Eating two or more servings of vegetables a day may slow a person's mental decline by about 40 percent compared with a person who consumes few vegetables, according to a six-year study of nearly 4,000 Chicago residents age 65 or older.

Consuming lots of fruit did not appear to offer the same mental protection, although fruit has been associated with a wide variety of other health benefits, said Martha Clare Morris, chief of Rush University Medical Center's Rush Institute for Healthy Aging.

The slowdown in the rate of cognitive decline experienced by people who ate 2.8 or more servings of vegetables a day is "equivalent to about five years of younger age" compared with people who ate less than one serving, Morris reported in Tuesday's issue of Neurology, the scientific journal of the American Academy of Neurology.

The study also suggested it may never be too late to reap the benefits of vegetable consumption. Older people who started eating more than two servings a day still showed a significant delay in mental decline, Morris said. One serving of a vegetable is generally equal to a cup.

The findings come on top of two earlier Rush studies indicating that the foods people eat may significantly affect their mental agility. Morris reported four years ago that eating foods high in vitamin E appeared to reduce the risk of Alzheimer's disease, and last year she found that eating fish had a similar effect.

Veggies full of antioxidants

Vegetables, especially those in the green leafy category, are brimming with antioxidant compounds like vitamin E, flavonoids and carotenoids that help snuff out cell-damaging free radicals, Morris said.

Eating vegetables with olive oil, vegetable oil or some other type of poly- or mono-unsaturated fats enhances the body's absorption of antioxidants, she added.

"This study is tremendously important," said Alberto Ascherio, associate professor of nutrition and epidemiology at the Harvard School of Public Health, who found similar results over a two-year period in the Nurses Health Study of more than 120,000 nurses. "It's not easy to capture the correlation between dietary behavior and cognitive function.

"This goes in line with previous evidence supporting the potential protective effect of vegetable consumption," he said. "Each of these studies is like a small step forward. In this field we don't have the critical experiment to answer the question once and for all. We have to get to the truth by small steps. It's a long process to try to understand what we can do to reduce cognitive decline."

Match people closely

In trying to figure out which specific food groups bestow important health benefits, epidemiologists match people as closely as possible so other factors in their lifestyles cancel each other out.

Matt Kaeberlein, who conducts research on the biochemical processes of aging at the University of Washington, was surprised the study didn't show any beneficial effect of eating fruit on cognitive decline.

Studies in animals, he said, show that berries—particularly blueberries, strawberries and cranberries—seem to protect memory in aging animals. And a diet high in fruits and vegetables has been linked to protection against heart disease, cancer, stroke, diverticulosis, diabetes and obesity.

Morris agreed that animal research indicates that berries may help preserve memory but that too few people in the study consumed berries regularly to determine if they helped preserve memory and other cognitive functions.

"The link between better cognition and vegetables is interesting and certainly real," Kaeberlein said. "But I wouldn't change my diet to stop eating fruits based on this study. There's plenty of evidence that for overall health you're going to be better off eating a diet that's high in both fruits and vegetables."

Further research is needed to document the exact role that vegetables play in cognitive health, Kaeberlein said. Learning which specific nutrients provide the greatest protection could lead to developing a pill people could take that would have the same benefits, he said.

Phyllis Chase, 84, a participant in Rush's Chicago Health and Aging Project, said she eats six to seven servings of vegetables and fruit every day. Chase, who reads, walks, works puzzles and pulls a cart for nine holes of golf, grows vegetables in her back-yard garden.

"I love vegetables. I've been a vegetable eater all my life," she said. "If I don't have anything cooked for dinner, I'll sauté two cups of mixed vegetables and I'll make a turkey wrap out of it."

Interviewed regularly

Like other study participants, Chase is regularly interviewed about the foods she eats and her daily activities. She is also regularly tested for dementia, memory, abstract thinking, speed of thinking, perceptual speed and visual-spatial skills. People in the study had their overall mental function tested in their homes at the start of their participation and then at three-year intervals.

"We were then able to say what each person's test score was each year that we measured it so that we knew whose scores stayed the same and whose got worse, and then we were able to match their scores with the highest vegetable consumers and the lowest," Morris said.

Chase appears to epitomize the Rush study results, which found that green leafy vegetables such as kale and collard greens had the greatest benefit, followed by carrots, squash and other yellow vegetables, and then cruciferous vegetables like broccoli and Brussels sprouts.

The study found no significant benefits from two categories—potatoes and legumes, or beans.

"The results are encouraging," Morris said. "It seems that two or more vegetables per day was responsible for a significant decrease in the rate of decline of thinking ability."