All kinds are considered a good source of this nutrient. Vitamin C helps your body heal and form blood vessels and other parts of the body. It’s also an antioxidant that fights free radicals. Those are molecules your body makes that can sometimes trigger damage. Experts recommend 90 milligrams of vitamin C a day. That’s the Daily Value, or the amount of a nutrient you should try to get every day. One average-sized onion has 9% to 18% of your Daily Value.