Onions contain sulfur compounds that help your liver detoxify, particularly the fat soluble toxins. They are also high in antioxidants, vitamins, folic acid and dietary fiber. Onions are the richest dietary source of quercetin, a potent antioxidant. The quercetin is more concentrated on or near the skin area. Antioxidants are compounds that help slow the oxidative damage to cells of the body.