Vitamin D Supplementation and Prevention of Type 2 Diabetes.

Randomized, placebo-controlled trial conducted at 22 academic medical centers in the United States. Adults at high risk for type 2 diabetes were enrolled if they met at least two ADA criteria for prediabetes and did not have diabetes at baseline; participants were not selected for vitamin D insufficiency. The vitamin D arm randomized 1211 participants and analyzed 1211 by intention-to-treat.

Vitamin D3 (cholecalciferol) was given orally as a 4000 IU soft-gel pill once daily during the trial. The active regimen was compared with placebo.

Vitamin D3 did not significantly reduce progression to type 2 diabetes over a median follow-up of 2.5 years. New-onset diabetes occurred in 293 participants in the vitamin D group versus 323 in the placebo group, corresponding to 9.39 and 10.66 events per 100 person-years. The hazard ratio was 0.88 (95% CI, 0.75 to 1.04; P = 0.12). Adverse events did not differ significantly between groups.

Follow-up was only about 2.5 years, which may be too short to detect a modest prevention effect. Participants were not selected for vitamin D deficiency, limiting applicability to deficient populations. The primary result was not statistically significant despite a numerical difference in diabetes incidence.