Antioxidants are molecules that work to prevent damage that occurs in cells and body tissues due to both normal bodily processes and exposure to some chemicals. The potential medical benefit of antioxidants may reside in their ability to prevent or slow the oxidation of molecules in the microscopic parts of the body, such as DNA or proteins.
Oxidation is a chemical process in which electrons from a substance are transferred to what is known as an oxidizing agent. Oxidation reactions may produce substances referred to as "free radicals." These molecules may cause chain reactions that can damage many cells in the body. Antioxidants may stop the chain reactions caused by free radicals, and therefore stop the process of this sort of damage occurring in the body. In other cases, antioxidants may stop other kinds of oxidation reactions by becoming oxidized themselves and thus sparing cells and tissues from damage.
The reactions that produce free radicals, called oxidation reactions, are critical for the body to function. However, the cumulative effect of many oxidation reactions may irreversibly damage the body. The human body may require a variety of antioxidants to reduce the impact of the free radicals created by oxidation reactions. If the body has too few antioxidants, then the stress of many oxidation reactions may damage or kill body cells. If enough of these cells are killed or damaged, then illness or disease may result.
Free radicals are considered dangerous because they are atoms with an unstable number of electrons, which makes them more chemically reactive than atoms with a stable number of electrons. These unstable atoms may take electrons from other atoms, such as the ones that make up DNA or proteins. When parts of cells lose electrons, the cell cannot function as well as it should. If affected by free radicals, then DNA may be mutated. Proteins that are affected by free radicals may degrade or unfold. Antioxidants neutralize the electrical charge that free radicals have due to their unstable number of electrons and thus protect body cells and tissues.
While oxygen is considered necessary for life, it may also be the most abundant free radical in the body, because it is highly unstable. The types of oxygen molecules that cause free radicals include hydrogen peroxide and hypochlorous acid. Free radicals created by oxygen include the hydroxyl radical (-OH) and the superoxide anion.
By chelating transition metals, some compounds may contribute to antioxidant defense against free radicals by chelating transition metals. Especially important in this system is the function of iron-binding proteins including transferrin and ferritin to sequester iron.
Antioxidants may be dependent upon one another to prevent oxidation; they may work together to stop free radicals in an quantity that both would not be able to achieve on their own.
Oxidation reactions may occur due to forces outside of the human body. For instance, exposure to tobacco smoke, radiation or environmental contaminants may result in oxidation reactions that damage cells.
Early research on antioxidants investigated their possible use in preventing edible fats from becoming rancid due to oxidation. Vitamins A, C and E were some of the first antioxidants identified by medical science as possibly beneficial to human health.
In addition to being available in a variety of fruits and vegetables, antioxidants are available as supplements. They are one of the most popular categories of supplements available today. Antioxidants are proposed to play a role in preventing many common health disorders, such as Alzheimer's disease, heart disease and some types of cancers. Some experts believe that antioxidants may be a treatment for stroke and medical conditions where the brain slowly loses its ability to function.
There is some debate as to whether consuming antioxidants from foods or in supplement form is actually beneficial to the health of patients. Doctors and other experts do not agree if these chemicals are actually helpful, and clinical trials have not all reached the same conclusion. There is also some concern that consuming too many antioxidants may have negative effects on a person's health. For instance, some experts believe that consuming too much vitamin C may result in kidney stones. However, antioxidants are the subject of increasing research because of the possible role they may play in preventing disease.
Antioxidants are molecules that work to prevent damage that occurs in cells and body tissues due to both normal bodily processes and exposure to some chemicals. The potential medical benefit of antioxidants may reside in their ability to prevent or slow the oxidation of molecules in the microscopic parts of the body, such as DNA or proteins.
Oxidation is a chemical process in which electrons from a substance are transferred to what is known as an oxidizing agent. Oxidation reactions may produce substances referred to as "free radicals." These molecules may cause chain reactions that can damage many cells in the body. Antioxidants may stop the chain reactions caused by free radicals, and therefore stop the process of this sort of damage occurring in the body. In other cases, antioxidants may stop other kinds of oxidation reactions by becoming oxidized themselves and thus sparing cells and tissues from damage.
