Threshold for Effects of Vitamin D Deficiency on Glucose Metabolism in Obese Female African-American Adolescents

Context: Vitamin D status can influence insulin resistance. Objective: The aim of the study was to determine the prevalence of vitamin D deficiency in obese African-American (AA) adolescent females in a southeastern latitude and to determine the relationship of 25-hydroxyvitamin D [25(OH) D] with insulin and glucose dynamics. Design: We conducted a cross-sectional study in a University Children's Hospital. Methods: Serum 25(OH) D, fasting glucose, PTH, serum calcium, serum lipids, serum transaminases, and C-reactive protein were assessed. Indices of insulin sensitivity and resistance were determined from an oral glucose tolerance test. Subjects were classified as vitamin D deficient or sufficient, based on the traditional vitamin D deficiency definition [serum 25(OH) D <20 ng/ml] and also by a lower 25(OH) D cut-point of 15 ng/ml or less. Results: A total of 51 AA adolescent females (body mass index, 43.3 +/- 9.9 kg/m(2); age, 14 +/- 2 yr) were studied. Serum 25(OH) D concentrations were 20 ng/ml or less in 78.4% and 15 ng/ml or less in 60.8% of subjects. There were no significant group differences in the metabolic outcomes when subjects were classified using the traditional vitamin D deficiency definition. The Matsuda index of insulin sensitivity was significantly lower (P = 0.02), and insulin area under the curve was significantly higher (P = 0.04) in subjects with 25(OH) D concentrations of 15 ng/ml or less vs. those with higher concentrations. Conclusions: Vitamin D deficiency is highly prevalent in obese, AA female adolescents and may promote insulin resistance. Our data suggest that a 25(OH) D concentration of 15 ng/ml or less may be the threshold by which vitamin D deficiency confers negative effects on insulin sensitivity. (J Clin Endocrinol Metab 94: 3200 -3206, 2009)