Cholecalciferol supplementation does not influence β-cell function and insulin action in obese adolescents: A prospective double-blind randomized trial

Background: There is increasing interest in the extraskeletal effects of vitamin D, particularly in the obese state with regard to the development of insulin resistance and diabetes. Objective: The objective of the study was to determine the effect of 2 doses of cholecalciferol (vitamin D₃) supplementation on insulin action (S_i) and pancreatic β-cell function in obese adolescents. Methods: We performed a 12-wk double-blind, randomized comparison of the effect of vitamin D₃ supplementation on S_i and β-cell function in obese Caucasian adolescents (body mass index > 95^th percentile). The subjects were randomly assigned to receive either 400 IU/d (n = 25) or 2000 IU/d (n = 26) of vitamin D₃. Each subject underwent a 7-sample 75 g oral glucose tolerance test, with glucose, insulin, and C-peptide measurements, to calculate S_i and β-cell function as assessed by the disposition index (DI), with use of the oral minimal model before and after supplementation. A total of 51 subjects aged 15.0 ± 1.9 y were enrolled. Included for analysis at follow-up were a total of 46 subjects (20 male and 26 female adolescents), 23 in each group. Results: Initial serum 25-hydroxyvitamin D [25(OH)D] was 24.0 ± 8.1μg/L. There was no correlation between 25(OH)D concentrations and S_i or DI. There was a modest but significant increase in 25(OH)D concentration in the 2000 IU/d group (3.1 ± 6.5 μg/L, P = 0.04) but not in the 400 IU/d group (P = 0.39). There was no change in S_i or DI following vitamin D₃ supplementation in either of the treatment groups (all P > 0.10). Conclusions: The current study shows no effect from vitamin D₃ supplementation, irrespective of its dose, on β-cell function or insulin action in obese nondiabetic adolescents with relatively good vitamin D status. Whether obese adolescents with vitamin D deficiency and impaired glucose metabolism would respond differently to vitamin D₃ supplementation remains unclear and warrants further studies.