Dietary oxidative stress and plant extracts antioxidant. Halliwell and Gutteridge (1995) attempted to define an antioxidant as “any substance that when present at low concentrations compared with those of an oxidizable substrate (every organic molecule found in vivo) signifi- cantly delays or prevents oxidation of that substrate.” However, this definition seems inadequate in certain cases. Plasma albumin is considered an important antioxidant molecule because it binds copper and protects extracellular targets such as low-density lipoproteins (LDLs) against oxidative damage. However, albumin is in considerable molar excess compared with LDLs (Halliwell 1995). Therefore, an antioxidant might be defined more simply as “any substance that delays, prevents or removes oxidative damage to a target molecule” (Halliwell and Gutteridge 2007). Antioxidants can be divided into categories according to specific characteristics. They include enzymes such as superoxide dismutase (SOD), catalase, glutathione reduc- tase, and glutathione peroxidase (GPX) and non-enzymatic metabolites such as glutathione, uric acid, vitamins, and polyphenols. Regarding their origin, various antioxidants such as glutathione, uric acid, catalase and SOD can be synthesized in vivo, whereas others, namely, polyphenols and β-carotene, are obtained from food. Based on their physical properties, antioxidants can be divided into water- soluble antioxidants such as uric acid, glutathione, and polyphenols or lipid-soluble antioxidants such as vitamins A and E and lipoic acid. The SOD enzyme catalyzes the dismutation of Ο2⋅− to Η2Ο2. It exists in mitochondrial form (MnSOD) and in cytoplasmic form (Cu/ZnSOD) that is primarily found in muscle cells (Das et al. 1997). Catalase is present in almost every kind of cell, but its concentration is higher in the erythrocytes and liver (Masters et al. 1986). Its subcellular localization is in peroxisomes, in mitochondria, and in the nucleus. It catalyzes the conversion of Η2Ο2, which is produced by SOD to Η2Ο and Ο2. The antioxidant activity of catalase is of great significance as it prevents the conversion of Η2Ο2 to the very harmful ΟΗ⋅. Furthermore, it has been demonstrated that catalase and SOD activities exhibit a linear correlation with life span in mammals (Cutler 1984). GPX, which requires selenium as a cofactor, is present in the cytoplasm and mitochondria and is an alternative route of Η2Ο2 degradation. Specifically, Η2Ο2 is converted to Η2Ο and Ο2 and oxidizes GSH (reduced form of glutathione) to GSSG (oxidized form of glutathione). Glutathione is considered one of the most important antioxidant metabolites and is the first line of defense against reactive species. At rest, glutathione is usually present in the reduced state. GSH is a tripeptide consisting of glutamic acid, cysteine, and glycine. It is the most abundant low-molecular-weight thiol-containing compound in biological fluids and tissues of mammals. In eukaryotic 13 cells, 90% of the intracellular GSH pool resides in the cytoplasm, and the remaining 10% is found in the mitochondria, endoplasmic reticulum, and the nucleus. However, the biosynthesis of GSH appears to occur exclusively in the cytoplasm. Intracellular GSH concentra- tion is estimated to range from 0.5 to 10 mM (Barycki 2007). GSH possesses potent antioxidant properties, main- taining the intracellular redox homeostasis due to the thiol group of cysteine which serves as a substrate of GPX (Halliwell and Gutteridge 2007). Glutathione is important for the regeneration of antioxidant vitamins E and C (May et al. 1996) and contributes to xenobiotic detoxification (Halliwell and Gutteridge 2007). In physiological condi- tions, GSH is in a dynamic equilibrium with GSSG. However, in the context of oxidative stress, GSH works with GPX to efficiently remove intracellular H2O2. This process protects biomolecules from oxidative modifica- tions, and GSH is converted to GSSG. Glutathione reductase reduces GSSG to GSH using NADPH as an electron donor, thus replenishing the GSH pool. The ratio of GSH/GSSG is often used as a biomarker of oxidative stress. It has been suggested that the GSH/GSSG ratio in the endoplasmic reticulum, in which the more oxidized environment aids protein folding, is as low as 1:1, in contrast to the mitochondria and the cytosol where the GSH/GSSG ratio is >10 (Barycki 2007). This is not always precise, however. As mentioned earlier, glutathione partic- ipates in several different biochemical processes, namely, in the regulation of protein and DNA synthesis and as an essential cofactor of many enzymes (Halliwell and Gutteridge 2007; Afzal et al. 2002). This fact can affect the ratio of GSH/GSSG and cause its conversion from the reduced to the oxidized form. Therefore, the use of the GSH/GSSG ratio as a biomarker to monitor oxidative stress lacks accuracy and probably needs to be reconsidered. Polyphenolic compounds constitute a crucial category of antioxidant metabolites. They are plant secondary metabo- lites which have at least one aromatic ring in their molecule and usually exist in the form of glycosides. They protect plants against harmful environmental conditions and the attack of microorganisms and contribute to the development of several characteristics such as color (Manach et al. 2004). More than 8,000 different polyphenolic compounds have been described. They are subdivided into non- flavonoids (e.g., hydrobenzoic acids, hydroxycinnamic acids, and stilbenes) and flavonoids (e.g., flavonols, flavanals, isoflavones, and anthocyanins). Flavonoids are composed of more than 4,000 different species that have two aromatic benzene rings linked through three carbons forming an oxygenated heterocycle. Plant polyphenols possess antioxidant properties as they inactivate free radicals by offering a hydrogen atom and an electron. They are converted into relatively harmless free radicals, which