A study published in JAMA Internal Medicine provides new evidence that a diet rich in olive oil, nuts, fish, fruit, vegetables and legumes may reduce the risk of cognitive decline.

Since the 1980s, a number of experts have highlighted the various health benefits of the Mediterranean diet, which emphasizes regular consumption of fish, legumes, whole grains, fruit and vegetables.

Olive oil is the primary source of fat, while red meat and full-fat dairy products are kept to a minimum, and red wine is allowed in moderation. The diet was even deemed part of the world's Intangible Cultural Heritage by UNESCO in 2010.

Previous observational studies have suggested the antioxidants found in the diet's staples -- namely fruit, vegetables and olive oil -- may help to delay neurodegeneration and the effects of aging on the brain.

But the paper published May 11 in JAMA Internal Medicine details the first clinical, randomized study on the subject, which involved 447 cognitively healthy participants aged 55 to 80.

The participants were divided into three groups. The first and second groups followed the Mediterranean diet, supplemented daily with five tablespoons of extra virgin olive oil or 30g of mixed nuts, respectively. The third group, the control, followed a low-fat diet. After following their assigned diets for four years, all of the participants were asked to take a series of neuropsychological tests.

The results indicate that those who followed the supplemented Mediterranean diet, whether with olive oil or nuts, scored higher than the control group, particularly when it came to the memory tests. Remarkably, the researchers even noted a decline in cognitive performance among those who followed the low-fat diet.

"It's never too late to change your dietary patterns to improve your health," explains Dr. Emilio Ros, who led the study at the Hospital Clinic at the University of Barcelona in Spain.

A previous study, carried out in May 2013 on the same group of participants, suggested that consuming nuts and olive oil had similar benefits for cognitive health, although it did not follow the participants through time.

The results are promising, the authors say, despite the small population size and the fact that it does not establish a significant correlation between the Mediterranean diet and mild cognitive impairment, one of the early signs of Alzheimer's disease.