By Denise Mann
Specifically, the team of scientists found it might increase heart disease risk among white or Chinese individuals, but it does not seem to pose any cardiovascular danger to black or Hispanic adults.
"We think that the differences are mainly due to biologic differences in vitamin D metabolism between race [and] ethnicity groups. However, future studies are needed to more carefully examine these potential differences," said study author Cassianne Robinson-Cohen, of the University of Washington, in Seattle. "Our results suggest that we should use caution in extrapolating results from studies conducted in European Americans to all race [and] ethnicity groups."
Vitamin D is known as the sunshine vitamin because the human body produces it when exposed to the sun's rays. In recent years, vitamin D deficiency has been linked to a host of illnesses, including heart disease, certain cancers, osteoporosis, diabetes, Alzheimer's disease, schizophrenia and some autoimmune disorders. The Institute of Medicine recommends 600 international units (IU) per day for everyone aged 1 to 70 and 800 IU a day for adults older than 70.
In the study, researchers measured vitamin D levels among more than 6,400 people of different ethnic backgrounds. None had evidence of heart disease when the study began.
After more than eight years of follow-up, 361 participants had had a heart attack or another heart-related event. White people were 26 percent more likely to experience a heart-related event for every drop in vitamin D levels of 10 nanograms per milliliter of blood (ng/mL), and Chinese people showed a 67 percent higher risk for the same drop. However, the same relationship was not seen among black and Hispanic people in the study.
Dr. Keith Norris, an associate professor of medicine at the David Geffen School of Medicine at University of California, Los Angeles, co-wrote an editorial accompanying the new study.
"This study brings up some interesting ideas about vitamin D and heart disease, and how it may differ in different populations," he said.
Ironically, blacks and Hispanics are known to have lower levels of vitamin D than their white or Asian counterparts, Norris said.
"We would think that the populations with lower vitamin D levels would be at greater risk for heart disease," but these findings suggest the complete opposite, he said.
In the study, black participants had the lowest vitamin D levels at 19.2 ng/mL. Hispanic participants had vitamin D levels of 24.6 ng/mL. By contrast, Chinese individuals had mean vitamin D levels of 26.7 ng/mL and white participants had a mean levels of 30.1 ng/ml.
The findings "reinforce the fact that we can't generalize from one group to another group whether blacks to whites, men to women or kids to adults, " Norris stressed. "We can't just grab a bunch of patients with low vitamin D, and put half on supplements and the other half on none."
The findings were published in the July 10 issue of the Journal of the American Medical Association.
While the study shows an association between low vitamin D levels and heart disease, it does not prove that there is a cause-and-effect relationship for any race or ethnicity.
The researchers did control for other heart disease risk factors such as diabetes, obesity and high blood pressure levels, but Norris speculated that "these other risk factors are not as powerful among white and Asian individuals, so vitamin D can express a more significant impact on its relationship with heart disease."
One expert said the study raises more questions than it answers.
"It just shows that in certain patients, low vitamin D might portend a higher heart disease risk while in others it is not so clear-cut, " said Dr. David Friedman, chief of heart failure services at North Shore-LIJ's Plainview Hospital, in Plainview, N.Y.
His advice is to see your doctor and get your vitamin D levels tested. If they are low, supplementing may make sense for bone health, and possibly heart health. "Make sure all of your other heart disease risks are in the normal range, too," he said.