The study, carried out on rat brain cells, adds strength to the theory that the risk of developing Alzheimer's and similar diseases may be reduced by dietary intervention, particularly by increasing one's intake of antioxidant-rich foods.
It showed that brain cells treated with the apple antioxidant quercetin had significantly less damage than those treated with vitamin C or not exposed to antioxidants. Like other antioxidants, quercetin has been associated with an increasing number of potential health benefits, including protection against cancer.
"On the basis of serving size, fresh apples have some of the highest levels of quercetin when compared to other fruits and vegetables and may be among the best food choices for fighting Alzheimer's," said study leader C.Y. Lee, professor and chairman of the department of Food Science & Technology at Cornell University.
He cautioned that protection against Alzheimer's using any food product is currently theoretical and added that genetics and environment are also believed to play a role in the disease.
But "eating at least one fresh apple a day might help", said Lee.
Results so far are limited to cell studies and more advanced research, particularly in animals, is still needed to confirm the findings. But Lee and his associates have also shown that apples may help protect against cancer.
For the current study, scheduled to appear in the 1 December issue of the Journal of Agricultural and Food Chemistry, the researchers exposed groups of isolated rat brain cells to varying concentrations of either quercetin or vitamin C.
The cells were then exposed to hydrogen peroxide to simulate the type of oxidative cell damage that is believed to occur with Alzheimer's. These results were then compared to brain cells that were similarly exposed to hydrogen peroxide but were not pre-treated with antioxidants.
Brain cells that were treated with quercetin had significantly less damage to both cellular proteins and DNA than the cells treated with vitamin C and the cells that were not exposed to antioxidants. This demonstrates quercetin's stronger protective effect against neurotoxicity, according to the researchers.
Scientists are not sure of quercetin's mechanism of action, but some suspect it might work by blocking the action of free radicals, an excess of which are thought to damage brain cells as well as other cell types over time. Further studies are however needed.
Even though quercetin is relatively stable during cooking, fresh apples are better sources of quercetin than cooked or processed apple products because the compound is mainly concentrated in the skin of apples rather than the flesh, Lee noted.
And in general, red apples tend to have more of the antioxidant than green or yellow ones, although any apple variety is a good source of quercetin.
Other foods containing high levels of quercetin include onions, which have some of the highest levels of quercetin among vegetables, as well as berries, particularly blueberries and cranberries.
There is currently no cure for Alzheimer's disease and no one is sure of its exact causes but some researchers are increasingly optimistic that dietary intervention using antioxidant-rich foods might help reduce the risk of developing the disease.
Alzheimer's affects an estimated 4.5 million people in the United States, according to the National Institute on Aging. That figure is expected to rise dramatically as the population ages.