Study Shows Racial Differences in Vitamin D–Binding Protein

Variations in the vitamin D–binding protein gene may help to explain differences in vitamin D levels and clinical vitamin D deficiency in black versus white individuals, according to a study in the New England Journal of Medicine.

The researchers analyzed data on total 25-hydroxyvitamin D, vitamin D–binding protein, parathyroid hormone, and bone mineral density in black and white adults from a United States population—based cohort study. The participants also underwent genotyping studies for the common rs7041 and rs4588 2 polymorphisms of the vitamin D–binding protein gene. The concentrations of bioavailable 25-hydroxyvitamin D were calculated in a subgroup of 1025 homozygous participants.

The black participants had a lower mean total 25-hydroxyvitamin D level: 15.6 ng/mL, compared with 25.8 ng/mL in white participants. The levels of vitamin D–binding protein were 168 and 337 µg/mL, respectively.

On adjusted analysis, the two polymorphisms accounted for close to 80 percent of the variation in vitamin D–binding protein levels and for 10 percent of the variation in total 25-hydroxyvitamin D. After genotype was accounted for, race explained less than 0.1 percent of the variation in vitamin D–binding protein. Despite their lower vitamin D levels, the black participants had a higher mean bone mineral density.

Study participants with lower total and bioavailable 25-hydroxyvitamin D levels had higher parathyroid hormone levels. However, at each level of parathyroid hormone, total 25-hydroxyvitamin D was lower among black participants. In the homozygous subgroup analysis, levels of bioavailable 25-hydroxyvitamin D were similar by race, and within categories of parathyroid hormone level.

The new results confirm that black persons have lower total 25-hydroxyvitamin D than do their white counterparts. However, because black individuals also have lower levels of vitamin D–binding protein, the levels of bioavailable 25-hydroxyvitamin D are not significantly different. The authors discuss the implications for assessing racial and ethnic differences in vitamin D levels, including the potential role of vitamin D–binding protein measurement [Powe CE, et al. Vitamin D–binding protein and vitamin D status of black Americans and white Americans. N Engl J Med 2013; 369:1991–2000].

January 2014 (Vol. 6, Number 1)