Cancers treated or prevented: Antioxidants are used to treat most types of cancer, although their effectiveness is questionable. They are usually taken for cancer prevention, although the link between antioxidant consumption and cancer prevention in humans is not proven.
Delivery routes: Oral, as tablets, caplets, capsules, powder, or tea; some fruits and vegetables are high in antioxidants
How these compounds work: Free radicals are formed during normal cellular metabolism. Free radicals are compounds that are unstable because they contain an unpaired electron. This unpaired electron causes the free radical to react with other molecules in order to gain another electron, creating an electron pair and a more stable molecule. The process of gaining an electron is called oxidation. Antioxidants in the body react with free radicals and make them harmless to cells. Cells naturally make many antioxidants such as glutathione and coenzyme Q10 (ubiquinone). When the number of free radicals exceeds the antioxidants available to neutralize them, however, the body develops a condition called oxidative stress. Oxidative stress appears to make cells especially susceptible to damage.
Free radical formation and oxidation are normal processes. Oxidation and oxidative stress, however, are thought to contribute to aging, as well as to cancer, cardiovascular disease, and other chronic diseases. In the absence of adequate antioxidant compounds, free radicals most often combine with and damage DNA, cellular proteins, and molecules in cell membranes. This damage can cause gene mutations and change cellular metabolism, which may lead to cancer and other diseases. Exposure to ultraviolet (UV) light, radiation, cigarette smoke, and other known carcinogens increases the number of free radicals that are formed, suggesting another link between free radicals, oxidative stress, and cancer.
In theory, increasing the amount of antioxidants in the body should decrease the amount of damage to DNA and cells and reduce the risk of cancer. Antioxidants have been shown to prevent or slow the development of some cancers in cell cultures grown in the laboratory and in some animal studies. The results in human studies have been mixed. Despite claims by some manufacturers of antioxidant dietary supplements, no clear link between antioxidant consumption and cancer prevention or treatment has been established in humans. The role of antioxidants in the prevention and treatment of cancer is of high interest to research scientists. Many clinical trials are being conducted, and there is no cost for qualified individuals to participate in a clinical trial.
The body makes some antioxidants, but others must be acquired through diet. Common antioxidants that the body does not make include vitamins A, C, and E, carotenoids, and flavonoids. Selenium is a mineral that is not strictly an antioxidant but is essential to many antioxidant reactions and often classed with the antioxidants. Coenzyme Q10 and glutathione are antioxidants naturally produced by the body that are also manufactured sold as high-dose supplements.
Vitamin A (retinol) is a fat-soluble vitamin found in liver, egg yolks, whole milk, and dairy products made with whole milk. It is also sold as a dietary supplement and is included in most multivitamin tablets and in special antioxidant formulations such as ACE, a combination of vitamins A, C, and E. Vitamin A is essential to health, but its role in cancer prevention and treatment is unclear. All-trans-retinoic acid (ATRA, Vesanoid) is a pharmaceutical drug that is a derivative of vitamin A. It is successfully used to treat promyelocytic leukemia and is being studied in individuals with breast and skin cancers. To date, however, vitamin A alone has not shown the same cancer-reducing effects as ATRA.
Alpha-carotene, beta-carotene, beta-cryptoxanthin, lutein, zeaxanthin, and lycopene are all carotenoids that show antioxidant activity in laboratory tests. These compounds are found in red, yellow, and orange plants such as carrots, cantaloupe, mango, and tomato. Beta-carotene and lycopene are also sold as dietary supplements either individually or in combination with other antioxidants. In the body, carotenoids are converted into vitamin A. An examination of twenty-one studies relating lycopene consumption and prostate cancer found that men with the highest lycopene intake had a modest decrease in the incidence of prostate cancer. The role of carotenoids in cancer prevention is actively being studied.
Vitamin C (ascorbic acid) is a water-soluble vitamin found in citrus fruits. For centuries, it has been known that vitamin C is essential for preventing the disease scurvy. Dietary supplements of vitamin C are often promoted as a preventive or treatment for the common cold, although clinical studies have not proved its effectiveness. Vitamin C has very strong antioxidant activities. Nevertheless, multiple human studies have failed to find a significant link between cancer prevention and vitamin C intake.
Vitamin E is a group of related compounds, the most active of which is alpha-tocopherol. Vitamin E is a fat-soluble vitamin with antioxidant properties. It is found in olive, sunflower, and safflower oils and in many nuts. The role of vitamin E in cancer prevention is controversial. The National Institutes of Health reported in 2013 that the antioxidant qualities of vitamin E make it possible to block the formation of carcinogenic qualities of various foods and damaging effects of free radicals. However, human trials and studies have not linked the intake of vitamin E with a reduction of cancer incidence. A 2014 study from the Public Health Sciences Division of the Fred Hutchinson Cancer Research Center elaborated on this by stating that high doses of the vitamin have the potential for a heightened risk for developing prostate cancer in some men.
Flavonoids are a group of antioxidant compounds found primarily in brewed tea, red wine, dark chocolate, apples, berries, and citrus fruits. These compounds have antioxidant activity. Traditional Chinese medicine has promoted the health effects of green tea for centuries, and recent studies show that dark (bitter) chocolate in small quantities may promote heart health, but the role of flavonoids in cancer prevention is unclear.
Selenium is a mineral found in fish, shellfish, grains, Brazil nuts, and many vegetables that are grown in selenium-rich soils. By itself, selenium is not an antioxidant, but it plays a critical role in antioxidant activity and is often classified with the antioxidants that must be acquired through diet. Selenium deficiency has been linked to increases in colorectal, lung, and prostate cancers. Increased selenium intake in people who are not selenium deficient, however, does not decrease the risk of developing cancer. Additionally, selenium is toxic in large doses.
Coenzyme Q10 and glutathione are the main antioxidants made by the body. They are also sold as dietary supplements. Studies of individuals who supplemented their diet with synthetic coenzyme Q10 or glutathione found no evidence that these supplements protected against cancer.
Individuals who maintain a healthy weight and eat a diet low in fats and high in fruits, vegetables, and whole grains develop cancer at a lower rate than individuals who eat a more traditional American diet high in fat and low in fruits and vegetables. The American Cancer Society and the National Cancer Institute recommend that individuals meet as many of their vitamin, mineral, and antioxidant needs as possible by eating a healthy, varied diet rather than by taking dietary supplements. As of 2014, neither organization recommended the use of antioxidant supplements to treat or prevent cancer.
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