DDM2 was a double-blind, randomized, placebo controlled clinical trial conducted in two sites (Tufts Medical Center, Boston, MA; VA Medical Center, Cincinnati, OH) to evaluate the effectiveness of oral daily vitamin D supplementation on pancreatic b-cell function and glycemia and its safety in US adults with well-controlled type 2 diabetes. A total of 127 patients (mean age, 60 years) with stable (HbA1c 7.5%) type 2 diabetes managed with lifestyle only or lifestyle plus metformin were given 4,000 IU of vitamin D3 (cholecalciferol) daily or placebo for 48 weeks. The primary outcome was insulin secretion rate (ISR) after a 3-hour 75-g oral glucose tolerance test done at baseline and week 24. Changes in HbA1c were assessed at 16, 24, 36, and 48 weeks.
Baseline mean blood 25-hydroxyvitamin D concentration was ~27 ng/mL, mean HbA1c was 6.6%, and 78% of patients were on metformin. At week 24, mean 25-hydroxyvitamin D improved by 20.5 and -1.6 ng/mL in the vitamin D and placebo groups, respectively. The vitamin D and placebo groups did not differ in change in ISR or HbA1c. Among patients treated with lifestyle only (n = 28), vitamin D supplementation reduced HbA1c compared with placebo (-0.1% vs 0.3%, respectively; P = 0.034) at week 24. We concluded that vitamin D3 at 4000 IU/d did not change ISR or HbA1c in patients with well-controlled type 2 diabetes on metformin not selected for vitamin D deficiency. However, vitamin D may have a small effect in patients not treated with any diabetes medications.
Results were published in the Journal of The Endocrine Society