For a long time, there has not been a satisfying explanation for the fact that blacks have higher bone mineral density but lower 25-hydroxy-vitamin D levels than whites. New research (NEJM 2013; 369: 1991-2000, editorial 22047-48) helps explain this paradox.
This study examined a community cohort of 2085 individuals in the “Healthy Aging in Neighborhoods of Diversity across the Life Span” study.
- Blacks had higher bone mineral density and lower 25-hydroxy-vitamin D levels than whites
- The calculated bioavailable levels of 25-hydroxy-vitamin D were similar to whites.
The editorial notes that the similar bioavailability is due to differences in the vitamin D-binding protein (aka GC-globulin). “GC1F is the most abundant form in persons of African ancestry whereas GC1S is most abundant in European populations.” Thus, it has been hypothesized that the vitamin D-binding protein in blacks has “increased affinity for vitamin D3, and thus able to transport vitamin D3 more efficiently from the skin to the liver for its metabolism to 25-hydroxy-vitamin D.”
Bottomline: This research in vitamin D metabolism may impact on how we determine vitamin D deficiency. The measurement of vitamin D-binding protein may need to be incorporated into the assessment.