Adhering closely to the Mediterranean-style diet — particularly one rich in vegetables and fish — is associated with higher cognitive function among older adults, according to a National Institutes of Health-funded study published this week in the journal Alzheimer’s & Dementia.

The study found no link, however, between the Mediterranean diet and slower cognitive decline.

These findings suggest that eating healthful foods may help keep our brains functioning at higher levels during the aging process, even if those levels aren’t quite as high as they were when we were younger.

For the study, researchers at the National Eye Institute (NEI) analyzed data from two major randomized clinical trials — the Age-Related Eye Disease Study (AREDS) and AREDS2 — that had previously investigated the effects of diet on age-related macular degeneration (AMD), an eye disease that gradually damages the retina, the light-sensitive tissue at the back of the eye. AMD is a leading cause of permanent vision loss and blindness in people aged 60 and older.

Both studies had reported that certain nutrients, particularly the antioxidants found in fruits and vegetables and the omega-3 fatty acids found in fish, were associated with a lower risk of developing AMD later in life. The authors of the current study wanted to see if the diets of the participants in the AREDS studies also had an effect on their cognitive function. Other research has shown an association between AMD and dementia, and the two conditions are known to share some environmental risk factors, such as smoking and high blood pressure.

“We do not always pay attention to our diets. We need to explore how nutrition affects the brain and the eye,” says Dr. Emily Chew, the study’s lead author and director of the NEI Division of Epidemiology and Clinical Applications, in a released statement.

Re-examining the data

For the study, Chew and her colleagues used data from 7,756 ARED participants who had completed cognitive tests while in those clinical trials. The participants were aged 55 to 80 when they entered the trials, and were followed for 10 years.

At the start of the trials, the participants filled out a detailed questionnaire designed to assess their diet over the previous year. Based on those questionnaires, the NEI researchers scored each participant on how closely they adhered to the Mediterranean diet, which emphasizes whole fruits, vegetables, whole grains, nuts, legumes, fish and olive oil, as well as reduced amounts of red meat and alcohol.

Then the researchers looked for associations between the participants’ diets and their cognitive functioning. They found that, in general, the people who most closely adhered to the Mediterranean diet had the highest cognitive function throughout the decade of the study. The differences were small, but still statistically significant.

The individual components of the diet that appeared to have the greatest protective effect on the brain were fish and vegetables. Fish was also the only food associated with slowing down the process of cognitive decline. At the 10-year mark, the people with the highest fish intake exhibited not only higher rates of cognitive functioning, but also the lowest rate of decline.

These findings held even after the researchers adjusted the data to account for education levels.

The benefits from the Mediterranean diet were similar for people with and without a gene — ApoE — known to raise the risk of late-onset Alzheimer’s disease. That finding suggests, say the researchers, that the diet’s influence on cognitive functioning is independent of genetic risk. The people with ApoE did, however, tend to have lower average scores for cognitive function than those without the gene. They also tended to show more cognitive decline.

Limitations and implications

This is an observational study, and therefore can’t prove a connection between diet and higher cognitive abilities. In addition, it relies on people self-reporting the foods they ate. Such reporting can be inaccurate.

In addition, most of the people in the study had some degree of AMD. Whether or not the findings can be generalized to other populations is unclear.

Still, the findings are provocative, for they support other observational studies that have found a link between the Mediterranean diet (or one that’s similar) and better cognitive function and slower cognitive decline.

“Scientists aren’t sure why the Mediterranean diet might help the brain,” explains the National Institute of Aging (NIA) on its website. “This primarily plant-based diet has been shown to improve cardiovascular health, which may, in turn, reduce dementia risk. In contrast, the typical Western diet increases cardiovascular disease risk, possibly contributing to faster brain aging.”

“In addition, this diet might increase specific nutrients that may protect the brain through anti-inflammatory and antioxidant properties,” the agency says.

FMI: You’ll find an abstract of the NEI study on the website for Alzheimer’s & Dementia, although the full paper  — despite being funded by the government — is behind a paywall. For more information on diet and the risk of dementia, go to the NIA’s website.