Low levels of total 25-Hydroxyvitamin D are common among black Americans. Vitamin D-binding protein ( DBP ) has not been considered in the assessment of vitamin D deficiency.
In the Healthy Aging in Neighborhoods of Diversity across the Life Span cohort of blacks and whites ( 2085 participants ), researchers measured levels of total 25-Hydroxyvitamin D, vitamin D-binding protein, and parathyroid hormone as well as bone mineral density ( BMD ).
Investigators have genotyped study participants for two common polymorphisms in the vitamin D-binding protein gene ( rs7041 and rs4588 ).
Levels of bioavailable 25-Hydroxyvitamin D in homozygous participants were estimated.
Mean levels of both total 25-Hydroxyvitamin D and vitamin D-binding protein were lower in blacks than in whites ( total 25-hydroxyvitamin D, 15.6 ng per milliliter vs 25.8 ng per milliliter, P less than 0.001; vitamin D-binding protein, 168 mcg per milliliter vs 337 mcg per milliliter, P less than 0.001).
Genetic polymorphisms independently appeared to explain 79.4% and 9.9% of the variation in levels of vitamin D-binding protein and total 25-Hydroxyvitamin D, respectively.
Bone mineral density was higher in blacks than in whites ( 1.05 g per square centimeter vs 0.94 g per square centimeter, P less than 0.001 ).
Levels of parathyroid hormone increased with decreasing levels of total or bioavailable 25-Hydroxyvitamin D ( P less than 0.001 for both relationships ), yet within each quintile of parathyroid hormone concentration, blacks had significantly lower levels of total 25-Hydroxyvitamin D than whites.
Among homozygous participants, blacks and whites had similar levels of bioavailable 25-Hydroxyvitamin D overall ( 2.9 ng per milliliter and 3.1 ng per milliliter, respectively; P=0.71 ) and within quintiles of parathyroid hormone concentration.
The study has shown that community-dwelling black Americans, as compared with whites, had low levels of total 25-Hydroxyvitamin D and vitamin D-binding protein, resulting in similar concentrations of estimated bioavailable 25-Hydroxyvitamin D.
Racial differences in the prevalence of common genetic polymorphisms provide a likely explanation for this observation. ( Xagena )
Powe CE et al, N Engl J Med 2013; 369:1991-2000