An antioxidant is a molecule capable of slowing or preventing the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons from a substance to an oxidizing agent. Oxidation reactions can produce free radicals, which start chain reactions that damage cells. Antioxidants terminate these chain reactions by removing free radical intermediates, and inhibit other oxidation reactions by being oxidized themselves. As a result, antioxidants are often reducing agents such as thiols or polyphenols.
The term "Antioxidant" refers to a group of organic substances, including vitamins C and E, vitamin A, selenium (a mineral), and a group know as carotenoids - beta-carotene being the most popular and well know carotenoid. Carotenoids are the pigment compounds that give many fruits and vegetables their vibrant colors.
Antioxidants are classified into two broad divisions, depending on whether they are soluble in water (hydrophilic) or in lipids (hydrophobic).
In general, water-soluble antioxidants react with oxidants in the cell cytosol and the blood plasma, while lipid-soluble antioxidants protect cell membranes from lipid peroxidation. These compounds may be synthesized in the body or obtained from the diet. The different antioxidants are present at a wide range of concentrations in body fluids and tissues, with some such as glutathione or ubiquinone mostly present within cells, while others such as uric acid are more evenly distributed. Some antioxidants are only found in a few organisms and these compounds can be important in pathogens and can be virulence factors.
Glutathione is a cysteine-containing peptide found in most forms of aerobic life. It is not required in the diet and is instead synthesized in cells from its constituent amino acids.
Melatonin is a powerful antioxidant that can easily cross cell membranes and the blood-brain barrier. Unlike other antioxidants, melatonin does not undergo redox cycling, which is the ability of a molecule to undergo repeated reduction and oxidation.
Vitamin E is the collective name for a set of eight related tocopherols and tocotrienols, which are fat-soluble vitamins with antioxidant properties. Of these, Plus/Minus-tocopherol has been most studied as it has the highest bioavailability, with the body preferentially absorbing and metabolizing this form.
The brain is uniquely vulnerable to oxidative injury, due to its high metabolic rate and elevated levels of polyunsaturated lipids, the target of lipid peroxidation. Consequently, antioxidants are commonly used as medications to treat various forms of brain injury. Here, superoxide dismutase mimetics, sodium thiopental and propofol are used to treat reperfusion injury and traumatic brain injury, while the experimental drug NXY-059 and ebselen are being applied in the treatment of stroke.
Antioxidants can cancel out the cell-damaging effects of free radicals. Furthermore, people who eat fruits and vegetables, which are good sources of antioxidants, have a lower risk of heart disease and some neurological diseases, and there is evidence that some types of vegetables, and fruits in general, probably protect against a number of cancers. These observations suggested that antioxidants might help prevent these conditions. There is some evidence that antioxidants might help prevent diseases such as macular degeneration, suppressed immunity due to poor nutrition, and neurodegeneration.
Relatively strong reducing acids can have anti-nutrient effects by binding to dietary minerals such as iron and zinc in the gastrointestinal tract and preventing them from being absorbed. Notable examples are oxalic acid, tannins and phytic acid, which are high in plant-based diets. Calcium and iron deficiencies are not uncommon in diets in developing countries where less meat is eaten and there is high consumption of phytic acid from beans and unleavened whole grain bread.
Many healthcare specialists claim that the elderly, especially those with a reduced food intake, heavy drinkers, frequent aspirin users, smokers and individuals with immune problems may benefit greatly by taking daily antioxidant supplements. It is also possible that higher level of antioxidants may aid in preventing certain life threatening diseases.
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