title: Insulin resistance indices are inversely associated with vitamin D binding protein concentrations.
abstract: BACKGROUND
  We hypothesized that, similar to the coordinated homeostatic regulation of most hormones, the concentration of free and bioavailable 25-hydroxyvitamin D [25(OH)D] will be tightly controlled by total 25(OH)D and vitamin D binding protein (VDBP) and that the VDBP concentrations will be associated with insulin resistance status.
  
  
  OBJECTIVE
  Our primary objective was to investigate associations between total, free, and bioavailable 25(OH)D and VDBP. We also evaluated the relationships of VDBP with insulin resistance indices.
  
  
  METHODS
  The study design was cross-sectional in the setting of a university children's hospital. The relative concentration of bioavailable 25(OH)D to total 25(OH)D [bioavailable 25(OH)D/total 25(OH)D was expressed as a percentage [percentage bioavailable 25(OH)D].
  
  
  RESULTS
  Subjects were 47, postmenarchal, female adolescents, with a mean age of 15.8±1.4 years, a mean body mass index of 23.1±4.0 kg/m2. The total 25(OH)D was strongly associated with VDBP (rho=0.57, P<.0001). At lower total 25(OH)D concentrations, the concentration of bioavailable 25(OH)D relative to total 25(OH)D was higher (23.8% vs 14.9%, P<.0001), whereas the relative concentration of free 25(OH)D was similar (P=.44). VDBP was inversely associated with fasting insulin (rho=-0.51, P=.0003) and homeostatic model assessment of basal insulin resistance (rho=-0.45, P=.002) and positively with whole-body insulin sensitivity (rho=0.33, P=.02); these relationships persisted after adjusting for percentage fat and attenuated after adjusting for race.
  
  
  CONCLUSIONS
  Our data suggest that VDBP concentrations are regulated by total 25(OH)D levels to maintain adequate concentrations of bioavailable 25(OH)D. VDBP concentrations are inversely associated with hyperinsulinemia and insulin resistance.
authors.affiliation: Division of Pediatric Endocrinology (A.P.A., X.W.), Department of Pediatrics, Center for Clinical and Translational Sciences (C.H.), and Department of Nutrition Sciences (B.A.G.), University of Alabama, Birmingham, Birmingham, Alabama 35233; and Division of Endocrinology (J.A.A.), Emory University School of Medicine, Atlanta, Georgia 30322.
authors.forename: Ambika P
authors.initials: AP
authors.lastname: Ashraf
authors.forename: Carrie
authors.initials: C
authors.lastname: Huisingh
authors.forename: Jessica A
authors.initials: JA
authors.lastname: Alvarez
authors.forename: Xudong
authors.initials: X
authors.lastname: Wang
authors.forename: Barbara A
authors.initials: BA
authors.lastname: Gower
mesh_terms: D000293:Adolescent; D001823:Body Composition / Q000502:physiology; D015992:Body Mass Index; D003430:Cross-Sectional Studies; D005260:Female; D006305:Health Status Indicators; D006801:Humans; D007328:Insulin / Q000097:blood; D007333:Insulin Resistance / Q000502:physiology; D009994:Osmolar Concentration; D014807:Vitamin D / Q000031:analogs & derivatives / Q000097:blood; D014808:Vitamin D Deficiency / Q000097:blood / Q000453:epidemiology; D014809:Vitamin D-Binding Protein / Q000097:blood*
chemical_list: D007328:Insulin; D014809:Vitamin D-Binding Protein; D014807:Vitamin D; C104450:25-hydroxyvitamin D