Explaining the Vitamin D Paradox

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.

Key Findings:

  • 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.

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