Sticking to a Mediterranean diet rich in olive oil, red wine, fish and fresh produce may lower the odds of developing Alzheimer's disease as much as 68%.
The diet, with little red meat and dairy, was considered most likely to influence Alzheimer's risk by protecting the cardiovascular system, lead investigator Nikolaos Scarmeas said in the Archives of Neurology.
The study of 1,984 people in northern Manhattan builds on data published in April.
Cardiovascular risk factors such as diabetes, high blood pressure, heart disease and stroke didn't change the results, scientists added.
Previous studies tied the low-fat Mediterranean diet to a reduced risk of those illnesses. The antioxidants in the food or its effect of reducing inflammation may play important roles, said Scarmeas, an assistant professor of neurology at Columbia University Medical Center in New York.
"If we can clarify the exact mechanism by which diet protects, then we could even improve on the diet and bring up to the surface the most positive components," he said, adding "we can even have more insight about mechanisms that could affect Alzheimer's disease."
Researchers divided the study subjects, with an average age of 76.3, into three groups. People who followed the diet most closely had a 60 to 68% lower risk of developing the brain disease, compared with patients who ate the fewest foods in the diet.
People in the middle group had a 42 to 53% less risk, by comparison with the lowest group.
"The most impressive thing about this particular study is it's directing the attention to the diet itself and away from the blood vessels," said Sam Gandy, chairman of the medical and scientific advisory council for the Alzheimer's Association.
The study included 194 people with early-stage Alzheimer's and another 1,790 who had no dementia.