Vitamin D–Binding Protein and Vitamin D in Blacks and Whites

Powe et al. reported that black participants and white participants have different vitamin D requirements on the basis of the circulating concentration of vitamin D-binding protein.The authors used an assay for vitamin D-binding protein that is untested for the key measurement in their study.This assay is based on a primary monoclonal antibody produced against a single peptide fragment of the human vitamin D-binding protein.2][3] The problem is that the monoclonal-antibody assay gives lower values in blacks who have primarily the Gc1F variant of vitamin D-binding protein.The monoclonal antibody discriminates between the Gc1F and Gc1S variants to provide artificially low values for total vitamin D-binding protein in blacks, thus bringing the conclusion of this study into question.As shown in Figure S4 in the Supplementary Appendix (available with the full text of the article at NEJM.org), "calculated" concentrations of bio-available 25hydroxyvitamin D correlate well with physically measured concentrations in participants homozygous for either the Gc1F or Gc1S variant, but the slopes of the relationships are very different.Specifically, for any given physically measured level of bio-available 25hydroxyvitamin D, the authors are overestimating bio-available 25-hydroxyvitamin D by 2 to 2.5 times owing to underestimation of vitamin D-binding protein in blacks.