Effect of moderate-dose vitamin D supplementation on insulin sensitivity in vitamin D-deficient non-Western immigrants in the Netherlands: a randomized placebo-controlled trial

Background: Low serum 25-hydroxyvitamin D [25(OH)D] concentrations have been associated with insulin resistance, the metabolic syndrome, and type 2 diabetes. Because many non-Western immigrants in the Netherlands are vitamin D deficient, obese, and at high risk of diabetes, vitamin D supplementation may contribute to prevent diabetes and insulin resistance. Objective: We examined the effect of vitamin D supplementation on insulin sensitivity and 13 cell function in overweight, vitamin D-deficient, non-Western immigrants at high risk of diabetes. Design: The study was a 16-wk, randomized, placebo-controlled trial. A total of 130 non-Western immigrants with prediabetes (fasting glucose concentration >5.5 mmol/L or random glucose concentration from 7.8 to 11.1 mmol/L) and vitamin D deficiency (serum 25[OH]D concentration <50 nmol/L) were randomly assigned after stratification by sex to receive either cholecalciferol (1200 IU/d) or a placebo for 16 wk. All participants received 500 mg Ca/d as calcium carbonate. The primary outcome was the difference in the area under the curve of insulin and glucose after a 75-g oral-glucose-tolerance test after 4 mo of treatment. Secondary outcomes were insulin-sensitivity variables, beta cell function variables, and metabolic syndrome. Results: Mean serum 25(OH)D concentrations increased significantly in the vitamin D compared with placebo groups. After 4 mo of therapy, the mean between-group difference was 38 nmol/L (95% Cl: 32.1, 43.9 nmol/L; P < 0.001). There was no significant effect on insulin sensitivity and beta cell function. In a post hoc analysis, when patients with diabetes at baseline were excluded, a significant increase in the insulinogenic index was observed in participants who obtained a 25(OH)D concentration >= 60 nmol/L (P = 0.040). Conclusions: Vitamin D supplementation in non-Western vitamin D deficient immigrants with prediabetes did not improve insulin sensitivity or beta cell function or change the incidence of metabolic syndrome. However, after the exclusion of diabetic subjects, an improvement in the insulinogenic index was observed in participants who obtained a 25(OH)D concentration >= 60 nmol/L.