The skin of an apple contains about half of the apple's overall dietary fiber content.
A medium apple also delivers 9 milligrams of vitamin C, 100 IUs of vitamin A, and 200 grams of potassium. By removing the peel, you lose about a third of those nutrients.
The peel also has four times more vitamin K than its flesh; about 5 percent of your daily value.
Vitamin K—also prevalent in meat and in spinach and other green veggies—helps you form blood clots that patch you up when you have a bad scrape and helps activate the proteins your body needs for cell growth and healthy bone maintenance.
An apple's skin boasts potential benefits beyond its vitamin content.
An antioxidant called quercetin, found mostly in the apple's skin, can help lung function, ease breathing problems and protect your lungs from irritants. Quercetin is also believed to fight off brain tissue damage and protect your memory.
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One study identified another compound that's found primarily in the peel, called triterpenoids, which appears to inhibit or kill certain types of cancer cells throughout the body.