Associations Between Serum 25-Hydroxyvitamin D, Insulin Sensitivity, Insulin Secretion, and β-Cell Function According to Glucose Tolerance Status

Background: The objective of this study was to determine whether glucose tolerance status influences the associations between serum 25-hydroxyvitamin D [25(OH)D], insulin sensitivity, insulin secretion, and beta-cell function. Methods: This cross-sectional study included 112 French Canadian postmenopausal women with normal glucose tolerance (NGT; n=65) or abnormal glucose tolerance (AGT; n=47). Estimates of insulin sensitivity [homeostasis model assessment of insulin sensitivity (HOMA%S) and glucose disposal rate (GDR)], insulin secretion [area under the curve of C-peptide (AUC C-peptide)], and beta-cell function (GDRxAUC C-peptide) were derived from a 2-hr euglycemic-hyperinsulinemic clamp and a 75-gram 3-hr oral glucose tolerance test (OGTT). Measures of adiposity were taken (waist circumference, body mass index, fat mass by the hydrostatic weighting technique, and computed tomography (CT)-derived total and visceral adiposity), questionnaires on physical activity, dietary calcium, and vitamin D intake were administered, and blood was sampled for measurement of parathyroid hormone, interleukin-6, and adiponectin. Results: AGT status was significantly associated with lower insulin sensitivity and beta-cell function (P <= 0.01 for all) but not with insulin secretion. Lower serum 25(OH)D concentrations were significantly associated with lower insulin sensitivity and secretion (P <= 0.01 for all) but not with beta-cell function. The interaction between glucose tolerance status and serum 25(OH)D concentration was not significant for either insulin sensitivity, insulin secretion, or beta-cell function, even after adjustment for potential confounders. Conclusion: Vitamin D and glucose tolerance status are both independently associated with measures of insulin sensitivity, insulin secretion, and beta-cell function. However, the association between serum 25(OH)D and these surrogate markers of type 2 diabetes mellitus risk is not influenced by glucose tolerance status.