It appears that the cancer protective effect largely stems from their flavonoid content -- effective antioxidants produced by plant metabolism -- and probably not from their vitamin C and beta carotene content.
It is a well-established fact that consumption of fruits and vegetables is associated with a lowered risk of lung cancer and some other malignancies, including skin cancer and colon cancer.
Moreover, not just any flavonoid made the biggest difference. The flavonoid quercetin -- largely from apples -- provided 95% of the total flavonoid intake in the population studied.
After adjusting for other fruits and vegetables in the diet, consumption of apples was inversely associated with lung cancer risk -- that is, the higher the apple intake, the lower the lung cancer risk.
The researchers found that the likelihood of lung cancer among those who ate the most apples fell by 58% relative to those who ate fewer apples. Much like rust attacking metal, oxidative stress is the molecular assault on body tissue from substances called free-radicals which arise during cellular processes that involve oxygen.
American Journal of Epidemiology (1997;146:223-230)