Adults with gum disease may be twice as likely as people with healthy gums to suffer a stroke, new research suggests.
It's not the first study to link gum disease and brain attacks caused by blood clots.
However, the new findings expand on that knowledge by demonstrating a "dose-response" relationship.
"The higher the level of gum disease, the worse the risk," explained study author Dr. Souvik Sen, chair of neurology at the University of South Carolina School of Medicine, in Columbia.
Stroke risk rose with the level of gum disease; it was 1.9 times, 2.1 times and 2.2 times higher for people with mild, moderate and severe gum disease, respectively, the findings showed.
One stroke expert said that was the most intriguing discovery in the study.
"The fact that it is a dose-effect relationship, it's an important finding," said Dr. Maurizio Trevisan. He is dean of the City University of New York School of Medicine in New York City.
"Unfortunately, it still does not prove the cause/effect relationship because it's an observational study," said Trevisan. However, he was involved in the first major study, published in 2000, showing a relationship between poor oral health and stroke risk.
Researchers still don't know why people with gum disease have a higher stroke risk. The levels of inflammation found in both gum disease and hardening of the arteries may play a role.
Sen explained that "when that hardening of the blood vessels happens in the brain or the neck, it can lead to a stroke."
But there may be other reasons. It could be that people who neglect their oral health are also less likely to go to the doctor for medical conditions or take medications as prescribed, he added.
"The question still remains whether, if we treated gum disease, can we prevent strokes and heart attacks? -- or not," Sen said.
He and his team used data from a large prospective analysis sponsored by the U.S. National Heart, Lung, and Blood Institute.
More than 6,700 adults who had not had a stroke were categorized based on their level of gum disease and followed for 15 years. The participants were mostly white and 55 percent were female, with an average age of 62.
Nearly 300 strokes were recorded over the study period.
Even after adjusting for other factors -- including age, race and various health factors -- stroke risk was higher among those with greater levels of gum disease.
The link between increasing levels of gum disease and stroke was strongest for two types of clotting, or ischemic, strokes. Nearly half (47 percent) were thrombotic strokes. These are due to clot formation in an artery of the brain. A quarter (26 percent) were cardioembolic strokes, which occur when a blood clot moves from the heart to the brain.
Trevisan said, "I don't think we should tell people that they should floss their teeth in order to prevent heart disease."
But given the strong level of evidence linking gum disease and stroke, and the importance of oral health as people age, "the message is you should take care of your mouth no matter what," he said.
The findings were to be presented Thursday at the International Stroke Conference in Houston. Research presented at meetings is viewed as preliminary until published in a peer-reviewed journal.
Sen said his team was to present two other related studies at the stroke conference. One of those studies found that people who make annual visits to the dentist have a lower risk of ischemic stroke than people who go to the dentist from time to time as needed, he said.