Exogenous Antioxidants are Double-edged Swords
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The balance between oxidation and antioxidation is believed to be critical in maintaining healthy biological systems. Under physiological conditions, the human antioxidative defense system, including superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), and glutathione (GSH), allows the elimination of excess reactive oxygen species (ROS), including superoxide anions (O-2), hydroxyl radicals (OH), and alkoxyl radicals (RO). However, our endogenous antioxidant defense systems are incomplete without exogenously originating reducing compounds, such as vitamin C, vitamin E, carotenoids, and polyphenols, which play an essential role in many antioxidant mechanisms in living organisms. Therefore, there is continuous demand for exogenous antioxidants to prevent oxidative stress. However, higher doses of isolated compounds may be toxic, owing to pro-oxidative effects at high concentrations or their potential to react with beneficial concentrations of ROS normally present at physiological conditions that are required for optimal cellular functioning. In this review, synthetic antioxidants among supplementary antioxidants, fruit rich nutrition and various supplementary products to strengthen our body will be considered and discussed under the light of experimental and epidemiological evidence. Antioxidants in physiological dose ranges are considered to be safe in healthy people and overdoses cause damage via pro-oxidative effects; therefore, the importance of doses of antioxidants, the lower preventive doses that protect healthy individuals from illnesses, and higher therapeutic doses that treat cancer patients will be emphasized in this review.