Author Interviews, Diabetes, Vitamin D / 09.06.2019 Interview with: [caption id="attachment_49667" align="alignleft" width="142"]Anastassios G. Pittas, M.D MS Professor Co-Director, Diabetes and Lipid Center; Tufts University School of Medicine Dr. Pittas[/caption] Anastassios G. Pittas, M.D MS Professor Co-Director, Diabetes and Lipid Center; Tufts Medical Center What is the background for this study? What are the main findings? Response: Observational studies have consistently reported an association between low blood vitamin D level and development of type 2 diabetes. However, whether vitamin D supplementation lowers risk of developing diabetes is not known. We designed and conducted the Vitamin D and diabetes (D2d) study to answer this question.  We randomized 2,423 people with prediabetes to 4,000 IU/day of vitamin D3 or placebo and followed them for new-onset diabetes with blood tests every 6 months for an average of 2.5 years. About 80% of participants had sufficient vitamin D level at baseline (25-hydroxyvitamin D level >= 20 ng/mL). The trial was designed to show a reduction of 25% or more in diabetes risk with vitamin D. The study was unable to show a reduction of 25% or more. At the end of the study, there was a 12% reduction in risk of developing diabetes with vitamin D, which missed statistical significance (hazard ration 0.88; 95% confidence interval 0.75 to 1.04). In a small subgroup of participants with vitamin D deficiency at baseline (25-hydroxyvitamin D level < 12 ng/mL) there was 62% reduction in risk of diabetes with vitamin D (hazard ration 0.38; 95% confidence interval 0.18 to 0.80).