While doctors routinely perform vitamin D blood tests during annual physicals, many of the 70 to 90 percent of African Americans who are diagnosed as vitamin D deficient may actually have healthy levels and could be taking supplements unnecessarily.

That’s the conclusion of a new study from Massachusetts General Hospital, which found that tests to measure total vitamin D levels don’t reflect the genetic differences that lead some people—often African Americans and Asians from warmer climates—to carry more of the “free” form of the nutrient.

This free vitamin D—which is really a hormone—is readily available for the body to use to maintain strong bones and other cellular functions. Vitamin D that’s bound to a protein can only be used by the body as a last resort.

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In the study, published Thursday in the New England Journal of Medicine, researchers from MGH and the National Institutes of Health measured vitamin D levels in more than 2,000 African Americans and Caucasians ages 30 to 64 living in Baltimore, Md. Their research found that African Americans tended to have lower levels of vitamin D and less vitamin D–binding protein when compared with Caucasians, which was linked to a gene mutation. African Americans also had denser bones and higher levels of calcium.

“We shouldn’t be calling people with low vitamin D levels ‘deficient’ if they have strong bones and significant amounts of free [vitamin] D hormone,” said study senior author Dr. Ravi Thadhani, chief of nephrology at Mass. General Hospital.

Thadhani said he would like to see companies develop commercial tests to measure free vitamin D levels in order to more accurately identify those with true deficiencies. In the meantime, he said, doctors should generally recommend vitamin D supplements only to African Americans who have a history of bone fractures or osteoporosis.

Some vitamin D researchers argue that there’s no harm in giving supplements to those who have low vitamin D levels on currently used tests—just to be on the safe side. But there are side effects associated with supplementation such as a small increased risk of kidney stones, especially in those who also take calcium supplements. Other preliminary research suggests that pushing vitamin D levels too high in African Americans could be associated with more calcium deposits on coronary arteries, possibly raising their risk for heart disease.