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J Clin Endocrinol Metab. 2016 May;101(5):2226-34. doi: 10.1210/jc.2016-1104. Epub 2016 Mar 23.

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

Author information

1
Bone & Mineral Unit (C.M.N., Y.W., C.M.S., E.S.O.), Oregon Health & Science University, Portland, Oregon 97239; School of Public Health (C.M.N., J.L.), Oregon Health & Science University, Portland, Oregon 97239; Medical Research Council Human Nutrition Research (K.S.J., A.P., I.S.), Cambridge, UK CB1 9NL; Department of Orthopedics (R.F.C.), University of California, Los Angeles, California 90095; Pacific Northwest National Laboratory (J.M.J., R.D.S., T.S., Y.G., A.A.S.), Richland, Washington 99354; Institute of Metabolism and Systems Research (M.H.), University of Birmingham, Birmingham, UK B15 2TT; University of California (J.S.A.), Los Angeles, California 90095; School of Medicine (C.M.S., C.G.L., E.S.O.), Oregon Health & Science University, Portland, Oregon 97239; Portland Veterans Affairs Medical Center (C.G.L.), Oregon 97239; Laboratory of Diagnostic Medicine (D.V.), KU Leuven, 3000 Belgium; Laboratory of Clinical and Experimental Endocrinology (D.V., R.B.), KU Leuven, 3000 Belgium; Department of Cardiovascular Sciences (S.P.), KU Leuven, Belgium 3000; Department of Laboratory Medicine (S.P.), University Hospitals Leuven, 3000 Belgium; MRC Keneba (A.P.), Keneba, The Gambia; and Department of Epidemiology (J.M.Z., J.A.C.), University of Pittsburgh, Pennsylvania 15261.

Abstract

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.

PMID:
27007693
PMCID:
PMC4870848
DOI:
10.1210/jc.2016-1104
[Indexed for MEDLINE]
Free PMC Article

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