The finding, published in the early online addition of Annals of Neurology, comes from a study of 2,258 older adults in New York. At the study's start, participants were in their 70s, on average, and none had dementia. Alzheimer's disease is the most common form of dementia in seniors.
Participants took a 61-item survey about the foods they typically ate. They also took a battery of tests every 1.5 years for four years to screen for Alzheimer's. Those tests covered mental skills including memory, language, and reasoning.
Participants weren't asked to change their eating habits. During the study, the group had 262 cases of Alzheimer's, with fewer cases seen in participants on Mediterranean-style diets.
Lower Rates of Alzheimer's Disease
"The main finding of the study is that higher adherence to a Mediterranean diet type of food consumption is associated with decreased risk for developing Alzheimer's disease," neurologist Nikoloas Scarmeas, MD, tells WebMD.
Scarmeas, who worked on the study, is an assistant professor of neurology at New York's Columbia University Medical Center.
Based on participants' food surveys, Scarmeas and colleagues gave each participant a score for adherence to a Mediterranean-style diet. Scores ranged from 0-9, with higher scores showing greater adherence to a Mediterranean diet.
Compared with those with the lowest scores, those with middle scores were 15% less likely to have been found to have developed Alzheimer's disease, and those with the highest scores were 40% less likely to have been found to have Alzheimer's disease.
Adjusting for age, ethnicity, education, genetic factors linked to Alzheimer's disease, and caloric intake didn't change the results.
What Did They Eat?
The Mediterranean diet included high intake of certain foods:
- Fruits including apples, oranges, orange or grapefruit juice, peaches, apricots, plums, and bananas
- Vegetables including tomatoes, broccoli, cabbage, cauliflower, Brussels sprouts, raw or cooked carrots, corn, yams, spinach, collard greens, and yellow squash
- Legumes including peas, lima beans, lentils, and beans
- Cereals including cold breakfast cereals, white or dark bread, rice, pasta, and potatoes (baked, broiled, or mashed)
- Monounsaturated fatty acids, such as those found in olive oil
The Mediterranean diet also includes moderate amounts of fish of all types, low intake of meat and poultry, low to moderate amounts of dairy products, and a moderate amount of alcohol (usually wine served at meals).
Looking at the Big Picture
Past studies have focused on isolated nutrients, Scarmeas notes.
"The novel approach of this study is that we looked at the combination of foods into a food pattern… because people do not consume dietary elements in isolation, but only as part of their overall diet," he says.
Possibly, it's the combination of nutrients, not single nutrients, "that would be carrying beneficial results," Scarmeas says.
"When we looked at individual elements of this diet in isolation, we could not detect much beneficial effect, while when we looked at all of them together, the effect was there and it was quite prominent," Scarmeas says.
"This underlines again the importance of looking at combinations of foods and nutrients when we look at the diet, rather than individual ones."
The Mediterranean diet isn't a diet in the sense of a temporary dietary change. It's about eating healthfully in the long run, not following flash-in-the-pan food fads.
"We looked into our data, and adherence to these dietary habits seems to be a longstanding pattern," Scarmeas says. "It seems that people do not change their diet preferences and this is what they have been following for years."
"In particular for Alzheimer's disease, we do not know exactly when the disease starts," Scarmeas says. "There are data that show that small changes in the brain may occur decades before the clinical onset of the symptoms. So it seems that it's important that whatever dietary elements are beneficial that they are taken as early as possible and for a long time."
Observational studies like this one don't prove that participants' eating habits solely prevented Alzheimer's. Even after adjusting for other factors, it's possible that people favoring Mediterranean diets had other traits working in their favor.
"Since it's the first study relating this diet to Alzheimer's disease, it's a little bit premature to make recommendations to people," Scarmeas says. "It has to be replicated and shown that it is beneficial by other investigators and in other studies. That will increase our confidence that this is a true finding."