Antioxidants are substances that prevent oxygen molecules from interacting with other molecules in a process called oxidation. In the body, antioxidants combine with potentially damaging molecules called free radicals to prevent the free radicals from causing damage to cell membranes, DNA, and proteins in the cell. Common antioxidants important to human health are vitamins A, C, E; beta-carotene; and selenium. By some estimates, up to 68% of North Americans and Europeans were taking at least one antioxidant dietary supplement in 2015.
The role of antioxidants in the body is complex and not completely understood. Antioxidants combine with free radicals so that the free radicals cannot react with, or oxidize, other molecules. In this way, antioxidants help slow or prevent damage to cells. Antioxidant enzymes present in the body also work to prevent oxidation. Damage caused by free radicals is thought to cause or contribute to cardiovascular disease, cancer, Alzheimer's disease, age-related changes in vision, and other signs of aging. However, no direct cause-and-effect relationship between antioxidant intake and disease prevention has been proven. Antioxidants unrelated to those of importance in the body have commercial uses in the preservation of processed food and in many industrial processes.
Oxygen is essential to many reactions that occur within cells. Free radicals form mainly as a result of normal cellular metabolism involving oxygen. They can also form in abnormally large amounts when the body is exposed to radiation, ultraviolet (UV) light, and toxins such as cigarette smoke or certain chemicals.
The common feature of free radicals is that their molecular structure contains an unpaired electron. Free radical molecules with an unpaired electron are unstable and have a strong tendency to react with other molecules by “stealing” an electron from them to form a more stable electron pair. This reaction is called oxidation (even when it happens with molecules other than oxygen). In the body, free radicals cause damage when they react with deoxyribonucleic acid (DNA), proteins, and lipids (fats). Antioxidants are molecules that react with free radicals in ways that neutralize them so they are no longer able to “steal” electrons and cause damage.
When free radicals build up faster than antioxidants can neutralize them, the body develops a condition called oxidative stress. Oxidative stress reduces the body's ability to deal with damage to cells and is thought to play a role in the development of chronic diseases such as cardiovascular disease, cancer, and Alzheimer's disease. Researchers know that a diet high in fruits and vegetables containing antioxidants promotes health and decreases the risk of developing some chronic diseases such as atherosclerosis (hardening of the arteries). In the early 2000s, dietary supplements containing antioxidants were popularized as a way to reduce oxidative stress; prevent health problems such as cancer, stroke, heart attack, and dementia; and live longer. Research has since shown that although there are relationships between antioxidant levels and health, antioxidant dietary supplements are not adept at preventing age-related diseases. In fact, some evidence now suggests that high intakes of antioxidants may actually have harmful effects in promoting the development of cancers and preventing the prophylactic effects of certain cholesterol-reducing medications.
One problem in determining whether there is a cause-and-effect relationship between oxidative stress and disease is that it is often unclear whether oxidative stress causes a disease or if the disease brings about oxidative stress. Also, everyone develops oxidative stress as they age, but not everyone develops the same diseases. The interactions among an individual's diet, environment, genetic make-up, and health are complex and still not well understood. Antioxidants remain of great interest to researchers seeking ways to prevent and cure chronic disease. Many clinical trials are underway to determine safety and effectiveness of different antioxidants, both alone and in combination with other drugs and supplements.
CARDIOVASCULAR DISEASE. The strongest link between antioxidant levels and health is related to the development of cardiovascular disease. Low-density lipoprotein cholesterol (LDL or “bad” cholesterol) appears to react with free radicals. This changes the LDL cholesterol in a way that allows it to accumulate in cells lining the blood vessels. These cholesterol-loaded cells are precursors to the development of plaque, hard deposits that line blood vessels and cause cardiovascular disease, heart attack, and stroke.
