Sometimes your Vitamin D test is clubbed with other tests (for example, blood sugar levels) and hence, your doctor may recommend that you submit your blood sample before and after eating something. Particularly for Vitamin D test, it is not necessary that you fast. In case you are taking some medication and if that medication can have a temporary effect on the levels of Vitamin D in your body, then you may need to consult your doctor regarding whether to abstain taking the medicine for a while or continue taking it.