People with a large pot belly in midlife are at increased risk for dementia in their later years, according to a new study, and the bigger the belly the higher the risk.

“People should be concerned not only about weight, but about where they carry it,” said Rachel A. Whitmer, the study’s lead author and a research scientist at the research division of Kaiser Permanente in Oakland, Calif. “And midlife is not too early to start thinking about it.”

It is well known that fat around the waist, even in people who are not overweight, is a significant risk factor for cardiovascular disease and diabetes, but this is the first finding of an association with dementia.

The study, which appeared online last week in Neurology, measured the sagittal abdominal diameter, or S.A.D. (in laymen’s terms, how far the pot belly sticks out), of 6,583 men and women in their 40s from 1964 to 1973, then looked at their records an average of 36 years later, when they found 1,049 dementia cases.

Compared with those in the lowest one-fifth in S.A.D., those in the highest one-fifth were almost three times as likely to have dementia. Even among people of normal weight, those with an S.A.D. above 9.8 inches — which corresponds roughly to a 40-inch waist — were almost twice as likely to be demented as those with the smallest bellies. The association held after controlling for high blood pressure, diabetes, high cholesterol and other variables.