Granted you're no Galloping Gourmet, but this question has psychological as well as nutritional implications. (Catholic mothers are quite underrated when it comes to instilling childhood neuroses.) Is it really true that potato skins, apple peels, and carrot outsides are good for you? What value do they have? According to my mom, peeling apples, carrots, potatoes, and the like leads to vitamin and fiber deficiencies and, worse, spiritual laxity. As a result I suffer pangs of conscience every time I peel a carrot, spit out an apple peel, or leave behind a potato skin. But ever since I discovered all that stuff she told me about sex was wrong, I've been suspicious. Just what are the dire consequences of peeling one's fruits and vegetables? --Rita Hoffman, Washington, D.C.
A complicated story, my little muskrat, even if we leave your mental health out of it. Generally speaking it's a good idea to keep the skins on your fruits and vegetables, but not because they're a great source of vitamins and minerals--quite the contrary. In potatoes, for instance, the skin, which is a dark corky layer called the "periderm," consists mostly of dead cells filled with a waxy, largely vitaminless substance whose chief function is to protect the potato's insides. On the other hand, skin does keep vitamins from being boiled off during cooking, certainly a useful property. It's also a good source of dietary fiber, about which more in a moment.
If it's any comfort to mom, you could make the case that the region near the skin contains slightly more vitamins than the rest of the potato, carrot, or whatever. The water content of a fruit or vegetable generally is concentrated in the center, whereas the exterior is comparatively dry. If you've got less water, you've got more of everything else, micronutrients included. Ergo, per unit of volume, the outside's arguably got more vitamins than the inside. But the difference is small--there's a fair quantity of micronutrients distributed throughout the "flesh," as it's called. Since most Americans don't have any grave deficiencies in the vitamins and minerals department (at least the ones you can get a lot of in fruits and vegetables, such as vitamins A and C), what difference there is isn't worth worrying about.
What's more important is that skin contains much of the total content of dietary fiber in many fruits and vegetables. This is particularly true of apples. One medium apple with peel contains about 3.3 grams of fiber, while a peeled apple contains only 1.5 grams. This matters because the average American's diet is seriously deficient in fiber. Although there are no official daily fiber requirements, authorities say most people get only 10-20 grams per day, even though they could use 20-35.
There are several types of dietary fiber; the one abundant in apples, and to some extent in all fruits and vegetables, is called pectin. It's water soluble, and water-soluble fiber has been shown to reduce blood cholesterol levels. What happens apparently is that each particle of fiber grabs an armful of bile salts on its way through the intestine (a process called "binding") and carries it out of the body during excretion. The body then has to make new bile salts and evidently uses cholesterol as the raw material, thus keeping the blood cholesterol level down.
It has not been conclusively shown that an apple a day keeps the heart specialist away, which is to say they haven't proved that eating apples and other pectin-rich fruits and vegetables reduces heart disease. It's also true that if you eat too much water-soluble fiber it'll bind with (and subsequently carry away) micronutrients your body needs. But since most people could stand to double their dietary fiber intake, it's probably a good idea to leave your apples and other fruits and vegetables unpeeled . . . unless of course the whole idea just grosses you out of existence. Nutritionists would rather have you peel your veggies and at least get the benefit of the complex carbohydrates and starches than not peel and consequently not eat them at all.
Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.