title: Defining physiologically "normal" vitamin D in African Americans.
abstract: The relationship between serum 25(OH)D and intact parathyroid hormone (iPTH) was evaluated in the Multicenter Osteoarthritis Study (MOST). No further change in iPTH was observed for African Americans with 25(OH)D levels above 20 ng/ml, suggesting that compared to Caucasians, lower vitamin D targets for sufficiency may be appropriate for African Americans.
  
  
  BACKGROUND
  Vitamin D levels ≥30 ng/ml are commonly considered "normal" based upon maximal suppression of iPTH; however, this has recently been challenged and the optimal 25(OH)D level among non-Caucasians is unclear. We evaluated the cross-sectional relationship between serum 25(OH)D and iPTH in a sample of Caucasian and African American adults.
  
  
  METHODS
  We used baseline serum samples of participants from the Multicenter Osteoarthritis Study (MOST) for this analysis and used three methods to model the relationship between 25(OH)D and iPTH: ordinary least squares regression (OLS), segmented regression and Helmert contrasts.
  
  
  RESULTS
  Among Caucasians (n = 1,258), 25(OH)D and iPTH ranged from 4 to 51 ng/ml and 2 to 120 pg/ml and from 3 to 32 ng/ml and 3 to 119 pg/ml in African Americans (n = 423). We observed different thresholds between African Americans and Caucasians using each analytic technique. Using 25(OH)D as a categorical variable in OLS, iPTH was statistically higher at lower 25(OH)D categories than the 24-32 ng/ml referent group among Caucasians. However, in African Americans, the mean iPTH was only significantly higher at 25(OH)D levels below 15 ng/ml. Using segmented regression, iPTH appeared to stabilize at a lower 25(OH)D level in African Americans (19-23 ng/ml) compared to in Caucasians (>32 ng/ml). Helmert contrasts also revealed a lower threshold in African Americans than Caucasians.
  
  
  CONCLUSIONS
  Among MOST participants, the 25(OH)D thresholds at which no further change in iPTH was observed was approximately 20 ng/ml in African Americans versus approximately 30 ng/ml in Caucasians, suggesting optimal vitamin D levels in Caucasians may not be applicable to African Americans.
authors.affiliation: Department of Epidemiology, University of Alabama at Birmingham, University Blvd, Birmingham, AL 35294, USA.
authors.forename: N C
authors.initials: NC
authors.lastname: Wright
authors.forename: L
authors.initials: L
authors.lastname: Chen
authors.forename: J
authors.initials: J
authors.lastname: Niu
authors.forename: T
authors.initials: T
authors.lastname: Neogi
authors.forename: K
authors.initials: K
authors.lastname: Javiad
authors.forename: M A
authors.initials: MA
authors.lastname: Nevitt
authors.forename: C E
authors.initials: CE
authors.lastname: Lewis
authors.forename: J R
authors.initials: JR
authors.lastname: Curtis
mesh_terms: D001741:Black or African American; D000368:Aged; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010281:Parathyroid Hormone / Q000097:blood*; D012016:Reference Values; D012621:Seasons; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood; D014808:Vitamin D Deficiency / Q000097:blood / Q000208:ethnology; D044465:White People
chemical_list: D010281:Parathyroid Hormone; D014807:Vitamin D; C104450:25-hydroxyvitamin D