Ajcn011684 486..494

Background: A suboptimal vitamin D and calcium status has been associated with higher risk of type 2 diabetes in observational studies, but evidence from trials is lacking. Objective: We determined whether vitamin D supplementation, with or without calcium, improved glucose homeostasis in adults at high risk of diabetes. Design: Ninety-two adults were randomly assigned in a 2-by-2 factorial-design, double-masked, placebo-controlled trial to receive either cholecalciferol (2000 IU once daily) or calcium carbonate (400 mg twice daily) for 16 wk. The primary outcome was the change in pancreatic b cell function as measured by the disposition index after an intravenous-glucose-tolerance test. Other outcomes were acute insulin response, insulin sensitivity, and measures of glycemia. Results: Participants had a mean age of 57 y, a body mass index (BMI; in kg/m) of 32, and glycated hemoglobin (Hb A1c) of 5.9%. There was no significant vitamin D · calcium interaction on any outcomes. The disposition index increased in the vitamin D group and decreased in the no–vitamin D group (adjusted mean change 6 SE: 3006 130 compared with21266 127, respectively; P = 0.011), which was explained by an improvement in insulin secretion (62 6 39 compared with2366 37 mU · L · min, respectively; P = 0.046). Hb A1c increased less, but nonsignificantly, in the vitamin D group than in the no–vitamin D group (0.06 6 0.03% compared with 0.14 6 0.03%, respectively; P = 0.081). There was no significant difference in any outcomes with calcium compared with no calcium. Conclusion: In adults at risk of type 2 diabetes, short-term supplementation with cholecalciferol improved b cell function and had a marginal effect on attenuating the rise in Hb A1c. This trial was registered at clinicaltrials.gov as NCT00436475. Am J Clin Nutr 2011;94:486–94.