Leeks and onions are both healthful, low-fat foods that are commonly used together in soups, stews and other dishes. Both foods provide very little protein, so you should supplement these vegetables with lean protein sources to ensure a balanced diet. The method of cooking will affect the nutritional value of leeks or onions -- frying will add fat content to either vegetable.
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Nutrition Data: Onions
According to the USDA nutrition database, 100 g of raw onion contains 40 calories, 1.1 g of protein, 0.1 g of fat, 9.3 g of carbohydrate, 1.7 g of dietary fiber and 4.2 g of sugars. Trace minerals including calcium, iron, magnesium, phosphorous, potassium, sodium, zinc, manganese, copper, fluoride and selenium are also present in the typical raw onion. A 100-g serving of raw onion is equivalent to one small to medium onion.
Nutrition Data: Leeks
According to the USDA nutrition database, 100 g of raw leek bulb and lower-leaf portion contains 61 calories, 1.5 g of protein, 0.3 g of fat, 14.1 g of carbohydrate, 1.8 g of dietary fiber and 3.9 grams of sugar. A typical individual leek weighs roughly 89 grams, so a 100-gram serving is slightly more than one typical leek. Trace levels of minerals including calcium, iron, phosphorous, potassium, magnesium, sodium, zinc, copper, fluoride, manganese and selenium are also present in leeks.
Benefits of Onions
Eaten raw, onions are lower in calories and fat than leeks. If you are restricting your calorie intake with the goal of losing or maintaining weight, an onion will contribute fewer calories to your daily total than a leek. However, both leeks and onions are low-calorie and low-fat foods -- both are mostly fat free. One potential advantage of onions over leeks arises from the onion's flavor, which is typically stronger than that of a leek. You may eat less onion, by weight, in a meal because of its stronger flavor. Onions are lower in sodium than leeks, with 4 mg of sodium in every 100 g of onion but 20 mg of sodium in every 100 g of leeks.
Benefits of Leeks
Leeks are richer in certain vitamins and minerals than onions. In 100 g of leeks, there are 2.1 g of iron -- 10 times the amount in the same weight of onion. According to USDA data for raw onions and raw leeks, the calcium content of leeks ranks higher, with the exception of green onions. Iron is necessary to prevent anemia, and calcium supports bone health. Leeks also are significantly higher in vitamin C content than onions, and leeks contain vitamin A whereas onions do not.