Vitamin D–Binding Protein and Vitamin D Status of Black Americans and White Americans
Are there differences in vitamin D–binding protein genotypes and concentrations of circulating vitamin D–binding proteins between black and white Americans and, if present, may these differences account for observed racial differences in manifestations of vitamin D deficiency?
This was a cross-sectional study of community-dwelling participants in the Healthy Aging in Neighborhoods of Diversity across the Life Span (HANDLS). Data were collected between 2004 and 2008 from participants living in Baltimore at the time of enrollment and who self-identified themselves as black or white. Participants underwent laboratory analysis for total 25-hydroxyvitamin D (D2 and D3), vitamin D–binding protein, and parathyroid hormone. DNA samples from the participants were genotyped for two common single-nucleotide polymorphisms in the coding region of the vitamin D–binding protein. Concentrations of bioavailable 25-hydroxyvitamin D were calculated in homozygous participants.
Unadjusted levels of both 25-hydroxyvitamin D and vitamin D–binding protein were lower in blacks than in whites (total 25-hydroxyvitamin D, 15.6 ± 0.2 ng/ml vs. 25.8 ± 0.4 ng/ml, p < 0.001; vitamin D–binding protein, 168 ± 3 µg/ml vs. 337 ± 5 µg/ml, p < 0.001). Genetic polymorphisms in the vitamin D–binding protein gene (rs7041 and rs4588) could explain 79.4% of the variation in levels of vitamin D–binding protein. Calculated levels of bioavailable 25-hydroxyvitamin D were similar in blacks and whites (2.9 ± 0.1 ng/ml and 3.1 ± 0.1 ng/ml, respectively; p = 0.71).
While black adults had lower levels of 25-hydroxyvitamin D and vitamin D–binding protein (likely owing to a genetic variant) when compared to white adults, bioavailable 25-hydroxyvitamin D was equivalent in the two races.
The limitations of this cross-sectional study aside, the current analysis provides compelling evidence that community-dwelling black adults may be misclassified as being vitamin D deficient based only on low total 25-hydroxyvitamin D levels. The routine measurement of vitamin D–binding protein may be beneficial to more accurately identify truly vitamin D–deficient individuals.
Keywords: Polymorphism, Genetic, Vitamin D Deficiency, Vitamin D-Binding Protein, European Continental Ancestry Group, Cardiovascular Diseases, Baltimore, Parathyroid Hormone, Genotype, African Continental Ancestry Group, Vitamin D, United States
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