Antioxidant is the descriptive term given to a group of organic substances that generally function within the body to promote health through their counteraction against the potentially destructive effect of oxygen in human tissues, as carried out by agents known as free radicals. The best known anti-oxidants are vitamin C (ascorbic acid), vitamin E, selenium (a mineral), and the carotenoids, the class of the pigmentation substances that color many fruits and vegetables. An example of a carotenoid is betacarotene, the substance that produces the color in carrots.

Human diet is subdivided into two general classifications for the purpose of basic nutritional assessment. The first classification is macronutrients, the term that refers to all of the food products that contain carbohydrate, protein, and/or fat. The second classification is micronutrients, the term that defines all of the vitamins and most of the minerals that are absorbed into the body through food digestion. Phytochemicals are the trace substances found in plants and plant food products that are absorbed into the body as a part of either macronutrients or micronutrients; many phytochemicals also possess antioxidant properties.

An understanding of the function of antioxidants begins with the process of oxidation within the body. Oxidation occurs in a number of biological processes that are beneficial to the body. The essential role of oxygen within the aerobic energy system, and the oxidation of various harmful bacteria that invade the body are two examples of oxygen and its positive relation to human cell function.

Oxidation has a negative impact on the health of cells in certain conditions. While the oxygen molecule, described by the chemical equation O2, is often present in the body, it is not always as a stable, electrically neutral substance, but as the chemical structure known as free radicals, in which either atoms or molecules have one or more unpaired electron. When the cells have used oxygen, free radicals are often created as a byproduct of that process.

Atoms or molecules in which all electrons within the structure are paired tend to be stable; free radicals are unstable and are therefore highly reactive with other molecules. The free radicals seek out atoms or molecules from which they may "steal" an electron to create electrochemical balance within the radical; this results in instability in the unbalanced molecule, which will in turn seek to address its newly unstable state. This process will lead to a chain reaction of electron theft, which produces damage to both the membrane of cells and the contents of cells. Heart disease, increased incidence of stroke, diabetes, cancer, macular degeneration (a disease affecting the function of the macula, a small but vital area of the retina within the eye), and the acceleration of the general aging process have all been scientifically linked to the actions of free radicals within the body.

The micro-effects of oxidation within the body are similar to commonly observed oxidation in the world at large, such as the formation of rust (iron oxide) when bare steel is exposed to the elements.

Antioxidants do not attack these free radicals so much as they are scavengers of them. Antioxidants possess the ability to "donate" electrons, typically from a present hydrogen atom with the antioxidant structure, to electrically neutralize the free radical. Different types of antioxidants perform specialized types of scavenging and free radical-neutralizing work within the body.

Vitamin A is a fat-soluble vitamin, meaning that it may be stored within the body, either in the adipose (fat storage) tissue or in the liver. Common sources of vitamin A and other betacarotenoids are carrots, squash, and broccoli, as well as all other brightly colored fruits. Vitamin E is also fat soluble; it operates to specifically protect the free fatty acids in the bloodstream and the cell structures from oxidation; the presence of these fatty acids places an important role in the overall healthy function of cells. Vitamin E is present in many types of nuts, seeds, vegetable oils, and many types of whole grains.

The antioxidant role performed by vitamin C within the body is one of the many important functions of this chemical; vitamin C is essential to the manufacture of collagen, the protein that is the primary building material in bones and connective tissue. Vitamin C is water soluble, and it acts as a scavenger against waterborne free radicals before these agents can act against fat-soluble compounds, an event that creates the damaging chain reactions associated with cellular damage.

The chemical reaction between vitamin C and the encountered free radical results in the donation of a hydrogen atom to the oxidizing radical. The presence of enzymes creates a recycling effect on the vitamin C, permitting it to scavenge other radicals. The rate at which vitamin C can scavenge free radicals is increased by a factor of approximately ten times if the scavenging process occurs in the presence of phosphates. Vitamin E has been to shown to work as an excellent co-antioxidant with vitamin C. Vitamin C is present in all varieties of citrus fruits, as well as broccoli, strawberries, and tomatoes.

Selenium is plentiful in fish, red meats, and grains. It most commonly acts as an antioxidant in conjunction with glutathione peroxidase, a nonessential amino acid (an amino acid produced by the body), to protect white blood cells from the adverse effects of free radicals.

The antioxidants that are contained within various phytochemicals have also been the subject of specialized research in recent years. The best known of these substances are the flavanoids, a group of polyphenols (compounds with a hydrocarbon-based structure), found in soy, red wine, raw grapes, and cranberries. Lycopene, the chemical that is responsible for the red pigmentation of tomatoes and watermelon, is a powerful carotenoid. Lignan, a chemical found in flax, barley, and oatmeal, has also been identified as a water-soluble antioxidant.

Lutein is the antioxidant linked to the onset of macular degeneration; research suggests that a lutein deficiency is an underlying cause of the degenerative condition.

The body is capable of producing its own anti-oxidant defenses through its manufacture of two different enzymes, glutathione peroxidase, and superoxide dismutase (SOD). Enzymes are proteins manufactured by the body, primarily in the liver. Enzymes function as catalysts for a specific chemical reaction or series of reactions. When it comes into contact with a free radical oxygen molecule, SOD is the fastest reacting antioxidant available, in that it can neutralize more free radicals more quickly than any other substance within the body.

Significant publicity has been generated worldwide as to whether the consumption of antioxidants through dietary supplements, in larger quantities than are otherwise available through regular diet, will have a proportionately greater and beneficial effect on health. Numerous antioxidant products are marketed on the basis of claims that the products will either eliminate or reduce the risks associated with various types of cardiovascular disease, cancer, and a multitude of conditions that occur as a result of aging.

Scientific research has proved inconclusive as to whether this type of supplementation in fact achieves an antioxidant result. Studies that were specifically directed about the relationship between supplement consumption and the effect on the low density lipoproteins (LDLs) present in the blood vessels were conducted between 2003 and 2005 in the United States. LDL molecules were known to be particularly vulnerable to oxidation by free radicals, tending to cause the LDL molecules to proliferate on arterial walls, leading to the formation of plaque and creating stenosis, an unhealthy narrowing of the blood vessel that renders the individual more vulnerable to arteriosclerosis and stroke. The research in this area has proved to be inconclusive. Further studies where the test subjects were provided with large doses of vitamin A, vitamin C, and vitamin E as supplements failed to establish any evidence of increased disease prevention.

It is well established in the diet and nutritional sciences that the best manner in which to ingest nutrients of any kind and to have the nutrients effectively absorbed into the body is through food, and not supplements. The reduction of LDL and other unhealthy cholesterols in the bloodstream is most effectively addressed by examining the components of the foods consumed, as opposed to eating poor foods and then addressing a perceived cholesterol or LDL concern. Most nations of the world have published food guidelines that are similar in their scope to those advanced by the United States Department of Agriculture (the "Food Pyramid") and the American Heart Association. In general terms, the guidelines state that, as an alternative to antioxidant supplements, a healthy adult, eating six or more servings of grain products (bread, pastas, etc.), five servings of fruit or vegetables, between two to four servings of low fat dairy products, and a maximum of 6 oz (200 g) of lean meat or fish, will ingest 100% of the recommended daily allowances of the vitamins and minerals with antioxidant properties.

SEE ALSO Cardiovascular system; Minerals; Phytochemicals; Vitamin C; Vitamin E.