Free radicals are generated by exposure to harmful environmental factors, such as: smog, ozone, chemicals, drugs, smoking and radiation, as well as during normal physiological processes, especially in defense against microbes and other foreign substances. Free radicals can damage DNA, proteins and lipids within cells, altering or inhibiting cellular function. Dietary antioxidants, such as vitamins E and C, carotenoids and others, function to help control or eliminate free radicals by donating electrons, thus, maintaining cellular health by offsetting the damage they can potentially cause. Antioxidants that are available through the diet are considered exogenous antioxidants and include: amino acids [N-acetyl-L-cysteine (NAC), taurine and L-glutathione (tripeptide)], vitamins C and E, carotenoids (beta-carotene, lycopene, lutein and zeaxanthin), and minerals (selenium and zinc). Antioxidants produced in the body are considered endogenous antioxidants and include: ubiquinone/ubiquinol (CoQ10), alpha lipoic acid and the master cellular defense enzyme, superoxide dismutase (SOD) and many more. However, many factors have been found to compromise the levels of endogenous antioxidants in the body making supplementation essential, such as the normal aging process, certain medications and oxidative stress.