Effects of Vitamin D Supplementation on Insulin Sensitivity and Secretion in Prediabetes

Context Vitamin D regulates glucose homeostasis pathways, but effects of vitamin D supplementation on beta-cell function remain unclear. Objective To investigate the effects of vitamin D-3 supplementation on insulin sensitivity and beta-cell function. Methods This is a prespecified secondary analysis of the Vitamin D and Type 2 Diabetes study. Overweight/obese adults at high risk for type 2 diabetes (prediabetes) were randomly treated with vitamin D-3 4000 IU or matching placebo daily for 24 months. Main Outcome Disposition index (DI), as an estimate of beta-cell function, was calculated as the product of Homeostasis Model Assessment 2 indices derived from C-peptide values (HOMA2%S-cpep) and C-peptide response during the first 30 minutes of a 75-g oral glucose tolerance test (OGTT). Results Mean age was 60.5 +/- 9.8 years and body mass index was 31.9 +/- 4.4 kg/m(2). Mean serum 25(OH)D level increased from 27.9 +/- 10.3 ng/mL at baseline to 54.9 ng/mL at 2 years in the vitamin D group and was unchanged (28.5 +/- 10.0 ng/mL) in the placebo group. The baseline DI predicted incident diabetes independent of the intervention. In the entire cohort, there were no significant differences in changes in DI, HOMA2%S-cpep, or C-peptide response between the 2 groups. Among participants with baseline 25(OH)D level <12 ng/mL, the mean percent differences for DI between the vitamin D and placebo groups was 8.5 (95% CI, 0.2-16.8). Conclusions Supplementation with vitamin D-3 for 24 months did not improve an OGTT-derived index of beta-cell function in people with prediabetes not selected based on baseline vitamin D status; however, there was benefit among those with very low baseline vitamin D status.