When I cook I don't often peel my veggies, but I've been around people who do and always wondered if its a nutritional faux pas.
It seems that in general peeling lowers nutrient content but not by a ton:
"According to the United States Department of Agriculture, a large red apple with its skin intact contains about 5 grams of fiber, 13 milligrams of calcium, 239 milligrams of potassium, and 10 milligrams of vitamin C. But remove the skin, and it still contains about 3 grams of fiber, 11 milligrams of calcium, 194 milligrams of potassium, and plenty of its vitamin C and other nutrients."*
What the USDA is not accounting for the in example above are the micronutrients present in the skin:
"Notice that many fruits and veggies have skins that are more colorful than the flesh - potatoes, apples, pears, peaches, etc. The colors are usually a sign that the skin contains nutritious phytochemicals likecarotenoids and flavonoids. Apple skins are red due to the presence of anthocyanins which are not present in the flesh. Anthocyanins are powerful antioxidants. So if you don't eat the apple skin, you don't get the antioxidants. Grape skins contain most of the resveratrol, etc."*
Potatoes are another example where there is a ton of nutrient loss by removing the skin.
So what is the bottom line?
Particularly if you are buying organic/local farmed raised produce and don't mind the flavor of the skin keep it on. Save yourself an added step in the cooking process.
If you are using chemically sprayed produce its best to remove the top layer as it will reduce your toxicity exposure, but only by a small fraction. Be aware that apples, celery, cucumbers, nectarines, peaches, potatoes, and zucchini are some of the heaviest sprayed crops.
So what do you say? To peel or not to peel?