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Black Americans had lower levels of total 25-hydroxyvitamin D and vitamin D-binding protein than whites, but they had similar levels of bioavailable 25-hydroxyvitamin D and higher bone density, a population-based cohort study reported.

Mean (± SE) levels of total 25-hydroxyvitamin D were lower in blacks than in whites, (total 25-hydroxyvitamin D, 15.6 ± 0.2 ng per ml versus 25.8 ± 0.4 ng per ml, P<0.001). The same was true of vitamin D-binding protein (168 ± 2 mcg per ml in blacks versus 337 ± 5 mcg per ml, P<0.001) in whites.

Bone mineral density (BMD), however, was higher in blacks than in whites (1.05 ±0.01 g per cm² versus 0.94 ± 0.01 g per cm², P<0.001). Adjusted mean calcium levels were also greater in blacks than whites (9.11 ± 0.01 mg per deciliter versus 8.99 ± 0.01 mg per deciliter, P<0.001), wrote , of the National Institute on Aging, and her co-authors, in an article published online Nov. 21 in The New England Journal of Medicine.

The study used cross sectional data collected between 2004 and 2008 from 2,085 black and white Baltimore residents 30 to 64 years of age who were enrolled in the Healthy Aging in Neighborhoods of Diversity Across the Life Span (HANDL) study.

The researchers measured levels of total 25-hydroxyvitamin D, vitamin D-binding protein, and parathyroid hormone, as well as BMD. They also genotyped the participants for two common polymorphisms in the vitamin D-binding protein gene. Levels of bioavailable 25-hydroxyvitamin D were based on estimates.

The findings may be explained by variants in the vitamin D-binding protein gene, which often differ in those of African and European descent. When the researchers genotyped study participants, they found genetic polymorphisms that accounted for 79.4% of the variation in levels of vitamin D binding protein, and 9.9% of the variation in levels of vitamin D-binding protein and total 25-hydroxyvitamin D.

"Low levels of vitamin D-binding protein in blacks may provide protection against the manifestations of vitamin D deficiency despite low levels of total 25-hydroxyvitamin D," the authors wrote.

Low 25-hydroxyvitamin D is often seen as a sign of vitamin D deficiency, for which skeletal disorders are the strongest evidence. That blacks have higher bone mineral density but lower 25-hydroxyvitamin D levels than whites has been "a perplexing paradox," wrote , in an accompanying editorial.

"On the basis of the current guidelines (suggesting a threshold for sufficiency of 20 or 30 ng per milliliter), 77% to 96% of our black participants would be classified as vitamin D deficient," wrote the authors. That characterization, they said, is "inconsistent with the observation that they had higher BMD, higher calcium levels, and only slightly higher parathyroid levels their white counterparts."

This study, in addition to previous ones, they said, suggests that low total 25-hydroxyvitamin D levels don't necessarily indicate vitamin D deficiency, and "calls into question routine supplementation in persons with low levels of both total 25-hydroxyvitamin D and vitamin D-binding protein who lack other traditional manifestations of this condition."

More research is needed to "fully appreciate what bioavailable versus total vitamin D status means -- for 25-hydroxyvitamin D as well as 1,25-dihydroxyvitamin D," said Holick.

There were some limitations in the study. Among them were the fact that, because the study was cross-sectional and observational, the researchers were unable to predict the effects of vitamin-D binding protein levels on fracture risk. Results were based on calculation of bioavailable 25-hydroxyvitamin D, rather than direct measurement.

The authors did not have data on participants' use of vitamin D supplements. And "measurement of bone-turnover markers, levels of 1,25-dihydroxyvitamin D, and urinary calcium excretion might have provided additional insight into the effect of vitamin D-binding protein on mineral metabolism," the authors said.

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