Vitamin D deficiency reduces insulin secretion and turnover in rats.

Vitamin D deficiency can result in low serum insulin levels, which lead to impaired glucose tolerance. The present study was conducted to assess the mechanisms of this impairment. Pancreatic secretion of insulin and insulin turnover in vitamin D-deficient rats were studied. Insulin secretion was found to be significantly decreased in vitamin D-deficient rats compared to control rats. Supplementation with calcium alone could reverse the defect. The impaired insulin secretion observed in vitamin D deficiency was not specific to glucose stimulation but was also seen in response to arginine. Insulin turnover was studied in vivo using A14-(125)I-insulin. The pharmacokinetic parameters (elimination rate constant K, insulin biological half-life, t1/2, plasma concentrations of insulin at time zero I0) were calculated from the plasma radioactivity. The vitamin D-deficient rats had a lower degradation rate than controls. These findings suggest that vitamin D deficiency leads to reduction in both insulin secretion and turnover, and that the reduced turnover could be an adaptive mechanism to conserve the low amounts of insulin secreted.