The reactions that produce free radicals, called oxidation reactions, are critical for the body to function. However, the cumulative effect of many oxidation reactions may irreversibly damage the body. The human body may require a variety of antioxidants to reduce the impact of the free radicals created by oxidation reactions. If the body has too few antioxidants, then the stress of many oxidation reactions may damage or kill body cells. If enough of these cells are killed or damaged, then illness or disease may result.
Free radicals are considered dangerous because they are atoms with an unstable number of electrons, which makes them more chemically reactive than atoms with a stable number of electrons. These unstable atoms may take electrons from other atoms, such as the ones that make up DNA or proteins. When parts of cells lose electrons, the cell cannot function as well as it should. If affected by free radicals, then DNA may be mutated. Proteins that are affected by free radicals may degrade or unfold. Antioxidants neutralize the electrical charge that free radicals have due to their unstable number of electrons and thus protect body cells and tissues.
While oxygen is considered necessary for life, it may also be the most abundant free radical in the body, because it is highly unstable. The types of oxygen molecules that cause free radicals include hydrogen peroxide and hypochlorous acid. Free radicals created by oxygen include the hydroxyl radical (-OH) and the superoxide anion.
By chelating transition metals, some compounds may contribute to antioxidant defense against free radicals by chelating transition metals. Especially important in this system is the function of iron-binding proteins including transferrin and ferritin to sequester iron.
Antioxidants may be dependent upon one another to prevent oxidation; they may work together to stop free radicals in an quantity that both would not be able to achieve on their own.
Oxidation reactions may occur due to forces outside of the human body. For instance, exposure to tobacco smoke, radiation or environmental contaminants may result in oxidation reactions that damage cells.
Early research on antioxidants investigated their possible use in preventing edible fats from becoming rancid due to oxidation. Vitamins A, C and E were some of the first antioxidants identified by medical science as possibly beneficial to human health.
In addition to being available in a variety of fruits and vegetables, antioxidants are available as supplements. They are one of the most popular categories of supplements available today. Antioxidants are proposed to play a role in preventing many common health disorders, such as Alzheimer's disease, heart disease and some types of cancers. Some experts believe that antioxidants may be a treatment for stroke and medical conditions where the brain slowly loses its ability to function.
There is some debate as to whether consuming antioxidants from foods or in supplement form is actually beneficial to the health of patients. Doctors and other experts do not agree if these chemicals are actually helpful, and clinical trials have not all reached the same conclusion. There is also some concern that consuming too many antioxidants may have negative effects on a person's health. For instance, some experts believe that consuming too much vitamin C may result in kidney stones. However, antioxidants are the subject of increasing research because of the possible role they may play in preventing disease.
Diet
The term "antioxidants" describes a variety of molecules. The most popularly used antioxidants today are ascorbic acid (vitamin C), glutathione, lipoic acid, carotenes, α-tocopherol (vitamin E), and ubiquinone (Coenzyme Q10).
Experts have suggested that in order for antioxidants to be possibly beneficial, a person must consume them on a regular basis over a period of many years as the proposed benefits of antioxidants may occur by possibly stopping many oxidation reactions over long periods of time. Therefore, consuming a large number of antioxidant supplements over a day, week or month may not reverse the damage that has already been done by free radicals.
Antioxidants fit into two main classifications: antioxidants created by the body or antioxidants provided by foods that are eaten. Antioxidants that dissolve in water are known as hydrophilic and react with oxidants in the blood and in the free spaces inside cells. Antioxidants that dissolve in fats are known as hydrophobic and help to protect the outer part of the cell, known as the membrane, from a process known as lipid peroxidation. Each type of antioxidant may be more present in some body fluids and tissues than others. For instance, one type of antioxidant may be abundant in the kidneys, but almost absent from the heart while the opposite may be true for another antioxidant. Some antioxidants may occur at about the same concentration in every part of the body.
Antioxidants are found in varying amounts in foods such as vegetables, fruits, grain cereals, legumes, and nuts. Some antioxidants such as lycopene and ascorbic acid can be destroyed by long-term storage or prolonged cooking. Other antioxidant compounds are more stable, such as the polyphenolic antioxidants in foods such as whole-wheat cereals and tea. In general, processed foods are thought to contain less antioxidants than fresh and uncooked foods since preparation processes may expose the food to oxygen.
