<\/p>\n
https:\/\/www.cancer.gov\/about-cancer\/causes-prevention\/risk\/diet\/antioxidants-fact-sheet<\/p>\n
What are free radicals, and do they play a role in cancer development?<\/p>\n
Free radicals<\/a>\u00a0are highly reactive chemicals that have the potential to harm cells. They are created when an atom or a molecule (a chemical that has two or more atoms) either gains or loses an electron (a small negatively charged particle found in atoms). Free radicals are formed naturally in the body and play an important role in many normal cellular processes (1<\/a>,\u00a02<\/a>). At high concentrations, however, free radicals can be hazardous to the body and damage all major components of cells, including DNA, proteins, and cell membranes. The damage to cells caused by free radicals, especially the damage to DNA, may play a role in the development of cancer and other health conditions (1<\/a>,\u00a02<\/a>).<\/p>\n Abnormally high concentrations of free radicals in the body can be caused by exposure to\u00a0ionizing radiation<\/a>\u00a0and other environmental toxins. When ionizing radiation hits an atom or a molecule in a cell, an electron may be lost, leading to the formation of a free radical. The production of abnormally high levels of free radicals is the mechanism by which ionizing radiation kills cells. Moreover, some environmental toxins, such as cigarette smoke, some metals, and high-oxygen atmospheres, may contain large amounts of free radicals or stimulate the body\u2019s cells to produce more free radicals.<\/p>\n Free radicals that contain the element oxygen are the most common type of free radicals produced in living tissue. Another name for them is \u201creactive oxygen species<\/a>,\u201d or \u201cROS\u201d (1<\/a>,\u00a02<\/a>).<\/p>\n What are antioxidants?<\/p>\n Antioxidants<\/a>\u00a0are chemicals that interact with and neutralize\u00a0free radicals<\/a>, thus preventing them from causing damage. Antioxidants are also known as \u201cfree radical scavengers.\u201d<\/p>\n The body makes some of the antioxidants that it uses to neutralize free radicals. These antioxidants are called\u00a0endogenous<\/a>\u00a0antioxidants. However, the body relies on external (exogenous) sources, primarily the diet, to obtain the rest of the antioxidants it needs. These exogenous antioxidants are commonly called dietary antioxidants. Fruits, vegetables, and grains are rich sources of dietary antioxidants. Some dietary antioxidants are also available as\u00a0dietary supplements<\/a>\u00a0(1<\/a>,\u00a03<\/a>).<\/p>\n Examples of dietary antioxidants include\u00a0beta-carotene<\/a>,\u00a0lycopene<\/a>, and vitamins A, C, and E (alpha-tocopherol<\/a>). The mineral element\u00a0selenium<\/a>\u00a0is often thought to be a dietary antioxidant, but the antioxidant effects of selenium are most likely due to the antioxidant activity of proteins that have this element as an essential component (i.e., selenium-containing proteins), and not to selenium itself (4<\/a>).<\/p>\n In laboratory and\u00a0animal studies<\/a>, the presence of increased levels of exogenous\u00a0antioxidants<\/a>\u00a0has been shown to prevent the types of\u00a0free radical<\/a>\u00a0damage that have been associated with cancer development. Therefore, researchers have investigated whether taking dietary antioxidant\u00a0supplements<\/a>\u00a0can help lower the risk of developing or dying from cancer in humans.<\/p>\n Many\u00a0observational studies<\/a>, including\u00a0case\u2013control\u00a0studies<\/a>\u00a0and\u00a0cohort studies<\/a>, have been conducted to investigate whether the use of dietary antioxidant supplements is associated with reduced risks of cancer in humans. Overall, these studies have yielded mixed results (5<\/a>). Because observational studies cannot adequately control for biases that might influence study outcomes, the results of any individual observational study must be viewed with caution.<\/p>\n Randomized\u00a0controlled clinical trials<\/a>, however, lack most of the biases that limit the reliability of observational studies. Therefore, randomized trials are considered to provide the strongest and most reliable evidence of the benefit and\/or harm of a health-related intervention. To date, nine randomized controlled trials of dietary antioxidant supplements for cancer prevention have been conducted worldwide. Many of the trials were sponsored by the National Cancer Institute. The results of these nine trials are summarized below.<\/p>\n Overall, these nine randomized controlled clinical trials did not provide evidence that dietary antioxidant supplements are beneficial in primary cancer prevention. In addition, a systematic review of the available evidence regarding the use of vitamin and mineral supplements for the prevention of chronic diseases, including cancer, conducted for the United States Preventive Services Task Force (USPSTF) likewise found no clear evidence of benefit in preventing cancer (27<\/a>).<\/p>\n <\/p>\n It is possible that the lack of benefit in clinical studies can be explained by differences in the effects of the tested antioxidants when they are consumed as purified chemicals as opposed to when they are consumed in foods, which contain complex mixtures of antioxidants, vitamins, and minerals (3<\/a>). Therefore, acquiring\u00a0a more complete\u00a0understanding of the antioxidant content of individual foods, how the various antioxidants and other substances in foods interact with one another, and factors that influence the uptake and distribution of food-derived antioxidants in the body are active areas of ongoing cancer prevention research.<\/p>\n Should people already diagnosed with cancer take antioxidant supplements?<\/p>\n Several randomized\u00a0controlled trials<\/a>, some including only small numbers of patients, have investigated whether taking\u00a0antioxidant<\/a>\u00a0supplements during cancer treatment alters the effectiveness or reduces the toxicity of specific therapies (28<\/a>). Although these trials had mixed results, some found that people who took antioxidant supplements during cancer therapy had worse outcomes, especially if they were smokers.<\/p>\n In some preclinical studies, antioxidants have been found to promote tumor growth and metastasis in tumor-bearing mice and to increase the ability\u00a0of circulating\u00a0tumor cells to metastasize (29<\/a>\u201331<\/a>). Until more is known about the effects of antioxidant supplements in cancer patients, these supplements should be used with caution. Cancer patients should inform their doctors about their use of any\u00a0dietary supplement<\/a>.<\/p>\n If you would like to reproduce some or all of this content, see\u00a0Reuse of NCI Information<\/a>\u00a0for guidance about copyright and permissions. In the case of permitted digital reproduction, please credit the National Cancer Institute as the source and link to the original NCI product using the original product’s title; e.g., \u201cAntioxidants and Cancer Prevention was originally published by the National Cancer Institute.\u201d<\/em><\/p>\n<\/div>\n<\/div>\n<\/div>\n