Free 25-Hydroxyvitamin D: Impact of Vitamin D Binding Protein Assays on Racial-Genotypic Associations

Research output: Contribution to journalArticle


  • Carrie Nielson
  • Kerry Jones
  • Rene Chun
  • Jon Jacobs
  • Ying Wang
  • John Adams
  • Christine Swanson
  • Christine Lee
  • Dirk Vanderschueren
  • Steven Pauwels
  • Ann Prentice
  • Richard Smith
  • Shi Tujin
  • Gao Yuqian
  • Athena Schepmoes
  • Joseph Zmuda
  • Jodi Lapidus
  • Jane Cauley
  • Inez Schoenmaker
  • Roger Bouillon
  • Eric Orwoll

Colleges, School and Institutes

External organisations

  • Pacific NW Natl Lab
  • University of Colorado
  • Portland VA Med Ctr
  • KU Leuven Univ
  • MRC Human Nutrition Research
  • University of Pittsburgh
  • Oregon Health & Science University


CONTEXT: Total 25-hydroxyvitamin D (25OHD) is a marker of vitamin D status and is lower in African Americans than in whites. Whether this difference holds for free 25OHOD (f25OHD) is unclear, considering reported genetic-racial differences in vitamin D binding protein (DBP) used to calculate f25OHD.

OBJECTIVES: Our objective was to assess racial-geographic differences in f25OHD and to understand inconsistencies in racial associations with DBP and calculated f25OHD.

DESIGN: This study used a cross-sectional design.

SETTING: The general community in the United States, United Kingdom, and The Gambia were included in this study.

PARTICIPANTS: Men in Osteoporotic Fractures in Men and Medical Research Council studies (N = 1057) were included.

EXPOSURES: Total 25OHD concentration, race, and DBP (GC) genotype exposures were included.

OUTCOME MEASURES: Directly measured f25OHD, DBP assessed by proteomics, monoclonal and polyclonal immunoassays, and calculated f25OHD were the outcome measures.

RESULTS: Total 25OHD correlated strongly with directly measured f25OHD (Spearman r = 0.84). Measured by monoclonal assay, mean DBP in African-ancestry subjects was approximately 50% lower than in whites, whereas DBP measured by polyclonal DBP antibodies or proteomic methods was not lower in African-ancestry. Calculated f25OHD (using polyclonal DBP assays) correlated strongly with directly measured f25OHD (r = 0.80-0.83). Free 25OHD, measured or calculated from polyclonal DBP assays, reflected total 25OHD concentration irrespective of race and was lower in African Americans than in US whites.

CONCLUSIONS: Previously reported racial differences in DBP concentration are likely from monoclonal assay bias, as there was no racial difference in DBP concentration by other methods. This confirms the poor vitamin D status of many African-Americans and the utility of total 25OHD in assessing vitamin D in the general population.


Original languageEnglish
Pages (from-to)2226-34
Number of pages9
JournalJournal of Clinical Endocrinology and Metabolism
Issue number5
Early online date23 Mar 2016
Publication statusPublished - May 2016


  • Adult, African Continental Ancestry Group, Aged, Cross-Sectional Studies, European Continental Ancestry Group, Humans, Male, Vitamin D, Vitamin D-Binding Protein, Journal Article