements at a higher rate than those without the genetic factor. This confounds the results and makes this type of study useless as it does not measure actual vitamin-D levels.