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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.

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. And the ursolic acid in apple skin has been shown by studies to stimulate muscle growth, increase skeletal muscle and decrease risk of obesity.

A potato's skin packs more nutrients—iron, calcium, potassium, magnesium, vitamin B6 and vitamin C—ounce-for-ounce than the rest of the potato. For example, 100 grams of potato peel packs seven times more calcium and 17 times more iron than the same amount of potato flesh. Ditch the skin and you'll also lose up to 90 percent of a potato's iron content and half of its fiber. And don't forget the skin of a sweet potato is loaded with a significant amount of beta-carotene, which converts to vitamin A during digestion. Vitamin A is essential for cell health and immune system regulation, and it is extremely useful in maintaining organ function. Citrus (Oranges, Lemons, Grapefruits, Limes) The peel of an orange packs in twice as much vitamin C as what's inside. It also contains higher concentrations of riboflavin, vitamin B6, calcium, magnesium and potassium. The peel's flavonoids have anti-cancer and anti-inflammatory properties. (Citrus fruit also boosts iron absorption.) As nutritious as citrus peels are, you're unlikely to start eating oranges whole. The entire peel is bitter and difficult to digest. Instead, grate the peel using a microplane or another tool and sprinkle it on top of salads, or in a vinaigrette dressing. Citrus shavings make a good pairing with ice cream and chocolate as well.

The dark green skin contains the majority of a cucumber's antioxidants, insoluble fiber and potassium. The cucumber peel also holds most of its vitamin K. The next time you have a Greek salad, ask the chef not to peel your cukes. Kiwi You've probably been spooning out the green flesh inside for years, but a kiwi's fuzzy exterior is also edible. In fact, the skin contains more flavonoids, antioxidants and vitamin C than the insides—and double the fiber. So ditch the spoon, wash the kiwi and eat it like a peach. If you find the fuzz unappetizing, scrape it off first.

An eggplant's purple hue comes from a powerful antioxidant called nasunin, which helps protect against cancerous development, especially in the brain and other parts of the nervous system. Nasunin is also believed to have anti-aging properties. Eggplant skin is also rich in chlorogenic acid, a phytochemical that boasts antioxidant and anti-inflammatory properties, and also promotes glucose tolerance. Although the eggplant interior contains chlorogenic acid, it's much more prevalent in the skin.

Researchers found that mango skin contains properties similar to resveratrol, which helps burn fat and inhibits the production of mature fat cells. Mango flesh extracts were also tested, but did not produce the same results, which suggests that one needs to eat mango skin in order to get this beneficial property. A mango's peel also contains larger quantities of carotenoids, polyphenols, omega-3, omega-6 and polyunsaturated fatty acids than its flesh. Another study found compounds more heavily concentrated in mango's skin that fight off cancer, diabetes and heart disease. Mango skin also has quercetin.The skin of a mango can be eaten raw, or cooked along with the insides. Another way to eat both flesh and skin is to pickle the entire mango.

Since the skin of a carrot is the same color as what's directly beneath it (like a tomato or a red pepper), the peel and its flesh have similar nutritional properties. However, the highest concentration of phytonutrients is found in a carrot's skin or immediately underneath. Just rinse the carrot thoroughly rather than peeling it. 

All watermelon contains citrulline, which has antioxidant properties and converts to arginine, an essential amino acid that is beneficial to the heart, immune system and circulatory system. But most of that citrulline is found in the rind. Eating a rind might sound unappetizing, but it can be pickled (like a cucumber), or simply sautéed and seasoned. Or throw it in a blender with the watermelon flesh, and add some lime.

Like apple skin and mango skin, the outside of an onion's skin contains quercetin. Although that skin is not directly edible, you can draw out some of those nutrients by adding it to stock.

Pineapple contains bromelain, an enzyme that can help reduce inflammation, especially in the nose and sinuses. One study found that a pineapple's core and peel yielded the highest amount of bromelain in the fruit, at 40 percent by weight. The skin and core of a pineapple straight-up would be tough on your digestive system, so try putting them through a juicer or sauté them for a few minutes in a pan.

A banana's peel contains way more fiber than its flesh, and is likewise richer in potassium. The peel also contains lutein, a powerful antioxidant that plays a role in maintaining healthy eye function. An amino acid called tryptophan is more highly concentrated in the peel than the insides. Among other things, tryptophan is believed to ease depression by increasing the body's levels of serotonin, a neurotransmitter in the brain that affects mood. Although the peel has a bitter taste and tough, ropey consistency most people aren't used to, an overripe banana (brown or black) becomes thinner, sweeter and easier to chew. You can also put the peel (ripe or overripe) through a juicer with the rest of the banana. Or you can boil the peel for several minutes to make it softer, or throw it in the frying pan. If you want to get really creative, bake a banana peel in the oven for 20 minutes or so, or until it becomes dried out, then use it to make tea.