title: Non-linear association between 25-hydroxyvitamin D and the incidence of type 2 diabetes: a community-based nested case-control study.
abstract: OBJECTIVE
  To examine the nature of the association between 25-hydroxyvitamin D [25(OH)D] and newly diagnosed type 2 diabetes.
  
  
  METHODS
  Serum 25(OH)D concentrations were measured for 761 participants (aged 20-83 years) in the Tehran Lipid and Glucose Study, selected for a 1-to-3 nested case-control study. Cases were 191 cases of Type 2 diabetes diagnosed during a median follow-up of 3.6 years. A total of 570 participants were matched with these cases with regard to age, sex, BMI, and month of entering the study. Diabetes was defined according to the American Diabetes Association criteria, 2003. Serum 25(OH)D was measured using the enzyme immunoassay method. Odds ratios for Type 2 diabetes were obtained from conditional logistic regression models for tertiles of serum 25(OH)D concentrations [tertile-1: 2.82-11.02 (reference), tertile-2: 11.03-21.80, and tertile-3: ≥ 21.82 ng/ml]. The multivariate model was adjusted for age, sex, family history of diabetes, systolic blood pressure, triglyceride-to- HDL cholesterol ratio, waist-to-height ratio, lifestyle modification intervention, leisure time physical activity, and fasting plasma glucose at baseline. Non-linearity in the associations between baseline 25(OH)D and Type 2 diabetes, was examined by using restricted cubic splines.
  
  
  RESULTS
  Unadjusted odds ratios (95% confidence intervals) of diabetes were 0.73 (0.74-1.13), 0.54 (0.34-0.85) for the second and third tertiles, respectively. Multivariate adjusted odds ratios were 0.47 (0.25-0.90) and 0.43 (0.23-0.82), respectively. Below the cutoff of ~ 10 ng/ml the risk of newly diagnosed Type 2 diabetes increased dramatically.
  
  
  CONCLUSIONS
  It was found that 25(OH)D concentrations contributed to the Type 2 diabetes incidence rate in a non-linear fashion, with the risk beginning to increase sharply for values < 10 ng/ml.
authors.affiliation: Prevention of Metabolic Disorders Research Center, Research Institute for Endocrine Sciences, Shahid Beheshti University of Medical Sciences, Tehran, Iran.
authors.forename: M
authors.initials: M
authors.lastname: Tohidi
authors.forename: M
authors.initials: M
authors.lastname: Bozorgmanesh
authors.forename: R
authors.initials: R
authors.lastname: Mohebi
authors.forename: D
authors.initials: D
authors.lastname: Khalili
authors.forename: N
authors.initials: N
authors.lastname: Saadat
authors.forename: N
authors.initials: N
authors.lastname: Khorrami
authors.forename: F
authors.initials: F
authors.lastname: Azizi
authors.forename: F
authors.initials: F
authors.lastname: Hadaegh
mesh_terms: D015652:25-Hydroxyvitamin D 2 / Q000097:blood*; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002112:Calcifediol / Q000097:blood*; D016022:Case-Control Studies; D003924:Diabetes Mellitus, Type 2 / Q000453:epidemiology / Q000209:etiology*; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D007124:Immunoenzyme Techniques; D015994:Incidence; D007492:Iran / Q000453:epidemiology; D016015:Logistic Models; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D014808:Vitamin D Deficiency / Q000097:blood / Q000503:physiopathology*; D055815:Young Adult
chemical_list: D015652:25-Hydroxyvitamin D 2; D002112:Calcifediol