Insights into racial differences in vitamin D levels

Compared with whites, blacks consistently have lower levels of total vitamin D (25-hydroxyvitamin D) and elevated levels of parathyroid hormone (considered a sensitive marker of vitamin D deficiency), often leading to a diagnosis of vitamin D deficiency. Yet blacks have higher bone mineral density (BMD) than whites. A new investigation into racial differences in vitamin D-binding protein genotypes and concentrations of circulating vitamin D-binding protein suggest that racial differences in the prevalence of common genetic variants in the vitamin D-binding protein gene may explain this paradox.

Investigators measured levels of total vitamin D, vitamin D-binding protein, and parathyroid hormone as well as BMD in more than 2,000 black and white study participants. They examined participants for 2 common variants in the vitamin D-binding protein gene and estimated levels of bioavailable vitamin D.

As expected, BMD was higher in blacks than in whites, and levels of parathyroid hormone increased as levels of total or bioavailable vitamin D decreased. Levels of vitamin D-binding protein were lower in blacks than in whites, probably because of the high prevalence of genetic variants, which seemed to explain 79.4% of the variation in vitamin D-binding levels between blacks and whites. These genetic variants also accounted for 9.9% of differences in total vitamin D between blacks and whites. Alhough blacks had significantly lower levels of total vitamin D than whites, they had equivalent levels of bioavailable vitamin D, which seemed to be associated with their lower levels of vitamin D-binding protein (Powe CE, et al. N Engl J Med. 2013;369[21]:1991-2000).

COMMENTARY  The last 5 years have seen a flurry of studies describing the prevalence of vitamin D deficiency and its association with everything from diabetes mellitus to asthma to mental illness. We may now need to take a step back to look at these findings through the lens of this study, considering that what is a normal vitamin D level for one person may be abnormal for another. -Michael Burke, MD

MS FREEDMAN is a freelance medical editor and writer in New Jersey. DR BURKE, section editor for Journal Club, is chairman of the Department of Pediatrics at Saint Agnes Hospital, Baltimore, Maryland, and physician contributing editor for Contemporary Pediatrics. The editors have nothing to disclose in regard to affiliations with or financial interests in any organizations that may have an interest in any part of this article.