The Mediterranean diet typically bases meals around fruit, vegetables, whole grains, legumes, nuts and olive oil.
The study, published in JAMA Internal Medicine, was a randomized clinical trial that followed cognitive change over time among volunteers assigned to follow one of three different diets. Volunteers were cognitively healthy, had a high cardiovascular risk and an average age of 67.
Following the Mediterranean diet has been recommended by the Dietary Guidelines for Americans as a way to promote health and prevent disease. Emphasis is placed on eating primarily plant-based foods, basing every meal on foods such as fruits, vegetables, whole grains, legumes and nuts.
The diet discourages the use of saturated fats and trans fats, both associated with heart disease. Instead, healthier types of fat are obtained from sources such as olive oil, predominantly containing monounsaturated fat which can improve cholesterol levels.
Vegetables and healthier fats are also good sources of antioxidants that play an important role in counteracting oxidative stress. Oxidative stress occurs when the body is unable to detoxify itself fully, and the process is believed to play a significant role in cognitive decline.
"Oxidative stress and vascular impairment are believed to partly mediate age-related cognitive decline, a strong risk factor for development of dementia," write the authors. "Epidemiologic studies suggest that a Mediterranean diet, an antioxidant-rich cardioprotective dietary pattern, delays cognitive decline, but clinical trial evidence is lacking."
To test this hypothesis, Dr. Emilio Ros of the Institut d'Investigacions Biomediques August Pi Sunyer, Hospital Clinic, Barcelona, and coauthors examined the effects of Mediterranean diets supplemented with olive oil or nuts compared with a low-fat control diet.
A total of 155 participants were randomly assigned to follow a Mediterranean diet supplemented with one liter of extra virgin olive oil per week. Another 147 participants were assigned to follow a Mediterranean diet supplemented with 30 grams per day of mixed nuts (walnuts, hazelnuts and almonds).
The researchers measured cognitive change in the participants over time using multiple neuropsychological tests focusing on memory, global cognition and frontal cognition (attention and executive function). These participants were compared with a control group of 145 participants following a diet where they were advised to reduce dietary fat.
Extra olive oil or nuts 'may counteract age-related cognitive decline'
After a median of 4 years of the dietary intervention, follow-up cognitive tests were available in 334 participants. The researchers found there were 37 cases of mild cognitive impairment among the participants: 17 (13.4%) in the group that received extra olive oil, eight (7.1%) in the mixed nuts group and 12 (12.6%) in the control group.
Participants in the control group experienced significant decreases in each measured composite of cognitive function. The researchers noted, however, that the two Mediterranean diet arms of the study experienced different improvements in cognitive function.
"The group with nuts did better compared to the control group in memory tests, memorizing names or words, while the olive oil group did better on tests that require speed of thought, your frontal function, your executive function," explains Dr. Ros.
Although the study was a randomized clinical trial, it has its limitations. Not all participants received follow-up cognitive testing, and adherence to all three diets cannot be guaranteed. According to the researchers, further investigation is warranted.
"Our results suggest that in an older population, a Mediterranean diet supplemented with olive oil or nuts may counteract age-related cognitive decline," conclude the authors. "The lack of effective treatments for cognitive decline and dementia points to the need of preventive strategies to delay the onset and/or minimize the effects of these devastating conditions."
In an interview with JAMA, Dr. Ros states that the research team is currently conducting a study into the effects of walnuts on neurodegenerative disease, comparing a walnut diet with a control diet.
Last week, Medical News Today reported on a similar study in which researchers monitored the diets of older adults for 5 years and tested for cognitive decline. The researchers discovered that those who ate healthily experienced only a small drop in brain power.