TUESDAY, Feb. 10 (HealthDay News) -- Chalk up another endorsement for the so-called Mediterranean diet: The eating regimen, which is rich in fruits, vegetables, fish and olive oil, may help the brain stay sharp into old age, a new study suggests.

Following the healthful diet reduced the risk of getting mild cognitive impairment -- marked by forgetfulness and difficulty concentrating. And it also cut the risk of developing Alzheimer's disease if cognitive impairment was already present, said study lead author Dr. Nikolaos Scarmeas, an assistant professor of neurology at Columbia University Medical Center in New York City.

"We did two different types of analysis," Scarmeas said of the study, published in the February issue of Archives of Neurology.

Previous research has found that people who follow the Mediterranean are at less risk of developing a variety of diseases besides Alzheimer's, including heart disease, cancer and Parkinson's.

The Columbia researchers began the study by evaluating almost 1,400 people without cognitive impairment and 482 people with mild cognitive impairment, and then followed them for an average of 4.5 years. The participants -- average age 77 -- also completed a food frequency questionnaire, detailing what they had eaten during the past year.

The researchers divided the participants into three groups -- those who adhered somewhat or not at all to the Mediterranean diet, those who adhered moderately to it, and those who adhered regularly. Then they evaluated the participants' cognitive functioning.

They found that the diet helped in both cases -- preventing mild cognitive impairment and also the risk of further decline, even if people weren't entirely strict in their adherence to the diet.

"As compared to the group that ate very little or not at all of the Mediterranean diet, those who ate it to a moderate degree had 17 percent less risk of developing mild cognitive impairment," Scarmeas said. "Those who adhered a lot had a 28 percent less risk of developing mild cognitive impairment."

The diet also helped those who already had mild impairment. "Compared to those who adhered not at all or very little, those who ate the Mediterranean diet to a moderate degree had a 45 percent reduction in risk going from mild cognitive impairment to Alzheimer's disease. Those who adhered a lot had a 48 percent reduction in risk of going from mild cognitive impairment to Alzheimer's," he said.