WEDNESDAY, Nov. 16 (HealthDay News) -- People with certain blood types may be at heightened risk for stroke, a new study suggests.

Researchers analyzed data on nearly 62,000 women taking part in the Nurses' Health Study and about 28,000 men from the Health Professionals Follow-up Study. Participants were tracked for 20 to 26 years.

Over that time, both men and women with blood type AB had a 26 percent increased risk of stroke compared to those with blood type O.

In women, type B was also associated with a 15 percent heightened risk of stroke compared to women of a similar age with type O. A similar association was not found in men.

Blood type A wasn't linked with an added stroke risk in either gender, the investigators found.

"Blood group AB showed most consistent association with stroke," said study author Dr. Lu Qi, an assistant professor of medicine at Harvard Medical School and an assistant professor of nutrition at Harvard School of Public Health in Boston.

However, while the study did uncover an association between blood type and stroke, it did not prove a cause-and-effect relationship.

The study was slated for presentation Wednesday at the American Heart Association annual meeting in Orlando, Fla. Research presented at medical meetings should be viewed as preliminary until published in a peer-reviewed journal.

People can have one of eight possible blood types, which differs by the presence (or lack) of certain antigens, which are part of the immune system. Type O blood, known as the universal red cell donor, was the most common in whites in the group studied -- about 43 percent had O, 36 percent had A, 13 percent had B and about 7.5 percent had AB. Another antigen, the Rh factor, determines whether any blood type is either positive or negative.

Commenting on the study, Dr. Larry Goldstein, director of the Stroke Center at Duke University and an American Heart Association spokesman, said no one knows why certain blood types might add to stroke risk, or even if there is actually a relationship.

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.

More information

Learn more about blood types at the American Red Cross.