It's possible that blood type is standing in for some other factor that influences stroke risk, he noted.
"It's hard to know if blood type is a marker for something else or if there's a direct relationship," Goldstein said. "For example, maybe there is some other genetic factor that is traveling along with the blood type or that's associated with that blood type that may affect stroke risk."
In the study, researchers took into account age, smoking status and physical activity levels, but not other factors such as cholesterol levels or diabetes that could also influence stroke risk, Goldstein added.
A study published in January in The Lancet found that blood type O may offer some protection from heart attacks. But other research looking at blood type and disease risk haven't consistently shown an association, Goldstein pointed out.
Future research will look at stroke risk and blood type among other ethnic groups, as well as trying to figure out what biological mechanism might explain the association, Qi added.
Learn more about blood types at the American Red Cross.
SOURCES: Lu Qi, M.D., Ph.D., assistant professor, medicine, Harvard Medical School and assistant professor, nutrition, Harvard School of Public Health, Boston; Larry Goldstein, M.D., director, Stroke Center, Duke University, Durham, N.C., and spokesman, American Heart Association; Nov. 16, 2011, presentation, American Heart Association annual meeting, Orlando, Fla.
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