Peel back the papery skins of an onion and your eyes begin to mist. Smile through the tears because onions have no fat, sodium or cholesterol and provide you with an array of essential vitamins, minerals and other nutrients. Enjoy chopped, raw onions on top of a salad or saute them with a bit of olive oil for a light side dish.
The National Onion Association defines a serving size for onions as 1 cup or 160 grams. A serving has 64 calories, 15 grams of carbohydrates, 7 grams of naturally occurring sugar and around 5 grams of protein. One cup of onions also contains 20 percent of your daily requirement for vitamin C and 10 percent of your required vitamin B-6.
Nutrition information for a serving depends on the variety of onion and how you measure. One hundred and sixty grams of sweet onion has 51 calories, 12 grams of carbohydrate, 1.2 grams of protein and 8 grams of sugar. The same weight of green onions -- such as you might use for a garnish -- has a mere 43 calories, 1.5 grams of protein and 9 grams of total carbohydrates. If you simply scooped up a cup of green onions, you'd see an even bigger difference. A cup of chopped green onions has only 19 calories, according to the USDA National Nutrient Database. For accuracy, use weight to measure serving size.