Antioxidants are man-made or natural substances that may prevent or delay some types of cell damage. Antioxidants are found in many foods, including fruits and vegetables. They are also available as dietary supplements. Examples of antioxidants include

• Beta-carotene
• Lutein
• Lycopene
• Selenium
• Vitamin A
• Vitamin C
• Vitamin E

Vegetables and fruits are rich sources of antioxidants. There is good evidence that eating a diet with lots of vegetables and fruits is healthy and lowers risks of certain diseases. But it isn't clear whether this is because of the antioxidants, something else in the foods, or other factors.

High-dose supplements of antioxidants may be linked to health risks in some cases. For example, high doses of beta-carotene may increase the risk of lung cancer in smokers. High doses of vitamin E may increase risks of prostate cancer and one type of stroke. Antioxidant supplements may also interact with some medicines. To minimize risk, tell you of your health care providers about any antioxidants you use.

Environmental pollutants and toxins are all around us, and they have been convincingly linked to a wide variety of diseases and defects. While it's impossible to completely eliminate pollutants from your life, it is possible to limit your exposure:

• Drink filtered tap water.
• Eat organic foods when possible, especially strawberries and tomatoes.
• Use nontoxic household cleaners, fertilizers, pesticides, and herbicides.
• Minimize exposure of foods to plastics. Store foods in glass or ceramic containers. Don't microwave foods in plastic containers or with plastic wrap covering them.
• Limit your use of paper products that have been chlorine bleached to make them white.
• Keep windows open, even just a crack, to provide ventilation and protect against indoor pollutants.
• Women who may become pregnant, pregnant women, nursing mothers, and young children should avoid eating shark, swordfish, king mackerel, and tilefish because they contain high levels of mercury.

You can help make the environment cleaner and safer by evaluating your purchases based on their environmental impact, encouraging the use of reusable products, and taking personal responsibility for creating a sustainable community.

What are free radicals?

Free radicals are highly reactive atoms formed when oxygen interacts with certain molecules. They try to "steal" electrons from atoms in body tissues, and in doing so can damage cellular components such as cell membranes and DNA, and lead to heart disease, cancer, and other diseases.

Free radicals are formed in the normal processes of energy metabolism, but they are also created by environmental pollution, cigarette smoke, and toxic chemicals. To prevent free radical damage, your body has several enzyme systems that scavenge free radicals. Problems start when the body is inundated by so many free radicals that it can't cope with them all. Micronutrients, such as vitamin E, beta-carotene, and vitamin C, in addition to the thousands of phytochemicals now being discovered in many fruits and vegetables, can aid the body in eliminating damaging free radicals.

What are Phytonutrients? The prefix "phyto-" originated from the Greek word meaning plant. Phytonutrients sometimes referred to as phytochemicals, are a group of plant-based compounds that are believed to provide a wide range of health benefits to humans.

At least 900 different phytonutrients have been identified in plants. These include catechins, flavones, flavonols, isoflavones, lutein, lycopene and zeaxanthin. Fruits and vegetables such as spinach, soybeans, tomatoes, onions, carrots, oranges, berries and green tea are rich in these substances.

Studies have found the protective and disease-preventing benefits of phytonutrients are mainly related to their powerful antioxidative activities and anti-inflammatory properties. A common example, green tea, contains high amounts of epigallocatecin gallate or EGCG, one of the most powerful naturally occurring antioxidants.

Phytonutrients help protect from oxidative stress and chronic inflammation, the main causes of age-related chronic disease. Oxidative stress results from normal metabolism; it is increased by such factors as smog and tobacco smoke and decreased by antioxidants including the phytonutrients. Research has revealed that EGCG can decrease oxidative stress as well as inflammation in the arteries, reducing the risk of developing atherosclerosis.

Phytonutrients in foods such as apples, grapes, berries, plums and other vegetables have been found to improve blood vessel function, improve the negative effects of LDL ("bad" cholesterol), and prevent coronary artery disease through their anti-inflammatory effects. Oxidative stress can also contribute to osteoporosis which causes bones to become porous and break easily. Osteoporosis affects more than 10 million people in the United States and costs more than $17 billion in direct annual healthcare expenses.

