Effects of vitamin D deficiency and repletion on insulin and glucagon secretion in man

Summary We have studied the effects of vitamin D deficiency on pancreatic A- and B-cell function. Four subjects with vitamin D deficiency and 10 healthy subjects were studied. Pancreatic B-cell function was assessed by the insulin response to an oral glucose tolerance test. An insulin tolerance test was used to evaluate pancreatic A-cell function. The patients were then treated with 2000 U/day of vitamin D₃ for 6 months, after which the clinical, metabolic, biochemical and radiological features of vitamin D deficiency resolved, and pancreatic A-and B-cell function was repeated. In the vitamin D-deficient subjects pre-treatment and post-treatment serum calcium levels (mean±SEM) were 2.22±0.01 mmol/l and 2.24±0.01 mmol/1 respectively, and 2.27 ± 0.02 mmol/l in healthy subjects (NS). The pre-treatment level of 1,25-dihydroxy vitamin D (1,25-(OH)₂D) of 29.7 ± 3.3 pg/ml in the vitamin D deficient subjects rose to 70.3±10.3pg/ml after treatment (p < 0.05). The 1,25-(OH)₂D level in the healthy subjects was 50.0 ± 13.7 pg/ml (p < 0.05 versus pre- and post-treatment values in the patients). Insulin secretion, calculated by the area under the insulin curve, was significantly lower before vitamin D₃ treatment in the patients (9.09±0.7 mU × min,p<0.05) compared with the healthy subjects (11.9±0.5 mU × min) and post-treatment values of the patients with vitamin D deficiency (13.7 ± 0.5 mU x min). Similar changes were seen in the insulogenic indicesΔ I/ΔG). WhileΔI/ΔG was 1.71±0.4 (mean ± SEM) during vitamin D deficiency, it increased to 2.48±0.3 with vitamin D repletion. The insulogenic index in the healthy subjects was 2.68±0.3. The glucose areas were not significantly different. Insulin-induced glucagon secretion was similar in all instances. The results of this study suggest that vitamin D deficiency reduces pancreatic insulin secretion but it does not affect pancreatic A-cell function.