The study of 6,583 people ages 40 to 45 in California had their abdominal fat measured and an average of 36 years later, 16 percent of them had been diagnosed with dementia.

Study author Rachel A. Whitmer of the Kaiser Permanente Division of Research in Oakland, Calif., found that those with the highest amount of abdominal fat were nearly three times more likely to develop dementia than those with the lowest amount of abdominal fat.

"Considering that 50 percent of adults in this country have an unhealthy amount of abdominal fat, this is a disturbing finding," Whitmer said in a statement.

A large abdomen increased the risk of dementia regardless of whether the participants were of normal weight, overweight, or obese, and regardless of existing health conditions, such as diabetes and cardiovascular disease, Whitmer added.

However, in observational studies, it's possible that the association of the abdominal obesity and dementia is not driven by the abdominal obesity, but a complex set of health-related behaviors, for which abdominal obesity is but one part, Whitmer noted.

The findings were published in the journal Neurology.