Onions are a nutrient-dense food, meaning that while they are low in calories they are high in beneficial nutrients like vitamins, minerals and antioxidants. One cup of chopped onion contains approximately 64 calories, 15 grams of carbohydrate, 0 grams of fat, 0 grams of cholesterol, 3 grams of fiber, 7 grams of sugar, 2 grams of protein and 10% or more of the daily value for vitamin C, vitamin B-6 and manganese. Onions also contain small amounts of calcium, iron, folate, magnesium, phosphorus and potassium and the antioxidants quercetin and sulfur.