I hate seeing someone peeling apple before eating. Their reason is simple. The peel never tastes good. However, I saw Americans on tv eating apples with peels intact. I supposed the peel is perfectly edible and should be eaten. Not thrown away.
It is true, the peel does not taste good. However, I developed the habit of eating it intact, as influenced by the biggest media (at that time).
During college days, I learned that fruit nutrients are more concentrated towards the peel. Why get off the peel when you can get more out of it. When the peel itself is edible, consume it. If necessary, peel as thinly as possible to avoid nutrient waste. The case of mango and papaya.
Apple cyanide… According to various literature, apple seeds should not be eaten. I contains poisonous cyanide. However, every seeds has protective coatings. Accidentally ingesting some seeds won’t do any harm. Stomach acid strength is not enough to digest it. Even if chewed intentionally, the low amount less likely do harm. Eating cassava has more chances of doing harm.
Apple seed against cancer… Not just apples. Various popular fruit seeds are claimed to have cancer curing effects. The same set of chemical responsible for cyanide formation is set to fight cancer. Of course, all of this are nothing but hearsay until proven and offered as medicine by big pharmaceutical companies.