In order to protect themselves from oxidative stress, living organisms have developed several antioxidant defense systems; many of which depend on minerals for proper functioning. The major enzymatic antioxidant defense systems in the body are: Superoxide Dismutase (SOD), which requires copper, zinc and manganese; glutathione peroxidase, which requires selenium; and catalase, which requires iron.
Although the elements selenium and zinc are sometimes called antioxidant nutrients, they do not actually perform a direct antioxidant action. Instead, the presence of antioxidant nutrients is sometimes necessary for antioxidant enzymes to function correctly.
Common Types And Sources Of Food Based Antioxidants
Chart adapted from International Food Information Council Foundation: Media Guide on Food Safety and Nutrition: 2004-2006.
May contribute to maintenance
of prostate health, and may
decrease risk of chronic
illnesses such as some
cancers and cardiovascular
disease.
Water soluble vitamins
Ascorbic acid (vitamin C)
rose hips, acerola cherry, black
currant, broccoli, papaya,
strawberry, orange, lemon,
spinach, cabbage
May protect against radicals
within the aqueous
compartments of cells and
extracellular fluids. May
regenerate other antioxidants
such as vitamin A and vitamin
E. As a reducing agent, it may
neutralize hydrogen peroxide
and other oxygen reactive
species.
vegetable and seed oils, whole
grains, green leafy vegetables
May protect cell membranes
from lipid peroxyl radicals.
Caffeic acid, Ferulic acid
apples, pears, citrus fruits,
some vegetables
May bolster cellular antioxidant
defenses; may contribute to
maintenance of healthy vision
and heart health.
Sulfides/Thiols
Diallyl sulfide, Allyl methyl trisulfide
garlic, onions, leeks, scallions
May enhance detoxification of
undesirable compounds; may
contribute to maintenance of
heart health and healthy
immune function.
Dithiolthiones
cruciferous vegetables such as
broccoli, cabbage, bok choy,
collards
May contribute to maintenance
of healthy immune function.
Theory/evidence
Experts are not sure if oxidants cause disease, or if oxidants are produced as a result of the disease. Regardless, the stress caused by oxidants over long periods of time is thought to play a role in the development of many types of diseases, including heart disease, rheumatoid arthritis, Alzheimer's disease, Parkinson's disease, some conditions associated with diabetes, and some types of diseases where the brain loses its ability to function properly. The role of free radicals in heart disease is better understood than in other types of diseases. The arteries may harden due to oxidation of bad cholesterol, and over time, this process may result in heart disease.
Measurement of antioxidants is not a straightforward process as this is a diverse group of compounds with different reactivities for different reactive oxygen species. In food science, the oxygen radical absorbance capacity (ORAC) has become the current industry standard for assessing antioxidant strength of whole foods, juices and food additives. Other measurement tests include the Folin-Ciocalteu reagent, and the trolox equivalent antioxidant capacity assay. In medicine, a range of different assays are used to assess the antioxidant capability of blood plasma and of these the ORAC assay may be the most reliable.
The first large randomized trial on antioxidants and cancer risk was the Chinese Cancer Prevention Study, which was published in 1993. This trial investigated the effect of a combination of beta-carotene, vitamin E and selenium on cancer in healthy Chinese men and women at high risk for gastric cancer. The study showed that the combination of beta-carotene, vitamin E and selenium significantly reduced incidence of both gastric cancer and cancer overall.
A 1994 cancer prevention study entitled the Alpha-Tocopherol (vitamin E/Beta-Carotene) Cancer Prevention Study (ATBC) demonstrated that lung cancer rates of Finnish male smokers increased significantly with beta-carotene and were not affected by vitamin E.
Another 1994 study, the Beta-Carotene and Retinol (vitamin A) Efficacy Trial (CARET), also demonstrated a possible increase in lung cancer associated with antioxidants.
The 1996 Physicians' Health Study I (PHS) found no change in cancer rates associated with beta-carotene and aspirin taken by U.S. male physicians.
