Vitamin D Supplementation in Patients With Type 2 Diabetes: The Vitamin D for Established Type 2 Diabetes (DDM2) Study

Context: Observational data support a role for vitamin D in type 2 diabetes, but evidence from trials is inconclusive. Objective: To evaluate the effect of vitamin D supplementation on (3-cell function and hemoglobin Alc (HbAlc) in patients with well-controlled type 2 diabetes. Design: Double-blind, randomized, placebo-controlled clinical trial. Setting: Tufts Medical Center, Boston, MA; VA Medical Center, Cincinnati, OH. Participants: A total of 127 patients (mean age, 60 years) with stable (HbAlc <7.5%) diabetes managed with lifestyle only or lifestyle plus metformin. Intervention: Subjects were given 4000 units of vitamin D3 (cholecalciferol) daily or placebo for 48 weeks. Main Outcome Measure: Insulin secretion rate (ISR) was estimated from peripheral plasma C peptide levels after a 3-hour 75-g oral glucose tolerance test done at baseline and week 24. Changes in HbAlc were assessed at 16, 24, 36, and 48 weeks. Results: Baseline mean plasma 25-hydroxyvitamin D [25(OH)D1 concentration was 26.6 ng/mL, mean HbAlc was 6.6%, and 78% of patients were on metformin. At week 24, mean 25(OH)D changed by 20.5 and 1.6 ng/mL in the vitamin D and placebo groups, respectively (P < 0.001). The vitamin D and placebo groups did not differ in change in ISR or HbAlc. Among patients treated with lifestyle only (n = 28), vitamin D supplementation reduced HbAlc compared with placebo (-0.1% vs 0.3%, respectively; P= 0.034) at week 24. This result was not observed at the other time points and could be due to chance. Conclusion: Vitamin D3 at 4000 IU/d did not change ISR or HbAlc in patients with well-controlled type 2 diabetes on metformin not selected for vitamin D deficiency. Copyright (C) 2018 Endocrine Society