There may be some truth to the old saying "An Apple Day Keeps the Doctor Away." Numerous studies have confirmed that apples and dietary phytochemicals in apples are associated with protective effects on a number of diseases including diabetes, cancer, heart disease, asthma, and Alzheimer's disease.

The reason may be the concentrated source of health-promoting compounds found in the peel (skin). Cornell researchers isolated over a dozen bioactive constituents from apple peels that fall into the category of triterpenoids. Some of these had never been described before. They also found that these compounds either inhibited or killed cancer cells. Previous research had shown the peel contains about 2-3 times more phytochemicals than the flesh. Apples are known to contain other phytochemicals (hydroxycinnamic acids derivatives, monomeric and oligomeric flavan-3-ols, flavonols, and dihydrocalcones, catechin, quercetin, anthocyanins) that show potent antioxidant activity. Cornell researchers applied for a patent to develop a powdered form of apple peel that involves drying and grinding apple peel from Idared, Rome Beauty, Cortland, and Golden Delicious varieties that concentrates and protects these bioactive chemicals.

He X, Liu RH. Triterpenoids isolated from apple peels have potent antiproliferative activity and may be partially responsible for apple's anticancer activity. J Agric Food Chem. 2007 May 30;55(11):4366-70. Epub 2007 May 8.