"Compared to people who consumed less than one serving of vegetables a day, people who ate at least 2.8 servings of vegetables a day saw their rate of cognitive change slow by roughly 40 percent," said study author Martha Clare Morris, ScD, with Rush University Medical Center in Chicago.
"This decrease is equivalent to about five years of younger age," she said.
Cognitive performance declines naturally with age, but the results of Chicago Health and Aging Project (CHAP) suggests that consuming a diet rich in vegetables, particularly the green leafy type, could slow this decline by about 40 per cent.
The prospective cohort study, funded by the National Institute of Aging, used data from 3,718 participants (62 per cent female, 60 per cent African American, average age 74). Dietary intakes were assessed using a 139-item food frequency questionnaire (FFQ), and mental function was assessed at by four different tests: the East Boston Tests of immediate memory and delayed recall, the Mini-Mental State Examination, and the Symbol Digit Modalities Test, taken at the start of the study and then again after three and six years.
After adjusting the results for potential confounders such as age, sex, race, educations, and cardiovascular risk factors, Dr. Morris and her colleagues found that consuming an average of 2.8 vegetable servings ever day was associated with a 40 per cent decrease in cognitive decline, compared to those who ate an average of 0.9 servings every day.
"The decrease in rate for persons who consumed greater than two vegetable servings per day was equivalent to about five years of younger age," wrote Morris in the the October 24 issue of the journal Neurology (Vol. 67, pp. 1370-1376).
"Of the different types of vegetables, green leafy vegetables had the strongest association," she said.
Surprisingly, no relationship was found between fruit consumption and cognitive decline.
"This was unanticipated and raises several questions," said Morris. "It may be due to vegetables containing high amounts of vitamin E, which helps lower the risk of cognitive decline.
"Vegetables, but not fruits, are also typically consumed with added fats such as salad dressings, and fats increase the absorption of vitamin E. Still, further study is required to understand why fruit is not associated with cognitive change," she said.
Indeed, the inverse association between vegetable consumption and cognitive decline was found to be insignificant when the researchers controlled for vitamin E intake in food or different types of fat.
The study does have several limitations, most notably that some dietary assessments were not performed at baseline, suggesting that some changes to diet may have occurred between starting the study and completing the questionnaire. The accuracy of FFQs is also dependent on the self-reporting of the individuals, and is susceptible to some error.
The Rush University researchers called for further research to understand the non-association with fruit, as well as exploring specific dietary components of citrus fruit on cognitive decline.
The "Five-a-day" message is well known, but applying this does not seem to be filtering down into everyday life. Recent studies have shown that the average American consumes about three portions a day.
A report from the European Union showed that global fruit and vegetable production was over 1 230 million tonnes in 2001-2002, worth over $50 billion (€ 41 000 million). Asia produced 61 per cent, while Europe and North/Central America both producing nine per cent.