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The free form of vitamin D

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 of the vitamin 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.

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In the study, published last Wednesday 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. 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. D.K.

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