By Amy Norton
WEDNESDAY, Nov. 20, 2013 (HealthDay News) -- Black adults typically have low levels of vitamin D in their blood, but they are on par with whites when it comes to the "active" form of vitamin D used by the body's cells, a new study finds.
Experts said the findings go a long way toward explaining a paradox: Blacks usually have fairly low vitamin D levels, but have greater bone mass than whites. Vitamin D is needed to maintain strong bones.
What's more, the results suggest that doctors may be overdiagnosing vitamin D deficiency in black patients, said lead researcher Dr. Ravi Thadhani, chief of nephrology at Massachusetts General Hospital, in Boston.
"We're suggesting that the definition of vitamin D deficiency needs to be rethought," said Thadhani, whose report appears in the Nov. 21 issue of the New England Journal of Medicine.
Right now, doctors use a blood test that measures a person's total 25-hydroxyvitamin D level. And if you consider just that total level, Thadhani said, up to 90 percent of black Americans would be labeled vitamin D deficient.
Among the nearly 1,200 black adults in his study, the average total vitamin D level was just shy of 16 nanograms per milliliter (ng/mL), versus almost 26 ng/mL among 900 white adults.
In general, levels below 20 ng/mL are considered a vitamin D deficiency.
But then Thadhani's team looked at study participants' levels of vitamin D-binding protein, which basically locks up the vitamin, away from body cells' use. It turned out that blacks also had lower levels of vitamin D-binding protein. So on balance, black and white adults had similar levels of "bioavailable" vitamin D -- the kind that their bodies can actually use.
And just like in past studies, black adults typically had greater bone mass and higher calcium levels than their white counterparts.
Why the discrepancy? Thadhani said that gene variations appeared to explain most of the difference in people's levels of the vitamin D-binding protein. Most blacks adults carried a gene variant linked to lower levels of the protein, while fewer than half of whites did.
"This highlights the role of genetics in determining our vitamin D status," Thadhani said. "Treating different populations as one big whole doesn't necessarily serve people well."
He said his team is not suggesting that doctors treat low vitamin D levels based on race. But, he added, "we shouldn't treat just based on total (vitamin D) levels alone."
Instead, Thadhani said, doctors could look at whether a black patient with low vitamin D has any other indicators of a problem -- such as low calcium levels or a very high level of parathyroid hormone.
Right now, there is no commercially available test that specifically gauges bioavailable vitamin D levels. Thadhani said he thinks it would be helpful to get such a test on the market.
But an expert not involved in the study said that while the results are "very interesting to scientists," it's not clear how they should be used in medical practice.
"We've known for a long time that blacks have lower levels of total vitamin D, but don't seem to show the consequences," said Dr. Michael Holick, a vitamin D researcher at Boston University Medical Center who wrote an editorial published with the study.
"This study may give us an explanation," Holick said.
But he added that as far as testing for and treating vitamin D deficiency, "it's not clear yet what it all means."
Holick noted that low levels of total vitamin D have been linked to health problems other than brittle bones. And, he said, some of those conditions, including diabetes and heart disease, are more common in blacks -- though it's not known whether vitamin D has anything to do with that.
Still, Holick said more research is needed before anyone diagnosed with vitamin D deficiency should stop taking supplements. "We can't make any definite comment yet on what people should do with this," he said.
In general, experts recommend that most adults get 600 international units (IU) of vitamin D per day. Meanwhile, those older than 70 should aim for 800 IU. The body makes vitamin D when the skin is exposed to the sun. The process is less efficient in darker-skinned people, which is why blacks typically have low total levels of vitamin D.