CHICAGO -- Eating two or more servings of vegetables a day could slow a person's mental decline by about 40 percent compared with someone who consumes few vegetables, according to a six-year study of nearly 4,000 Chicago residents age 65 and older.
Eating lots of fruit did not appear to offer the same mental protection, although fruit has been associated with a variety of other health benefits, said Martha Clare Morris, chief of Rush University Medical Center's Rush Center 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 vegetable per day, Morris reports in today's issue of Neurology, the scientific journal of the American Academy of Neurology.
The study suggested that it may never be too late to reap the benefits of vegetable consumption. Older people who started eating more than two vegetables a day still showed a significant slowdown in mental decline, Morris said. One serving of a vegetable is generally equal to a cup.
The new findings come on top of two earlier Rush studies indicating that the foods people eat can 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.
Vegetables, especially those in the green leafy category, are brimming with antioxidant compounds like vitamin E, flavonoids and carotenoids, Morris said, and vegetables contain more vitamin E than fruit does.
Eating vegetables with olive oil, vegetable oil or some other type of poly- or mono-unsaturated fats enhances the body's absorption of antioxidants, which help snuff out cell-damaging free radicals, she said.
"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 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."
In trying to figure out which food groups bestow important health benefits, epidemiologists match people as closely as possible so other factors in their lifestyles cancel out.
"When we controlled for all of those healthy lifestyle variables - physical exercise, age, sex, race, education, cognitive activity, participation - the effects of vegetables on cognition actually became stronger," Morris said.
Matt Kaeberlein, who conducts research on the biochemical processes of aging at the University of Washington, said he was surprised that the study did not show a similar slowdown in cognitive decline from eating fruit.
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.
While Morris agreed that animal research indicates that berries may help preserve memory, she said that too few people involved in her study ate berries to determine whether fruit 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 mental health, Kaeberlein said. Learning which specific nutrients provide the greatest protection could lead to developing a pill that people could take that would have the same benefits, he said.
"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."
Ronald Kotulak writes for the Chicago Tribune.