This study assessed the effect of vitamin D supplementation on the risk of diabetes. 130 men without diabetes and with vitamin D deficiency/insufficiency were given vitamin D or no supplementation. The results showed that the men administered with vitamin D had maintained insulin levels (the action of insulin is important in decreasing blood sugars) compared with men who did not receive vitamin D supplementation. More details can be found in the study.
S Tepper, DR Shahar, D Geva, S Ish-Shalom.