Cells continuously produce free radicals and reactive oxygen species as part of metabolic processes. These free radicals are neutralized by antioxidant defense system consisting of enzymes such as catalase, superoxide dismutase, glutathione peroxidase, and numerous non-enzymatic antioxidants, including vitamins A, E and C, glutathione, ubiquinone, and flavonoids. Antioxidants include both water and lipid soluble varieties. The lipid soluble antioxidants are located in the cellular membranes and lipoproteins, whereas the water soluble antioxidants are present in the aqueous environments, such as fluids inside cells and in the blood.