Onions are a good source of vitamin C, although they lose some of their vitamin C content when they are factory-processed by freezing or canning and also when they are cooked. You'll probably get the most vitamin C from onions if you enjoy them in their least-processed states, such as chopped and raw in a taco salad, or fresh and lightly steamed with other veggies.
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Vitamin C Amounts in Onions
Onions are a good dietary source of vitamin C according to the USDA National Nutrient Database for Standard Reference, 1/2 cup of raw, chopped onions provides 5.9 mg, which is 10 percent of the recommended daily allowance for vitamin C. However, onions lose some vitamin C content when they are canned, frozen, or, especially, cooked. Per USDA figures, 1/2 cup of canned onions contains 4.8 mg vitamin C, whereas about 1/2 cup, or 100 g, of frozen, chopped onions contains 3.3 mg of vitamin C. High-heat cooking, such as sautéing, seems to produce the greatest vitamin C losses, 1/2 cup of sautéed onions has only 0.8 mg vitamin C.
Vitamin C in Onion Food Products
Some food products containing onions, such as onion soup or onion rings, also provide some vitamin C. According to the USDA, a 1 cup serving of canned, prepared onion soup offers 1.2 mg of vitamin C, whereas 10 medium-sized onion rings contain 0.8 mg of vitamin C. However, as the soup is high in sodium and the onion rings contain a lot of saturated fat, eating these foods probably isn't the healthiest way to get more vitamin C in your diet. Even worse are onion-flavored potato chips which contain, per gram, more salt than canned onion soup and more fat than onion rings. Onion potato chips also have 1.2 mg of vitamin C, but again, salty, fatty foods aren't the most nutritious ways to eat onions.
Other Nutrients in Onions
Besides vitamin C, onions are also good sources of other important nutrients including dietary fiber, calcium, magnesium, potassium and phosphorus. Additionaly, according to Lexington, Kentucky news website, Kentucky.com, onions contain more the micronutrient quercitin than any other common fruit or vegetable. According to Dole Nutrition Institute nutrition researcher, Nicholas D. Gillitt, quercitin is a potent antioxidant that has been linked to a reduced risk of developing heart disease and certain types of cancers. Furthermore, onions provide phytochemicals such as disulfides and trisulfides which are linked to anti-cancer and anti-microbial activity, as well as inulin, a probiotic which promotes growth of "good" bacteria in the colon and may also aid in the absorption of essential minerals, says Gillitt.
Other Foods With Vitamin C
If onions really aren't your thing, there are still plenty of other healthy foods that provide vitamin C. The best dietary sources of vitamin C include: citrus fruits and their juices, red and green peppers, and kiwifruit. Strawberries, broccoli, cantaloupe, baked potatoes and tomatoes also provide some vitamin C. Although fruits and vegetables tend to lose a lot of their vitamin C content when they are cooked, most fruits and vegetables that provide vitamin C can be eaten raw, with the exception of onions, which typically aren't eaten raw in quantities large enough to provide much vitamin C. However, steaming or microwaving may lessen cooking losses of vitamin C, according to National Institutes of Health.