IMPROVEMENT IN PANCREATIC β-CELL FUNCTION WITH VITAMIN D AND CALCIUM SUPPLEMENTATION IN VITAMIN D-DEFICIENT NONDIABETIC SUBJECTS

Objective: There are varied reports on the effect of vitamin D supplementation on beta-cell function and plasma glucose levels. The objective of this study was to examine the effect of vitamin D and calcium supplementation on beta-cell function and plasma glucose levels in subjects with vitamin D deficiency. Methods: Nondiabetic subjects (N = 48) were screened for their serum 25-hydroxyvitamin D (25-OHD), albumin, creatinine, calcium, phosphorus, alkaline phosphatase, and intact parathyroid hormone (PTH) status. Subjects with 25-OHD deficiency underwent a 2-hour oral glucose tolerance test. Cholecalciferol (9,570 international units [IU]/day; tolerable upper intake level, 10,000 IU/day; according to the Endocrine Society guidelines for vitamin D supplementation) and calcium (1 g/day) were supplemented. Results: Thirty-seven patients with 25-OHD deficiency participated in the study. The baseline and postvitamin D/calcium supplementation and the difference (corrected) were: serum calcium, 9 +/- 0.33 and 8.33 +/- 1.09 mg/dL (-0.66 +/- 1.11 mg/dL); 25-OHD, 8.75 +/- 4.75 and 36.83 +/- 18.68 ng/mL (28.00 +/- 18.33 ng/mL); PTH, 57.9 +/- 29.3 and 36.33 +/- 22.48 pg/mL (-20.25 +/- 22.45 pg/mL); fasting plasma glucose, 78.23 +/- 7.60 and 73.47 +/- 9.82 mg/dL (-4.88 +/- 10.65 mg/dL); and homeostasis model assessment-2-percent beta-cell function C-peptide secretion (HOMA-2-% B C-PEP), 183.17 +/- 88.74 and 194.67 +/- 54.71 (11.38 +/- 94.27). Significant differences were observed between baseline and post-vitamin D/calcium supplementation serum levels of corrected calcium (Z, -3.751; P<.0001), 25-OHD (Z, -4.9; P<.0001), intact PTH (Z, -4.04; P<.0001), fasting plasma glucose (Z, -2.7; P<.007), and HOMA-2-% B C-PEP (Z, -1.923; P<.05) as determined by Wilcoxon signed rank test. Insulin resistance as measured by HOMA was unchanged. Conclusion: Optimizing serum 25-OHD concentrations and supplementation with calcium improves fasting plasma glucose levels and beta-cell secretory reserve. Larger randomized control studies are needed to determine if correction of 25-OHD deficiency will improve insulin secretion and prevent abnormalities of glucose homeostasis.