If you have not developed full blown diabetes mellitus type 2, but suffer insulin resistance, then vitamin D supplementation may help you.  A new Chinese study suggests that serum vitamin D is a predictor for insulin resistance in healthy people and vitamin D supplementation may help normalize the metabolic disturbance.Ding L, Wang C, Ma H, Tian Y, Lu Y, Pang S.  The study of serum vitamin d and insulin resistance in chinese populations with normal glucose tolerance. Int J Endocrinol. 2014;2014:870235.