The antioxidant phytonutrients can benefit bone health. Scientists at the Grand Forks Human Nutrition Research Center found phytonutrient-rich pinto bean hulls have high antioxidant activity. Supplements decreased bone loss and improved overall bone health. They also found green tea polyphenols to reduce oxidative stress and inflammation improving bone quantity and quality in animal models. They are also rich in calcium, magnesium, and potassium which are required for bone health and can help to prevent calcium loss from bones by neutralizing acids produced by normal digestion.

Most Americans do not consume the recommended numbers of servings of fruit and vegetables on a daily basis. For your good health including bone health, eating enough fruits and vegetable is important to maintaining a healthy lifestyle.

What are antioxidants?

During their normal activities, our bodies’ cells produce highly reactive substances that contain oxygen. Overproduction of these substances can lead to a condition called “oxidative stress,” which may damage cells and contribute to chronic diseases such as heart disease and cancer. Our bodies have preventive and repair mechanisms that regulate oxidative stress pathways. Research has shown that antioxidant enzymes are the most important components of this defense system. Phytochemicals that act as antioxidants may also play a role in helping regulate oxidative stress pathways. These small molecules are the focus of this fact sheet.

As research has progressed, it has become evident that antioxidants—especially in larger-than-usual amounts—may not always be beneficial. Large amounts of antioxidants may interfere with important functions in the cell, including its defense mechanisms and normal signaling. In addition, different types of antioxidants may not be interchangeable. Each of the many antioxidants found in the body has different properties.

Where do we get the antioxidants we need?

Our bodies make some of the antioxidants we need. Additional antioxidants come from foods, such as fruits, vegetables, and grains. Some antioxidants, such as vitamins C and E and beta-carotene, are also available as dietary supplements. 

Have studies been done on the health effects of antioxidants?

Yes. As will be discussed in more detail below, extensive studies have been done to evaluate the effects of antioxidant supplements on the risks of chronic diseases associated with aging. Much of this research focused on vitamin E and beta-carotene (a pigment found in vegetables and fruits that can be turned into vitamin A in your body). Other studies looked at vitamin C, carotenoids other than beta-carotene, or other antioxidants. Diseases that have been studied include cancer, cardiovascular disease, eye diseases associated with aging (macular degeneration and cataracts), and Alzheimer’s disease. 

In general, except for age-related macular degeneration, there is currently no evidence that antioxidant supplements have a positive impact on these diseases.

On the other hand, there is extensive evidence that people who eat more vegetables and fruits—foods that are rich sources of antioxidants—have lower risks of chronic diseases. For example:

• A 2017 review of 95 observational studies, with more than 2 million total participants, showed that people who had higher intakes of fruits and vegetables had lower risks of cardiovascular disease and cancer.
• A 2023 study from the United Kingdom in which 72,160 people were followed for an average of 9 years showed that higher intakes of fruits and vegetables were associated with a lower risk of cataracts.

There are several possible reasons why antioxidant supplements may not prevent cardiovascular disease and cancer even though antioxidant-rich vegetables and fruits are associated with reduced risk:

• The beneficial health effects of a diet high in vegetables and fruits or other antioxidant-rich foods may be due to other substances in these foods or other aspects of people’s lifestyles that are associated with eating these foods. For example, in a study of adults living in rural areas in the United States, eating at least five servings of fruits and vegetables daily was associated with several other factors that might affect health, such as getting at least moderate physical activity and having had a routine medical exam in the past year.
• Antioxidants consumed as purified chemicals might act differently than those consumed in foods, which contain complex mixtures of substances.
• The high doses of antioxidants in dietary supplements may have different effects than the smaller amounts in foods.

Can antioxidant supplements help to prevent cancer?

The current evidence does not support the idea that antioxidant supplements can prevent cancer. In fact, high doses of one antioxidant, beta-carotene, may increase the risk of lung cancer.

• In 2022, the U.S. Preventive Services Task Force, an independent panel of experts that makes evidence-based recommendations about disease prevention, recommended against the use of beta-carotene or vitamin E supplements for cancer prevention. They also concluded that the evidence is insufficient to make recommendations about supplements of other single nutrients or pairs of nutrients. These recommendations are consistent with the Task Force’s previous recommendations, issued in 2014.
  • The Task Force’s recommendation against the use of beta-carotene for cancer prevention is based on studies of beta-carotene supplementation involving more than 90,000 people, which showed that the harms of beta-carotene outweigh its benefits. Beta-carotene supplementation led to an increase in risk of lung cancer, with the strongest evidence of an increase in risk in people at high risk of this type of cancer (smokers and people with occupational exposure to asbestos), as well as an increased risk of death from cardiovascular disease.
  • The Task Force’s recommendation against the use of vitamin E for cancer prevention is based on a combined analysis of 5 studies (76,777 participants) of vitamin E supplementation, which showed no benefit of the supplements in preventing cancer.
  • The Task Force’s decision not to make a recommendation about the use of supplements of other single or paired nutrients—including antioxidants such as vitamin C and selenium—is based on a lack of adequate evidence on the benefits and harms of the supplements.

