The article An Apple a Day May Really Keep the Doctor Away mention:
Using colon cancer cells treated with apple extract, Liu and colleagues found that 50 milligrams of apple extracted from the skins decreased the cancer cell growth by 43%, while the same amount of extract from the flesh of the apple decreased cancer cell growth by 29%. Likewise, 50 milligrams of extract from apples with the skin on decreased liver cancer cell growth by 57%, compared to 40% for samples extracted from apples without the skin.
Many of the antioxidants in an apple are in the skin, which might have cancer-fighting properties. Researchers think it's better to get antioxidants from fruits and vegetables than from dietary supplements. These naturally occurring plant chemicals, or phytochemicals, may work together to fight cancer and other disease.