Morris has been looking at the eating habits of thousands of elderly Chicago residents for more than a decade. "Every 3 years we go into their homes and ask them all sorts of questions about their health and lifestyle," she explains. "But also, we administer tests that measure their thinking ability. So that we can look at changes in their thinking ability over time."

Morris had people record the kinds of fruits and vegetables they ate and how often. She found that people who ate more servings of vegetables per day had memories that deteriorated more slowly than those who didn't eat vegetables. "People who consumed two to three vegetable servings per day had a 40 percent reduction in the rate of their decline in their thinking ability, compared to people who consumed around one or no servings of vegetables a day." Eating fruits didn't do as much to preserve thinking ability as eating vegetables.

Morris found that some kinds of vegetables are better than others at preventing memory loss. She asked study participants about green leafy vegetables, yellow vegetables, cruciferous vegetables like broccoli andlegumes, or beans. "The more green leafy vegetables they consumed, the slower their rate of decline in thinking ability," she reports. "We also found evidence of association with the other types of vegetables, except for legumes. But the relation was not as strong as for green leafy vegetables." Morris believes the benefit was derived from those vegetables with especially high levels of vitamin E.