Onions are fat-free and cholesterol-free, low in sodium, high in vitamin C and a good source of fiber. Organosulfur and other compounds in onions help increase circulation, lower blood pressure and prevent blood clotting. They also detoxify the body and skin and fight the free radicals that can cause cell destruction and disease. Onions also contain generous amounts of a flavonoid called quercetin, a potent antioxidant with anti-allergy, antiviral and antihistamine properties. Studies show it may also protect against cataracts, cardiovascular disease and cancer.