By the current blood test for vitamin D, most African-Americans are deficient. That can lead to weak bones. So many doctors prescribe supplement pills to bring their levels up.

But the problem is with the test, not the patients, according to a newstudy. The vast majority of African-Americans have plenty of the form of vitamin D that counts — the type their cells can readily use.

The research resolves a long-standing paradox.

“The population in the United States with the best bone health happens to be the African-American population,” says Dr. Ravi Thadhani, a professor of medicine at Massachusetts General Hospital and lead author of the study. “But almost 80 percent of these individuals are defined as having vitamin D deficiency. This was perplexing.”

The origin of this paradox is a fascinating tale of genes interacting with geography. More on that later.

To unravel the mystery, Thadhani and his colleagues looked closely at various forms of vitamin D in the blood of 2,085 Baltimore residents, black and white. They focused on a form of the vitamin called 25-hydroxyvitamin D, which makes up most of the vitamin circulating in the blood. It’s the form that the standard test measures.

The 25-hydroxy form is tightly bound to a protein, and as a result, bone cells, immune cells and other tissues that need vitamin D can’t take it up. It has to be converted by the kidneys into a form called1,25-dihydroxyvitamin D.

For Caucasians, blood levels of 25-hydroxyvitamin D are a pretty good proxy for how much of the bioavailable vitamin they have. But not for blacks.

 

(read more here)