Regular doses of vitamin D (VTD)—about 2000 IU/d—early in life have been shown to reduce the risk of developing type 1 diabetes (up to an 80% reduction projected over the next 30 years).1 Vitamin D treatment has also improved glycemic control and insulin sensitivity in people with type 1 diabetes, people with type 2 diabetes, and normal individuals.2–4 Low VTD levels in elderly men have been associated with insulinemia and glucose intolerance.5 Low VTD levels in general have been associated with insulin resistance and obesity.6 Likewise, β-cell dysfunction and insulin resistance are also associated with low VTD levels.3 The following case reports demonstrate a substantial improvement in hemoglobin A1c (HbA1c) levels with the reversal of VTD deficiency. Changes in VTD levels and HbA1c levels are summarized in Table 1.

These cases support information that is already known about VTD and its effect on the islet cell. As discussed above, this might be true only for vitamin D3 and not vitamin D2, although vitamin D2 has been shown to improve bone health. Vitamin D insufficiency or deficiency is common, and repletion might improve glycemic control early in type 2 diabetes. Diabetes is one of the fastest growing chronic diseases worldwide. Vitamin D3 is inexpensive and readily available. Well-designed clinical studies are required to ascertain if improving 25(OH)D levels from an insufficiency or deficiency to sufficiency improves glycemic control in diabetes. These studies need to be properly designed: a randomized controlled trial with VTD deficiency or insufficiency identified in diabetic patients of various ethnic groups; VTD receptor genotyping; and VTD versus placebo repletion, with no other changes in diabetic management.