To the Editor: It is unclear whether circulating free or bioavailable 25-hydroxyvitamin D is a better marker of vitamin D status than is total 25-hydroxyvitamin D, especially in racially diverse populations. Until recently, the only method to compare the levels was to estimate the level of free or bioavailable 25-hydroxyvitamin D from total 25-hydroxyvitamin D, vitamin D-binding protein (also known as gc-globulin, encoded by the GC gene), and albumin, with or without the GC genotype. Powe et al. reported that levels of vitamin D-binding protein, as measured on a monoclonal enzyme-linked immunosorbent assay (ELISA, R&D Systems), were lower in black . . .
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