The 1999 Women's Health Study (WHS) tested effects of vitamin E and beta-carotene in the prevention of cancer and cardiovascular disease among women age 45 years or older. Among apparently healthy women, there was no benefit or harm observed from beta-carotene supplementation.
The Selenium and Vitamin E Cancer Prevention Trial (SELECT) is currently investigating whether taking selenium and/or vitamin E supplements may prevent prostate cancer in men age 50 or older.
The Physicians' Health Study II (PHS II) is a follow up of the earlier clinical trial by the same name and is currently investigating the effects of vitamin E, C and multivitamins on prostate cancer and total cancer incidence.
A qualified healthcare provider should be consulted before making decisions about therapies and/or health conditions.
Antioxidants that are relatively strong reducing acids can have anti-nutritional effects by binding to dietary minerals such as iron and zinc in the gastrointestinal tract and thereby 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.
Nonpolar (absence of electrical charge) antioxidants such as eugenol, a major component of the oil of cloves, have toxicity limits that can be exceeded with the misuse of undiluted essential oils. Toxicity associated with high doses of water-soluble antioxidants such as ascorbic acid are considered less of a concern as these compounds can be excreted rapidly in urine.
The Beta-Carotene and Retinol Efficacy Trial (CARET) involving lung cancer patients found that smokers given beta-carotene supplements had increased rates of lung cancer. Subsequent studies also found similar adverse effects in smokers given beta carotene. These harmful effects may also be seen in non-smokers. A recent meta-analysis including data from approximately 230,000 patients showed that beta-carotene, vitamin A or vitamin E supplementation may be associated with increased mortality, but saw no significant adverse effect from vitamin C. No health risk was seen when all the randomized controlled studies were examined together; an increase in mortality was detected only when the high-quality and low-bias risk trials were examined separately. However, as the majority of these low-bias trials dealt with either elderly people or people already suffering from disease, these results may not apply to the general population. These results are consistent with some previous meta-analyses that also suggested that Vitamin E supplementation may increase mortality, and that antioxidant supplements may increase the risk of colon cancer.
However, the results of this meta-analysis are inconsistent with other studies such as the SU.VI.MAX trial, which suggested that antioxidants have no effect on mortality. Overall, the large number of clinical trials carried out with antioxidant supplements suggest that either these products have no effect on health, or that they cause a small increase in mortality in elderly or vulnerable populations.
While antioxidant supplementation is widely used in attempts to prevent the development of cancer, it has been proposed that antioxidants may actually interfere with cancer treatments. This was thought to occur since the environment of cancer cells causes high levels of oxidative stress, making these cells more susceptible to further oxidative stress induced by treatments. As a result, by reducing the redox stress in cancer cells, antioxidant supplements were thought to decrease the effectiveness of radiotherapy and chemotherapy. However, this concern appears not to be valid, as it has been addressed by multiple clinical trials that indicate that antioxidants are either neutral or beneficial in cancer therapy.
Author Information
This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).
Bibliography
Blot WJ, Li JY, Taylor PR, et al. Nutrition intervention trials in Linxian, China: supplementation with specific vitamin/mineral combinations, cancer incidence, and disease-specific mortality in the general population. J Natl Cancer Inst. 1993 Sep 15;85(18):1483-92.
Hennekens CH, Buring JE, Manson JE, et al. Lack of effect of long-term supplementation with beta carotene on the incidence of malignant neoplasms and cardiovascular disease. N Engl J Med. 1996 May 2;334(18):1145-9.
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No authors listed. The effect of vitamin E and beta carotene on the incidence of lung cancer and other cancers in male smokers. The Alpha-Tocopherol, Beta Carotene Cancer Prevention Study Group. N Engl J Med. 1994 Apr 14;330(15):1029-35.
Omenn GS, Goodman G, Thornquist M, et al. The beta-carotene and retinol efficacy trial (CARET) for chemoprevention of lung cancer in high risk populations: smokers and asbestos-exposed workers. Cancer Res. 1994 Apr 1;54(7 Suppl):2038s-2043s.
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Yuan JM, Stram DO, Arakawa K, et al. Dietary cryptoxanthin and reduced risk of lung cancer: the Singapore Chinese Health Study. Cancer Epidemiol Biomarkers Prev. 2003 Sep;12(9):890-8.
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