From The New York Times:

People in a large area of the American South have long been known to have more strokes and to be more likely to die from them than people living elsewhere in the country.

Now, a large national study suggests the so-called stroke belt may have another troubling health distinction. Researchers have found that Southerners there also are more likely to experience a decline in cognitive ability over several years — specifically, problems with memory and orientation.

The differences to date in the continuing study are not large: Of nearly 24,000 participants, 1,090 in eight stroke-belt states showed signs of cognitive decline after four years, compared with 847 people in 40 other states.

But the geographic difference persisted even after the researchers adjusted for factors — like age, sex, race and education — that might influence the result. The most recent data from the study were published in Annals of Neurology.

None of the people with cognitive decline in the study had had detectable strokes. But some experts believe their memory problems and other mental issues could be related to the same underlying risk factors, including lifestyle patterns that contribute to hypertension, high cholesterol, diabetes and obesity.

Is it the fried food beloved by Southerners? Limited access to doctors? Too little exercise? Researchers are investigating those and other possible causes. Some experts also suggest that the participants could have had small, undetectable strokes that subtly affected brain function.

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Experts do not know exactly why more strokes occur in a region stretching across Alabama, Arkansas, Georgia, Louisiana, Mississippi, North Carolina, South Carolina and Tennessee (sometimes additional Southern states are included in the stroke belt). . . .

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In both stroke-belt and non-stroke-belt groups, older age, less education, and being African-American were associated with increased chances of cognitive decline. But even when those factors were accounted for, residents of the stroke belt still were 18 percent more likely to show impairment.

“These effects are so large, it overcomes these differences in the population,” Dr. Howard said. Most memory and orientation problems detected in the four years were subtle, he added, although a few cases were more severe.

Virginia Wadley, the study’s lead author and an associate professor of medicine at the University of Alabama at Birmingham, and her colleagues are now looking for possible causes, including genetic predisposition, nutrition, smoking, exercise, hypertension, high cholesterol, obesity, diabetes, emotional or work-related stress and environmental factors like air quality. “It’s likely a mixed bag,” Dr. Wadley said.