But the research also suggests that the same effect is not found in those who eat lots of fruit.

"It's a modest effect," said Martha Clare Morris, associate professor at Rush University Medical Center in Chicago, and lead author of the study. "People who consumed two or more vegetables a day had a 35 to 40 percent decrease in the decline in thinking ability over six years. That's the equivalent of being five years younger in age."

The study results are published in the Oct. 24 issue of the journal Neurology.

Morris' team studied 3,718 research participants 65 or older who live in the south side of Chicago. Sixty-two percent were black, 38 percent were non-Hispanic white, and 62 percent were female.

"We used a complete food questionnaire of 139 different food items," Morris said. "We asked about their usual intake and assessed the frequency of intake." During the six-year study, the participants received at least two cognitive tests that measured their memory and thinking speed.

"By far, the association with a slower rate of decline was found in the group that ate high amounts of green, leafy vegetables," Morris said. Such foods included lettuce and tossed salad, spinach, kale and collards.

The study also found that the slowdown in cognitive decline was greatest in the oldest people who ate at least two more vegetable servings a day.

Because the cognitive tests measured overall thinking ability, the benefits of eating vegetables may translate into an easier time with such everyday tasks as remembering phone numbers and names and balancing checkbooks, Morris said.

Morris suspects that vegetables may help protect memory and thinking speed because they contain high amounts of vitamin E, an antioxidant that can help reduce the damage caused by free radicals, unstable oxygen molecules generated by normal metabolism that can damage neurons in the brain and contribute to dementia.

"We had found in previous studies that vitamin E in food protected against cognitive decline and the development of Alzheimer's disease," she said.

Her previous research also had shown that consumption of healthy fats, such as the polyunsaturated and monounsaturated fats found in foods such as olive oil, were similarly protective.

"When we eat vegetables, we tend to put the good fats on them, such as an oil-based salad dressing on salads, healthy-fat mayonnaise on cole slaw, and healthy-fat margarine on vegetables," Morris said. "Such fats help us to absorb the vitamin E, and perhaps are also beneficial to the brain. So that's one plausible explanation of why vegetables are good for you."

Morris' study also found that high consumption of fruit had no effect on thinking ability. A similar large-scale study, the Nurses' Health Study, also found that high vegetable consumption, but not high fruit consumption, was associated with a slower rate of cognitive decline.

One of the most common antioxidants found in fruit, vitamin C, has not been consistently shown to protect the brain, Morris said. Most of her study participants consumed fruit such as oranges, grapefruits, apples and bananas, which are high in vitamin C.

 It's possible that some fruit may contain compounds that counteract antioxidants. Further studies are needed to determine whether fruit is brain-protective, she said.

As for eating vegetables, Morris said it's too soon to say for sure that they actually preserve the brain from age-related decline. "But it's encouraging to see that it appears to slow the rate of decline," she said. "We know that eating vegetables is important for chronic diseases. So this might be one more reason why you should eat your vegetables."

In her next study of the same group of Chicago residents, Morris hopes to examine whether high vegetable consumption helps protect against Alzheimer's disease. Results are expected in the next year, she said.

Dallas Anderson is program director for epidemiologic studies of Alzheimer's disease at the National Institute on Aging. "It may be premature to discount the role of fruit consumption in maintaining cognitive health," he said, citing recent research showing that weekly consumption of three or more servings of fruit and vegetable juices was associated with a reduced risk of Alzheimer's.

"Further research will be needed to take account of how the fruit is prepared, as peeling may greatly reduce the amounts of antioxidants available," Anderson said.

"I anticipate that further research will refine what we know about the relationship between fruit and vegetable consumption and cognitive function, helping to determine more definitively the types and amounts of foods that may preserve cognition," he added.