Can antioxidant supplements help to prevent cardiovascular disease?

The current evidence does not support the idea that antioxidant supplements can prevent cardiovascular disease. In fact, high doses of one antioxidant, beta-carotene, may increase the risk of death from cardiovascular disease.

• In 2022, the U.S. Preventive Services Task Force recommended against the use of beta-carotene or vitamin E supplements for prevention of cardiovascular disease. They also concluded that the evidence is insufficient to make recommendations about supplements of other single nutrients or pairs of nutrients. These recommendations are consistent with the Task Force’s previous recommendations, issued in 2014.
  • The Task Force’s recommendation against the use of beta-carotene for cardiovascular disease is based on studies of beta-carotene supplementation involving more than 90,000 people, which showed that the harms of beta-carotene outweigh its benefits. Beta-carotene supplementation led to an increase in risk of lung cancer, with the strongest evidence of an increase in risk in people at high risk of this type of cancer (smokers and people with occupational exposure to asbestos), and an increase in deaths from cardiovascular disease.
  • The Task Force’s recommendation against the use of vitamin E for cardiovascular disease prevention is based on a combined analysis of 4 studies (62,136 participants) of vitamin E supplementation, which showed no benefit of the supplements in preventing cardiovascular disease events.
  • The Task Force’s decision not to make a recommendation about the use of supplements of other single or paired nutrients—including antioxidants such as vitamin C and selenium—is based on a lack of adequate evidence on the benefits and harms of the supplements.

Can antioxidant supplements help to prevent cataracts?

No, according to a large body of research. A 2012 review of 9 high-quality studies (117,272 participants) that tested supplements of various antioxidants (vitamin E, vitamin C, beta-carotene) alone or in combinations found no evidence of an effect of the antioxidants on the occurrence or progression of cataracts.

Are antioxidant supplements helpful for age-related macular degeneration (AMD)?

Antioxidant supplements haven’t been shown to prevent AMD in people who don’t already have this disease, but certain supplements containing a combination of antioxidants and zinc have been shown to slow the progression of AMD.

• A 2017 review that examined the results of 5 studies (76,756 participants) did not find any significant benefit of vitamin E, vitamin C, or beta-carotene supplementation in preventing or delaying the onset of AMD.
• For people who already have AMD, supplements containing a combination of antioxidants and zinc may slow the progression of the disease. The evidence for this comes from two large studies sponsored by the National Institutes of Health—the Age-Related Eye Disease Study (AREDS) and Age-Related Eye Disease Study 2 (AREDS2).
  • AREDS evaluated the effects of a dietary supplement containing high doses of vitamins C and E, beta-carotene, zinc, and copper on the progression of AMD. Almost 4,800 people participated in this study, including 3,640 who had AMD. Among people with intermediate AMD, the supplement reduced the risk of developing advanced AMD by about 25 percent.
  • AREDS2 tested modifications to the original AREDS formula in about 4,200 people who were at high risk of progressing to advanced AMD. Participants were randomly assigned to groups that received the original formula or various modified versions. The modifications included removing beta-carotene and adding lutein and zeaxanthin (two carotenoids that are found in the eye). Because the link between beta-carotene and an increased risk of lung cancer in smokers was known before this study started, current smokers were not assigned to groups that received beta-carotene; only nonsmokers and former smokers were included in those groups. After 10 years of follow-up, lutein and zeaxanthin proved to be more effective than beta-carotene in reducing the risk of progression to advanced AMD. Beta-carotene increased lung cancer risk among former smokers, but lutein and zeaxanthin did not.

• A 2017 review concluded that supplements containing vitamin E with no other nutrients or lutein (with or without zeaxanthin) with no other nutrients may have little or no effect on progression to late AMD, but the evidence (from one study for vitamin E and six for lutein or lutein/zeaxanthin) was of low certainty.                       

