The difference found between elderly people who ate greens and those who did not was stunning: the equivalent of being 11 years younger in age, said the study in the journal Neurology.

While the research was based on survey responses and therefore fell short of proving cause and effect, researchers said it offers further evidence of the association between healthy eating and healthy aging. 

"Adding a daily serving of green, leafy vegetables to your diet may be a simple way to foster your brain health," said study author Martha Clare Morris of Rush University Medical Center in Chicago.

The study tracked 960 people with an average age of 81, and followed them for an average of nearly five years. None had dementia upon entering the study.

Participants completed questionnaires that asked how often they ate certain foods, including spinach, kale, collard greens and lettuce. They also had their thinking and memory skills tested once a year.

People who ate the most greens averaged about 1.3 servings per day. Those on the opposite end of the spectrum ate 0.1 servings per day. A serving is about a half cup, cooked.

© Provided by AFPRelaxNews One key aspect of the so-called Mediterranean diet is consuming plant-based foods, while limiting the intake of red meat. People who ate at least one serving daily "had a slower rate of decline on tests of memory and thinking skills than people who never or rarely ate these vegetables," said the study.

These results persisted even after researchers accounted for factors like smoking, high blood pressure, obesity, education level and mental and physical exercise.

"These observations are consistent with a broader body of evidence suggesting that people adhering to a Mediterranean diet may reduce their risk of dementia," said David Llewellyn, senior research fellow in clinical epidemiology at the University of Exeter, in England, who was not involved in the study.

One key aspect of the so-called Mediterranean diet is consuming plant-based foods, while limiting the intake of red meat. 

James Pickett, head of research at the Alzheimer's Society in London, pointed out that "the researchers did not directly look at dementia, so we cannot say that it would delay or prevent the onset of the condition.

"However, older people who ate one or two servings of Vitamin K-rich food per day performed better on memory tests than those who didn't," he added.

"A healthy diet rich in essential nutrients, combined with regular exercise and avoiding smoking, can help to reduce your risk of developing dementia."