That old saw about an apple a day keeping the doctor away may need an addendum: In tests on rats, researchers at Cornell University have found that an antioxidant, quercetin, naturally occurring in apples and other foods, could also help protect against brain-cell damage.

The study, published in the Journal of Agricultural and food Chemistry, used extracts of phytonutrients from red apples. Phytonutrients help apples fight off bacteria, viruses and fungi. They also give the fruit its antioxidant properties. The researchers singles out quercetin, a phytonutrient found in high levels in apples, as the chemicals largly responsible for the fruit’s antioxidant benefits. Antioxidants are though to counteract the damaging effects of free radicals, which have been associated with alzheimer’s diseases and other ailments

The Comell scientists compared phytonutrients from apples to vitamin C, another antioxidant. The rats brain cells trested with the apple extracts showed significantly less damage from exposure to hydrogen peroxide than those with no antioxidant or those treated with vitamin C. The higher the concentration of phytonutrients extracted from apples, the greater the protection was for the verve cells against oxidative stress.

“On the basis of serving size, fresh apples have some of the highest levels of quercetin when compared to other fruits and vegetables,” says C.Y. Lee, PhD, a food science professor who led the study. Apple skins contain the highest quercetin levels-meaning apple juice is not the best source of the antioxidant. Red apples typically have higher antioxidant levels than yellow or green apples. Other foods high in quercetin include blueberries and onions.

Lee stresses that his studies were conducted in the laboratory, not in clinical trials with humans. But he doesn’t hesitate to recommend more apples in the diet, as well as other fresh fruits and vegetables. He says,”Indeed, I have a reason to say an apple a day keeps the doctor away.”