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

Department of Pediatrics/Division of Pediatric Endocrinology and Metabolism, The Children’s Hospital (A.A., K.M.); Department of Nutrition Sciences (J.A., B.G.); School of Nursing (K.S.); and Department of Maternal and Child Health, School of Public Health (F.F.), University of Alabama at Birmingham, Birmingham, Alabama 35233

Address all correspondence and requests for reprints to: Ambika Ashraf, M.D., Children’s Hospital, University of Alabama at Birmingham, CPP 230, 1601 Fourth Avenue South, Birmingham, Alabama 35233. E-mail: AAshraf{at}peds.uab.edu.

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/m²; 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.

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