title: Bioavailable Vitamin D in Obese Children: The Role of Insulin Resistance.
abstract: BACKGROUND
  Studies examining vitamin D levels in association with childhood obesity usually do not consider the effect of insulin on vitamin D-binding protein and do not calculate the unbound, bioavailable vitamin D.
  
  
  OBJECTIVE
  This study aimed to evaluate in a group of children 1) the concentrations of both total 25-hydroxyvitamin D and bioavailable fraction, and 2) the potential role of insulin resistance in modulating the concentrations of bioavailable vitamin D. Design, Setting, and Patients or Other Participants: This was a cross-sectional study at a University Pediatric Department in which 63 obese children and 21 lean controls were enrolled.
  
  
  METHODS
  Total 25-hydroxyvitamin D and vitamin D-binding protein were measured, two single-nucleotide polymorphisms in the coding region of the vitamin D-binding protein (rs4588 and rs7041) were studied, and the vitamin D bioavailable fraction was calculated.
  
  
  RESULTS
  Obese children showed total 25-hydroxyvitamin D levels lower compared with nonobese children (21.3 ± 6.7 ng/mL vs 29.6 ± 11.7 ng/mL; P = .0004). Bioavailable 25-hydroxyvitamin D levels were not different among the two groups (3.1 ± 1.6 ng/mL vs 2.6 ± 1.2 ng/mL; P > .05). Insulin-resistant children showed higher bioavailable levels of 25-hydroxyvitamin D compared with noninsulin-resistant children (3.4 ± 1.4 ng/mL vs 2.0 ± 0.9 ng/mL; P = .013) and an inverse correlation between insulin resistance and vitamin D-binding protein was found (r:= -0.40; P = .024).
  
  
  CONCLUSIONS
  Obese children present levels of bioavailable 25-hydroxyvitamin D similar to those of normal-weight children due to reduced concentration of vitamin D-binding protein. The insulin resistance could play a role in this reduced concentration.
authors: Department of Woman, Child and of General and Specialized Surgery, Seconda Università degli Studi di Napoli, 80138 Napoli, Italy.
authors: Emanuele
authors: E
authors: Miraglia del Giudice
authors: Department of Woman, Child and of General and Specialized Surgery, Seconda Università degli Studi di Napoli, 80138 Napoli, Italy.
authors: Anna
authors: A
authors: Grandone
authors: Department of Woman, Child and of General and Specialized Surgery, Seconda Università degli Studi di Napoli, 80138 Napoli, Italy.
authors: Grazia
authors: G
authors: Cirillo
authors: Department of Woman, Child and of General and Specialized Surgery, Seconda Università degli Studi di Napoli, 80138 Napoli, Italy.
authors: Carlo
authors: C
authors: Capristo
authors: Department of Woman, Child and of General and Specialized Surgery, Seconda Università degli Studi di Napoli, 80138 Napoli, Italy.
authors: Pierluigi
authors: P
authors: Marzuillo
authors: Department of Woman, Child and of General and Specialized Surgery, Seconda Università degli Studi di Napoli, 80138 Napoli, Italy.
authors: Anna
authors: A
authors: Di Sessa
authors: Department of Woman, Child and of General and Specialized Surgery, Seconda Università degli Studi di Napoli, 80138 Napoli, Italy.
authors: Giuseppina Rosaria
authors: GR
authors: Umano
authors: Department of Woman, Child and of General and Specialized Surgery, Seconda Università degli Studi di Napoli, 80138 Napoli, Italy.
authors: Laura
authors: L
authors: Ruggiero
authors: Department of Woman, Child and of General and Specialized Surgery, Seconda Università degli Studi di Napoli, 80138 Napoli, Italy.
authors: Laura
authors: L
authors: Perrone
mesh_terms: D000293:Adolescent; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007333:Insulin Resistance / Q000502:physiology*; D008297:Male; D063766:Pediatric Obesity / Q000097:blood* / Q000235:genetics; D020641:Polymorphism, Single Nucleotide; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood; D014809:Vitamin D-Binding Protein / Q000097:blood* / Q000235:genetics
chemical_list: D014809:Vitamin D-Binding Protein; D014807:Vitamin D; C104450:25-hydroxyvitamin D