The diet, which was rich in omega-3 and omega-6 polyunsaturated fatty acids, vitamin E, and folate but poor in saturated fatty acids and vitamin B12, was similar to the Mediterranean diet.
Although the study could not prove a causal relationship, Scarmeas and his colleagues said that there are several ways the diet could protect against Alzheimer's disease.
Folate reduces circulating homocysteine levels, vitamin E has a strong antioxidant effect, and "fatty acids may be related to dementia and cognitive function through atherosclerosis, thrombosis, or inflammation via an effect on brain development and membrane functioning or via accumulation of beta-amyloid," they wrote.
The diet "may have the protective effect on Alzheimer's disease involving all these pathways," they wrote.
Researchers contacted by MedPage Todayand ABCNews.com noted that the findings could not prove causation.
"It may also be that eating healthy is a marker for other factors such as education, intellect, and income, which may be protective," said Dr. George Grossberg of St. Louis University.
Dr. Steven DeKosky, vice president and dean of the University of Virginia School of Medicine in Charlottesville, said there are several unknowns regarding the relationship between diet and Alzheimer's disease risk.
"At an individual level, we don't know how powerful an effect the foods might have on suppressing expression of Alzheimer's disease, or how long you would have to eat them to have an effect, or what interactions of nutrition or individuals' genes may occur and affect risk," DeKosky said.