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As the name implies, antioxidants are substances that are capable of counteracting the damaging, but normal, effects of the physiological process of oxidation in animal tissue. Antioxidants are nutrients (vitamins and minerals) as well as enzymes (proteins in your body that assist in chemical reactions). They are believed to play a role in preventing the development of such chronic diseases as cancer, heart disease, stroke, Alzheimer's disease, Rheumatoid arthritis, and cataracts.
Oxidative stress occurs when the production of harmful molecules called free radicals is beyond the protective capability of the antioxidant defenses. Free radicals are chemically active atoms or molecular fragments that have a charge due to an excess or deficient number of electrons. Examples of free radicals are the superoxide anion, hydroxyl radical, transition metals such as iron and copper, nitric acid, and ozone. Free radicals containing oxygen, known as reactive oxygen species (ROS), are the most biologically significant free radicals. ROS include the radicals superoxide and hydroxyl radical, plus derivatives of oxygen that do not contain unpaired electrons, such as hydrogen peroxide, singlet oxygen, and hypochlorous acid.
Because they have one or more unpaired electrons, free radicals are highly unstable. They scavenge your body to grab or donate electrons, thereby damaging cells, proteins, and DNA (genetic material). The same oxidative process also causes oils to become rancid, peeled apples to turn brown, and iron to rust.
It is impossible for us to avoid damage by free radicals. Free radicals arise from sources both inside (endogenous) and outside (exogenous) our bodies. Oxidants that develop from processes within our bodies form as a result of normal aerobic respiration, metabolism, and inflammation. Exogenous free radicals form from environmental factors such as pollution, sunlight, strenuous exercise, X-rays, smoking and alcohol. Our antioxidant systems are not perfect, so as we age, cell parts damaged by oxidation accumulate.
The Antioxidant Process
Antioxidants block the process of oxidation by neutralizing free radicals. In doing so, the antioxidants themselves become oxidized. That is why there is a constant need to replenish our antioxidant resources. How they work can be classified in one of two ways: Chain-breaking - When a free radical releases or steals an electron, a second radical is formed. This molecule then turns around and does the same thing to a third molecule, continuing to generate more unstable products. The process continues until termination occurs -- either the radical is stabilized by a chain-breaking antioxidant such as beta-carotene and vitamins C and E, or it simply decays into a harmless product. Preventive - Antioxidant enzymes like superoxide dismutase, catalase and glutathione peroxidase prevent oxidation by reducing the rate of chain initiation. That is, by scavenging initiating radicals, such antioxidants can thwart an oxidation chain from ever setting in motion. They can also prevent oxidation by stabilizing transition metal radicals such as copper and iron. The effectiveness of any given antioxidant in the body depends on which free radical is involved, how and where it is generated, and where the target of damage is. Thus, while in one particular system an antioxidant may protect against free radicals, in other systems it could have no effect at all. Or, in certain circumstances, an antioxidant may even act as a "pro-oxidant" that generates toxic oxygen species.
Types of Antioxidants
Antioxidants from our diet appear to be of great importance in controlling damage by free radicals. Each nutrient is unique in terms of its structure and antioxidant function.
The antioxidant enzymes superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) serve as your primary line of defense in destroying free radicals. SOD first reduces (adds an electron to) the radical superoxide (O2-) to form hydrogen peroxide (H2O2) and oxygen (O2).
2O2- + 2H --SOD--> H2O2 + O2
Catalase and GPx then work simultaneously with the protein glutathione to reduce hydrogen peroxide and ultimately produce water (H2O).
2H2O2 --CAT--> H2O + O2
H2O2 + 2glutathione --GPx--> oxidized glutathione + 2H2O
(The oxidized glutathione is then reduced by another antioxidant enzyme -- glutathione reductase.) Together, they repair oxidized DNA, degrade oxidized protein, and destroy oxidized lipids (fat-like substances that are a constituent of cell membranes). Various other enzymes act as a secondary antioxidant defense mechanism to protect you from further damage.
In addition to enzymes, vitamins, and minerals, there appear to be many other nutrients and compounds that have antioxidant properties. Among them is coenzyme Q10 (CoQ10, or ubiquinone), which is essential to energy production and can also protect the body from destructive free radicals. Also, uric acid, a product of DNA metabolism, has become increasingly recognized as an important antioxidant. Additionally, substances in plants called phytochemicals are being investigated for their antioxidant activity and health-promoting potential.
Do Antioxidants Improve Health?
Since antioxidants counteract the harmful effects of free radicals, you would think that we should consume as much as of them as possible. The truth is, although there is little doubt that antioxidants are a necessary component for good health, it is not clear if supplements should be taken and, if so, how much. Once thought to be harmless, we now know that consuming mega-doses of antioxidants can be harmful due to their potential toxicity and interactions with medications. Remember -- antioxidants themselves may act as pro-oxidants at high levels.
There are several possible explanations to account for the results.
Furthermore, literally hundreds, if not thousands, of observational studies (where investigators look for associations without giving participants supplements to take) have linked diets rich in antioxidant-rich fruits and vegetables to a lower risk for diseases like cancer, heart disease, stroke, cataracts, Parkinson's, Alzheimer's, and arthritis. So, despite the disappointing findings of trials, scientists remain certain of the many potential benefits of dietary antioxidants -- they simply haven't figured out exactly how the different antioxidant systems work together in our bodies to protect us from free-radical damage.