OBJECTIVE There has been conflicting evidence regarding the potential role of vitamin D in glucose homeostasis. This study was designed to investigate the effect of high-dose vitamin D3 treatment on β-cell function, insulin sensitivity, and glucose tolerance in subjects with prediabetes or diet-treated type 2 diabetes.

RESEARCH DESIGN AND METHODS Subjects (n = 44) were randomized to 30,000 IU vitamin D3 once weekly or placebo for 8 weeks. Hyperglycemic clamp assessed first-phase (0–12 min) and second-phase (12–120 min) insulin response, insulin sensitivity, and disposition index (DI). An oral glucose tolerance test assessed glucose tolerance and glycosylated hemoglobin assessed glycemic control.

RESULTS A total of 21 (vitamin D) and 22 (placebo) subjects completed the study, respectively. Season-adjusted 25-OH-vitamin D [25(OH)D] levels were doubled in the active treated group (43–82 nmol/L). No effect of vitamin D treatment, compared with placebo, was seen on first-phase or second-phase insulin secretion. There were no group differences in insulin sensitivity, DI, or any measures of glycemic control. No hypercalcemia or other adverse effects of vitamin D treatment were seen compared with placebo. Subgroup analyses of those with the lowest basal and greatest increase in 25(OH)D levels did not change these results.

CONCLUSIONS This study gives no support for any substantial effect of high-dose vitamin D treatment for 8 weeks in prediabetes or diet-treated type 2 diabetes on β-cell function, insulin sensitivity, or glycemic control.