An Apple a Day Keeps The Doctor Away

The proverb "An apple a day keeps the doctor away," addressing the health effects of the fruit, dates from 19th century Wales. Research suggests that apples may reduce the risk of colon cancer, prostate cancer and lung cancer. Compared to many other fruits and vegetables, apples contain relatively low amounts of Vitamin C as well as several other antioxidant compounds. The fiber content, while less than in most other fruits, helps regulate bowel movements and may thus reduce the risk of colon cancer. They may also help with heart disease, weight loss, and controlling cholesterol, as they do not have any cholesterol, have fiber, which reduces cholesterol by preventing reabsorption, and are bulky for their caloric content like most fruits and vegetables. 
There is evidence that in vitro apples possess phenolic compounds which may be cancer-protective and demonstrate antioxidant activity. The predominant phenolic phytochemicals in apples are quercetin, epicatechin, and procyanidin B2.
Apple juice concentrate has been found to increase the production of the neurotransmitter acetylcholine in mice, providing a potential mechanism for the "prevention of the decline in cognitive performance that accompanies dietary and genetic deficiencies and aging." Others studies have shown an "alleviation of oxidative damage and cognitive decline" in mice after the administration of apple juice.
The seeds are mildly poisonous, containing a small amount of amygdalin, a cyanogenic glycoside; usually not enough to be dangerous to humans, but it can deter birds.