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

To the Editor: Current assays for vitamin D sufficiency measure total circulating 25-hydroxyvitamin D. However, Powe et al. (Nov. 21 issue)(1) conclude that measurement of 25-hydroxyvitamin D that is not bound to the vitamin D-binding protein, which they refer to as bioavailable 25-hydroxyvitamin D, provides a better assessment of sufficiency. They report that the correlation between levels of bioavailable 25-hydroxyvitamin D and levels of parathyroid hormone is stronger than the correlation between levels of total circulating 25-hydroxyvitamin D and levels of parathyroid hormone when evaluated across races. However, they fail to reconcile their findings and the rationale underlying their conclusion ...