High dietary intakes and blood levels of antioxidant nutrients (e.g., beta-carotene) have been associated with lower risk of cardiovascular disease. This led researchers to hypothesize that increasing the amount of antioxidants in the blood by taking supplements would decrease the number of free radicals available to interact with LDL cholesterol, lowering the risk of developing cardiovascular disease. However, studies have found this not to be the case. A paper published in the Journal of the American Medical Association in 2007 analyzed 68 trials of antioxidant supplements, involving about 232,600 patients, and found that antioxidant supplements did not prolong life. In fact, when only rigorous, well-controlled studies were analyzed, the risk of dying increased 5%. This analysis is quite controversial, with some experts questioning the analytical methods used. However, the American Heart Association and similar organizations in other countries continue to advocate cardiovascular disease prevention through dietary consumption of antioxidants—in fruits, vegetables, whole grains, and nuts—instead of antioxidant supplements.
AGE-RELATED VISION IMPAIRMENT. Cataracts and age-related macular degeneration are two types of vision impairment common in older individuals. Cataracts develop because of changes in the protein in the lens of the eye. These changes cause the lens to become cloudy and limit vision. The changes may be due to damage by free radicals. Age-related macular degeneration is an irreversible disease of the retina that causes blindness. Two carotenoid antioxidants, zeaxanthin and lutein, are found in the retina and are essential to vision. However, study participants who took antioxidant supplements over several years did not have a reduced risk of developing these diseases.
ALZHEIMER'S DISEASE. Antioxidants have been associated with slowing the progression of memory loss and other cognitive disorders, including Alzheimer's disease, but results have been inconclusive. A 1997 study sponsored by the National Institute on Aging found that high doses of vitamin E prolonged the ability of participants to carry out activities of daily life when facing cognitive decline. However, subsequent studies have found conflicting evidence, and too much vitamin E may cause adverse side effects, including an increased risk of death. Vitamin E is sometimes prescribed to patients with Alzheimer's disease but should only be taken under close physician supervision. Again, the best sources of antioxidants are whole foods and not supplements.
Flavonoids found in berries, which are a class of antioxidants, have been linked to positive cognitive function, including delayed memory loss and lowered risk of Parkinson's disease, but a cause-and-effect relationship has not been established, and further research is needed.
The mixed results obtained in human studies of antioxidant supplements suggests that all antioxidants should come from foods and not from dietary supplements. There is also little information on the safety of antioxidant supplements in children and women who are pregnant or breastfeeding.
The amount of antioxidant in a supplement often is many times greater than that acquired by eating fruits and vegetables. Some antioxidants (e.g., vitamin A in the form of retinol) can build up to potentially dangerous levels in the body. In the case of supplementation, more is not necessarily better. Consult a physician before taking any antioxidant supplements.
The interaction among various antioxidants, enzymes, coenzymes, drugs, and herbal and dietary supplements is complex and not completely understood. Specific antioxidants may have known interactions with other substances, and any supplements should be discussed with a physician.
Antioxidants acquired by eating fruits and vegetables promote health. No complications are expected from antioxidants in food. Antioxidant dietary supplements may interact with other substances or increase the levels of antioxidants in the body in ways that cause undesirable side effects. Consult a physician prior to taking an antioxidant supplement.
Parents should encourage their children to eat a healthy and varied diet high in fruits, vegetables, and whole grains. There is no need to give children antioxidant dietary supplements. The safety of these supplements in children has not been studied.
See also Cancer-fighting foods ; Carotenoids ; Dietary supplements ; Phytonutrients ; Selenium ; Vitamin A ; Vitamin C ; Vitamin E ; Vitamins .
DeCava, Judith A. The Real Truth about Vitamins and Antioxidants. 2nd ed. Fort Collins, CO: Selene River Press, 2006.
Frei, Balz, ed. Natural Antioxidants in Human Health and Disease. San Diego: Academic Press, 2006.
Panglossi, Harold V., ed. Antioxidants: New Research. New York: Nova Science, 2006.
Paur, Ingvild, et al. “Antioxidants in Herbs and Spices: Roles in Oxidative Stress and Redox Signaling.” In Herbal Medicine: Biomolecular and Clinical Aspects, 2nd ed. Boca Raton, FL: CRC Press, 2011.
