Dr. Georgios Tsivgoulis of the University of Alabama at Birmingham and the University of Athens said the dietary information from 17,478 African-American and Caucasian people with an average age of 64 was reviewed to see how closely they adhered to a Mediterranean diet.
They were also given tests that measured memory and thinking abilities over an average of four years. A total of 17 percent of the participants had diabetes.
Seven percent of the participants developed impairments in their thinking and memory skills during the study, Tsivgoulis said.
"Since there are no definitive treatments for most dementing illnesses, modifiable activities, such as diet, that may delay the onset of symptoms of dementia are very important," Tsivgoulis said in a statement.
The study, published in Neurology, found that in healthy people, those who more closely followed the Mediterranean diet were 19 percent less likely to develop problems with their thinking and memory skills.
There was not a significant difference in declines between African-Americans and Caucasians, but the Mediterranean diet was not associated with a lower risk of thinking and memory problems in people with diabetes, the study said.