By Lauren Aaronson, published on December 10, 2004 - last reviewed on June 9, 2016
An apple a day keeps the doctor away, as the saying goes. And not just any doctor. An apple a day may help keep the neurologist away—along with the cognitive decline that often accompanies aging and neurodegenerative diseases like Alzheimer's and Parkinson's.
Food scientist Chang Y. Lee found that a nutrient in apples can protect rat brain cells from damage. The nutrient, an antioxidant called quercetin, provided even more protection than Vitamin C, which is known to combat neurodegenerative diseases in humans.
Quercetin belongs to a group of substances getting a lot of attention these days—flavonoids. These are naturally occurring chemical compounds that help give plants their color. Most flavonoids, as well as certain vitamins and minerals, are potent antioxidants; they neutralize harmful free radicals of oxygen, produced when cells burn oxygen for energy. If left unchecked, free radicals cause cumulative cell damage that may lead to cancer or, in the case of brain cells, Alzheimer's, Parkinson's or other age-related mental decline.
"Quercetin has much higher antioxidant activity compared to other flavonoids and Vitamin C," says Lee. His work has shown that not only may quercetin help brain cells, but also it may actually hurt cancer cells. In his lab experiments, quercetin blocked some of the pathways by which tumors grow out of control, and once again, in this task quercetin outperformed Vitamin C.
Apples are a primary source of quercetin. The same flavonoid is also found in onions, tea, blueberries and cranberries.
Other research suggests that quercetin may have a variety of benefits in combination with other nutrients. A Dutch study found that a high intake of black tea, which contains a variety of flavonoids in addition to quercetin, reduces the risk of heart disease, and a Finnish study found that people who ate the most whole apples had a lower risk of stroke than those who ate the least.
Substances that protect the heart and its circulation generally protect the brain as well. The common denominator is the network of blood vessels so crucial to both organs. Damage to blood vessels in the brain may impair the communication lines between nerve cells that underlie all mental activity.
Lee can't say for sure yet whether quercetin can halt neurodegeneration in people. Clinical trials or studies of large populations are needed to see whether his lab results hold up in the real world.
In the meantime, though, Lee heartily recommends apples for everyone. Given the potential benefits of quercetin, plus the other nutritious qualities of apples, he suggests that people add an apple to the mix of fruits and vegetables that they eat each day.
"One apple a day provides a significant amount of quercetin and flavonoids," he notes. The exact quantity of quercetin in an apple varies depending on the year, season and region the apple was grown in. But all apples—from red delicious to fujis—have a large amount.
Choose a fresh apple over applesauce or apple juice for a snack, Lee suggests, since quercetin resides primarily in the skin of the fruit. "Processed food may always have a chance to lose active compounds," he adds.
He also warns against supplements, because large doses of purified quercetin may prove dangerous. And it may turn out that quercetin works best just as nature intended it to be consumed—in concert with other substances in whole foods.
Although Lee has devoted years to studying apples, he admits that quercetin also shows up in blueberries, cranberries and onions. "But how much onion can you eat per day?" he asks.