Are antioxidants helpful for preserving cognitive function?

Two recent reviews looked at evidence related to this topic and found mixed results.

• A 2018 review examined 8 studies of antioxidant supplements (beta-carotene/vitamin A, vitamin C, and/or beta-carotene) that included about 47,000 total participants, all of whom were 40 years old or older at the beginning of the studies. There was low-certainty evidence of better overall cognitive function after taking beta-carotene for 18 years and after taking vitamin C for 5 to 10 years, but no effects were seen after shorter periods of supplementation or after taking vitamin E. The effects seen in the studies were small.
• Another 2018 review looked at studies of vitamin and mineral supplementation in people who already have mild cognitive impairment. Two of the studies involved antioxidants. In one study (516 participants), high-dose supplementation with vitamin E for 3 years did not have a significant effect on progression of mild cognitive impairment to dementia. In the other study, which involved combined vitamin E and C supplementation (256 participants), the evidence was too low in quality for any conclusions to be reached about cognitive effects. 

Are antioxidant supplements safe?

Not necessarily. Antioxidants can have harmful effects when taken at high doses.

• As discussed earlier, high-dose beta-carotene supplementation can increase people’s risks of lung cancer and cardiovascular disease. These effects have been seen primarily in people at high risk, such as smokers.
• High doses of vitamin E supplements may increase the risk of bleeding by reducing the blood’s ability to form clots after an injury. Vitamin E supplements may also interact with certain medicines, including anticoagulant or antiplatelet medicines.
• High doses of vitamin C can cause diarrhea, nausea, and stomach cramps. Vitamin C supplements may also interact with cancer treatments, such as chemotherapy and radiation therapy, and they can worsen iron overload in people with hemochromatosis, a condition that causes the body to store too much iron.
• Combinations of antioxidants may have undesirable effects. For example, in one study, a combination of vitamin E, vitamin C, selenium, and beta-carotene reduced the cholesterol-lowering effects of two drugs taken together for this purpose.

Tips To Consider

• Don’t use antioxidant supplements to replace a healthy diet or conventional medical care, or as a reason to postpone seeing a health care provider about a medical problem.
• If you have age-related macular degeneration, consult your health care providers to determine whether supplements of the types used in the AREDS or AREDS2 trials are appropriate for you.
• If you are considering a dietary supplement, first get information on it from reliable sources. Keep in mind that dietary supplements may interact with medications or other supplements and may contain ingredients not listed on the label. Your health care provider can advise you. If you are pregnant or nursing a child, or if you are considering giving a child a dietary supplement, it is especially important to consult your (or your child’s) health care provider.
• Take charge of your health—talk with your health care providers about any complementary health approaches you use. Together, you can make shared, well-informed decisions.

What are free radicals, and do they play a role in cancer development?

Free radicals are 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. 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.

Abnormally high concentrations of free radicals in the body can be caused by exposure to ionizing radiation and 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’s cells to produce more free radicals.

Free radicals that contain the element oxygen are the most common type of free radicals produced in living tissue. Another name for them is “reactive oxygen species,” or “ROS”.

What are antioxidants?

Antioxidants are chemicals that interact with and neutralize free radicals, thus preventing them from causing damage. Antioxidants are also known as “free radical scavengers.”

The body makes some of the antioxidants that it uses to neutralize free radicals. These antioxidants are called endogenous antioxidants. 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 dietary supplements.

Examples of dietary antioxidants include beta-carotene, lycopene, and vitamins A, C, and E (alpha-tocopherol). The mineral element selenium is 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.

Can antioxidant supplements help prevent cancer?

In laboratory and animal studies, the presence of increased levels of exogenous antioxidants has been shown to prevent the types of free radical damage that have been associated with cancer development. Therefore, researchers have investigated whether taking dietary antioxidant supplements can help lower the risk of developing or dying from cancer in humans.

Many observational studies, including case–control studies and cohort studies, 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. 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.

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 . Therefore, acquiring a more complete understanding 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.

Should people already diagnosed with cancer take antioxidant supplements?

Several randomized controlled trials, some including only small numbers of patients, have investigated whether taking antioxidant supplements during cancer treatment alters the effectiveness or reduces the toxicity of specific therapies. 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.

In some preclinical studies, antioxidants have been found to promote tumor growth and metastasis in tumor-bearing mice and to increase the ability of circulating tumor cells to metastasize. 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 dietary supplement.