Kaumudi J. Joshipura, lead author of a new study finding the link, says she can't explain the relationship.
"It is still a mystery," says Joshipura, who is an associate professor of epidemiology at the Harvard School of Public Health and the Harvard School of Dental Medicine. "We need more studies."
It's true that the report, which used data on 41,380 men in the Health Professionals' Follow-Up Study, did find that participants with fewer teeth generally were older, drank more alcohol, were less physically active, and were more likely to smoke, all of which are risk factors for stroke.
But those common risk factors aren't enough to explain the relationship, Joshipura says.
The report appears in tomorrow's issue of Stroke: Journal of the American Heart Association.
Indeed, the analysis came up with one counterintuitive finding: The association between tooth loss and stroke was higher among nonsmokers than among smokers.
Other studies have tended to find a similar association, but they have been controversial, Joshipura says. One strength of her study, Joshipura says, is that its population consisted largely of professionals -- dentists, veterinarians, pharmacists, optometrists, and the like, most white, mostly male -- who generally "behave properly" as far as health habits are concerned.
All of them filled out health questionnaires that were mailed to them every two years. Over 12 years, there were 349 ischemic strokes, the kind that happen when a clot blocks an artery leading to the brain. Men who had fewer than 25 teeth when they entered the study were 57 percent more likely to have an ischemic stroke than those with more than 25 teeth.
Breaking it down by numbers, the risk was 50 percent higher for men with 17 to 24 teeth, 74 percent higher for men with 11 to 16 teeth, and 66 percent higher for men with 10 or fewer teeth.
The number of teeth lost during the 12-year study didn't have much of an effect on the risk of stroke -- perhaps because tooth loss tended to be minor, perhaps because it takes many years for the effect of tooth loss to impact stroke risk, Joshipura says.
One hypothesis that is gaining attention is that the infection related to periodontal disease and tooth loss causes inflammation that injures the arteries, says Dr. Ralph L. Sacco, professor of neurology and epidemiology at Columbia Presbyterian Medical Center in New York City and a spokesman for the American Stroke Association.
"We have our own studies showing that tooth loss is related to carotid artery disease," Sacco says. The carotid artery is the main artery to the brain.
"Could chronic infection, periodontal disease being one of its manifestations, possibly be related to hardening of the arteries?" Sacco asks. That concept has led to growing attention to C-reactive protein, a marker of inflammation that studies have associated with heart disease, he says.
At any rate, Sacco says, the relationship is "another good reason to brush your teeth."
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