Consistent Inverse Associations of Total, "Bioavailable", Free, and "Non-Bioavailable" Vitamin D with Incidence of Diabetes among Older Adults with Lower Baseline HbA 1c (≤6%) Levels.

This large prospective cohort study of older Caucasian adults, in agreement with results from randomized trials and Mendelian randomization studies, supports a protective effect of vitamin D against development of diabetes. The "free hormone theory" may not be relevant in this context. However, our results underline the importance of adequate vitamin D status among those who have not yet shown any sign of impaired glucose tolerance.