Vitamin D–binding protein is the primary vitamin D carrier protein, binding 85 to 90% of total circulating 25-hydroxyvitamin D.19 The non–vitamin D–binding protein fraction (bioavailable 25-hydroxyvitamin D) consists primarily of albumin-bound 25-hydroxyvitamin D (10 to 15% of total 25-hydroxyvitamin D), with less than 1% of total 25-hydroxyvitamin D in the free form. Vitamin D–binding protein appears to inhibit some actions of vitamin D, because the bound fraction may be unavailable to act on target cells.20,21 Common genetic polymorphisms in the vitamin D–binding protein gene produce variant proteins that differ in their affinity for vitamin D.22,23 The prevalence of these polymorphisms differs between racial groups.24,25 Clinical assays measure the level of total 25-hydroxyvitamin D without distinguishing fractions bound to carrier proteins.
We conducted a study to determine whether vitamin D–binding protein genotypes and concentrations of circulating vitamin D–binding protein differ between black Americans and white Americans, possibly accounting for observed racial differences in manifestations of vitamin D deficiency.