Wildman, Robert E. C., ed. Handbook of Nutraceuticals and Functional Foods. 2nd ed. Modern Nutrition. Boca Raton, FL: CRC/Taylor & Francis, 2007.
Bjelakovic, Goran, et al. “Antioxidant Supplements for Prevention of Mortality in Healthy Participants and Patients with Various Diseases.” Cochrane Database of Systematic Reviews 3, art no. CD007176 (March 12, 2012). http://dx.doi.org/10.1002/14651858.CD007176 . pub2 (accessed March 13, 2018).
Bjelakovic, Goran, etal. “Mortality in Randomized Trials of Antioxidant Supplements for Primary and Secondary Prevention: Systematic Review and Meta-Analysis.” Journal of the American Medical Association 297, no. 8 (February 28, 2007): 842–57.
Feng-Jiao, Li, Liang Shen, and Hong-Fang Ji. “Dietary Intakes of Vitamin E, Vitamin C, and ß-Carotene, and Risk of Alzheimer's Disease: A Meta-Analysis.” Journal of Alzheimer's Disease 31, no. 2 (2012): 253–58.
Halliwell, Barry. “Free Radicals and Antioxidants: Updating a Personal View.” Nutrition Reviews 70, no. 5 (May 1, 2012): 257–65. http://dx.doi.org/10.1111/j.1753-4887 . 2012.00476.x (accessed March 15, 2018).
Piero, Dolara, Elisabetta Bigagli, and Andrew Collins. “Antioxidant Vitamins and Mineral Supplementation, Life Span Expansion, and Cancer Incidence: A Critical Commentary.” European Journal of Nutrition 51, no. 7 (October 5, 2012): 769–81. http://www.springerlink.com/content/l3m8913643721808 (accessed March 15, 2018).
Alzheimer's Association. “Medications for Memory Loss.” http://www.alz.org/alzheimers_disease_standard_prescriptions.asp (accessed March 15, 2018).
Consumer Reports on Health. “Vitamins, Supplements: New Evidence Shows They Can't Compete with Mother Nature,” March 2010. ConsumerReports.org . http://www.consumerreports.org/cro/2012/05/vitamins-supplements/index.htm (accessed March 15, 2018).
Dillner, Luisa. “Should I Take Antioxidant Supplements?” Dr. Dillner's Health Dilemmas (blog), The Guardian, August 8, 2011. http://www.guardian.co.uk/lifeandstyle/2011/aug/08/should-i-take-antioxidant-supplements (accessed March 15, 2018).
International Food Information Council Foundation. “Functional Foods Fact Sheet: Antioxidants.” Food Insight.org . http://www.foodinsight.org/Functional_Foods_Fact_Sheet_Antioxidants (accessed March 19, 2018).
MedlinePlus. “Antioxidants.” U.S. National Library of Medicine, National Institutes of Health. https://medlineplus.gov/antioxidants.html (accessed March 15, 2018).
National Cancer Institute. “Antioxidants and Cancer Prevention.” https://www.cancer.gov/about-cancer/causes-prevention/risk/diet/antioxidants-fact-sheet (accessed March 19, 2018).
Academy of Nutrition and Dietetics, 120 South Riverside Plz., Ste. 2000, Chicago, IL, 60606-6995, (312) 899-0040, (800) 877-1600, firstname.lastname@example.org, http://www.eatright.org .
American Heart Association, 7272 Greenville Ave., Dallas, TX, 75231, (800) 242-8721, http://www.americanheart.org .
National Cancer Institute, 6116 Executive Blvd., Ste. 300, Bethesda, MD, 20892-8322, (800) 4-CANCER (422-6237), http://www.cancer.gov .
Office of Dietary Supplements, National Institutes of Health, 6100 Executive Blvd., Rm. 3B01, MSC 7517, Bethesda, MD, 20892-7517, (301) 435-2920, Fax: (301) 480-1845, email@example.com, http://ods.od.nih.gov .
Tish Davidson, AM
Revised by David Newton