A Mediterranean diet with extra nuts and olive oil might help improve memory in older adults, a Spanish study suggests, though the results aren't definitive and more research is needed.

"This small study found that a Mediterranean diet, which is low in animal foods like meat and butter, and high in vegetables, legumes and whole grains, when supplemented with olive oil or nuts is associated with improved cognitive function," Samantha Heller, a nutritionist at New York University's Center for Musculoskeletal Care and Sports Performance who wasn't involved in the study, said by email.

While previous research has linked a Mediterranean diet to a reduced risk of heart disease and some cancers, as well as lower odds of developing Alzheimer's disease, scientists haven't conclusively proven that the diet itself is responsible, rather than other lifestyle choices made by people who eat this way.

In the current study, Dr. Emilo Ros of the lipid clinic, endocrinology and nutrition service at Hospital Clinic, Barcelona, and colleagues set out to establish a stronger link between the Mediterranean diet and better cognitive function.

They randomly assigned 447 older adults at risk for cardiovascular disease to follow one of three diets: a Mediterranean diet supplemented with one liter of extra virgin olive oil a week, a Mediterranean diet supplemented with 30 grams of nuts a day, or a low-fat diet.

At the start of the study, participants were around 67 years old and typically overweight, but not obese. Many of them had high blood pressure or high cholesterol.

All the subjects had cognitive function tests at the start of the study, but only 334 of them, or about 75 percent, completed a second round of brain evaluations at the end of the trial (after around four years of follow-up) to assess the impact of the diets. Dropouts were equally distributed among the three diet groups, and had slightly worse cognitive function at the start of the study than the participants who stuck with the experiment.

Based on the brain function tests done before and after the study, the group eating low-fat foods had a significant decrease in memory and cognitive function.

The group following a Mediterranean diet with supplemental nuts had significant improvements in memory, while the group adding extra virgin olive oil experienced significantly better cognitive function.

In terms of an actual diagnosis of mild cognitive impairment, however, the researchers only identified 37 people who developed that condition during the study, and the diets didn’t have a significant effect on the risk for that diagnosis.

To be sure, the study was small and the group receiving two cognitive function tests was even smaller, the researchers acknowledge in JAMA Internal Medicine. The data was also taken from a larger study that wasn't designed to examine brain function, and it's hard to say which aspect of the Mediterranean diet might help prevent cognitive decline, the authors wrote.

"This diet study is much better than purely observational ones, but it is far from one that provides definitive evidence," Dr. David Knopman, a neurologist at the Mayo Clinic in Rochester, Minnesota, said by email.

While the researchers describe the antioxidant-rich foods in a Mediterranean diet as potentially protecting against cognitive decline, "the evidence to data that pure antioxidants such as vitamin E have an impact on some specific process related to cognitive aging is very, very slim," said Knopman, who wasn't involved in the study.

The brain is 70 percent fat, and gets fats from foods people eat, Heller said. One of the most common fats in the brain is oleic acid, an unsaturated fat found in nuts, olive oil, sunflower seeds and avocados. Omega-3 fatty acids are also important for brain health, and found in fish, walnuts and soy foods.

"These healthy fats have been shown to improve cognitive function and brain health," Heller said. "Conversely, research suggests that eating unhealthy fats like trans fats found in processed foods, and saturated fats in animal foods accelerated cognitive decline, poor memory, and is linked with an increased risk of dementia."