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 β-cell function and hemoglobin A1c (HbA1c) 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 (HbA1c ≤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 HbA1c were assessed at 16, 24, 36, and 48 weeks.
Results: Baseline mean plasma 25-hydroxyvitamin D [25(OH)D] concentration was 26.6 ng/mL, mean HbA1c 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 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. 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 HbA1c in patients with well-controlled type 2 diabetes on metformin not selected for vitamin D deficiency.
Keywords: beta cell function; clinical trial; diabetes mellitus; insulin sensitivity; vitamin D.