Key Points

Vegetables and fruits are rich sources of antioxidants. There is good evidence that eating a diet that includes plenty of vegetables and fruits is healthy, and official U.S. Government policy urges people to eat more of these foods. Research has shown that people who eat more vegetables and fruits have lower risks of several diseases; however, it is not clear whether these results are related to the number of antioxidants in vegetables and fruits, to other components of these foods, to other factors in people’s diets, or other lifestyle choices.
Rigorous scientific studies involving more than 100,000 people combined have tested whether antioxidant supplements can help prevent chronic diseases, such as cardiovascular diseases, cancer, and cataracts. In most instances, antioxidants did not reduce the risks of developing these diseases.
Concerns have not been raised about the safety of antioxidants in food. However, high-dose supplements of antioxidants may be linked to health risks in some cases. Supplementing with high doses of beta-carotene may increase the risk of lung cancer in smokers. Supplementing with high doses of vitamin E may increase the risks of prostate cancer and one type of stroke.
Antioxidant supplements may interact with some medicines.
Tell all of your healthcare providers about any complementary and integrative health approaches you use. Give them a full picture of what you do to manage your health. This will help ensure coordinated and safe care.

About Free Radicals, Oxidative Stress, and Antioxidants

Free radicals are highly unstable molecules that are naturally formed when you exercise and when your body converts food into energy. Your body can also be exposed to free radicals from a variety of environmental sources, such as cigarette smoke, air pollution, and sunlight. Free radicals can cause “oxidative stress,” a process that can trigger cell damage. Oxidative stress is thought to play a role in a variety of diseases including cancer, cardiovascular diseases, diabetes, Alzheimer’s disease, Parkinson’s disease, and eye diseases such as cataracts and age-related macular degeneration.

Antioxidant molecules have been shown to counteract oxidative stress in laboratory experiments (for example, in cells or animal studies). However, there is debate as to whether consuming large amounts of antioxidants in supplement form benefits health. There is also some concern that consuming antioxidant supplements in excessive doses may be harmful.

Vegetables and fruits are healthy foods and rich sources of antioxidants. Official U.S. Government policy urges people to eat more vegetables and fruits. Concerns have not been raised about the safety of any amounts of antioxidants in food. For more information on antioxidants in foods, visit the U.S. Department of Agriculture webpage on antioxidants and phytonutrients.

Use of Antioxidant Supplements in the United States

A 2009 analysis using data from the National Health and Nutrition Examination Survey (1999–2000 and 2001–2002) estimated the amounts of antioxidants adults in the United States get from foods and supplements. Supplements accounted for 54 percent of vitamin C, 64 percent of vitamin E, 14 percent of alpha- and beta-carotene, and 11 percent of selenium intake.

Safety

High-dose antioxidant supplements may be harmful in some cases. For example, the results of some studies have linked the use of high-dose beta-carotene supplements to an increased risk of lung cancer in smokers and the use of high-dose vitamin E supplements to increased risks of hemorrhagic stroke (a type of stroke caused by bleeding in the brain) and prostate cancer.
Like some other dietary supplements, antioxidant supplements may interact with certain medications. For example, vitamin E supplements may increase the risk of bleeding in people who are taking anticoagulant drugs (“blood thinners”). There is conflicting evidence on the effects of taking antioxidant supplements during cancer treatment; some studies suggest that this may be beneficial, but others suggest that it may be harmful. The National Cancer Institute recommends that people who are being treated for cancer talk with their healthcare provider before taking supplements.

For more information about the safety of dietary supplements, see the National Center for Complementary and Integrative Health (NCCIH) fact sheet Using Dietary Supplements Wisely.

What the Science Says

Several decades of dietary research findings suggested that consuming greater amounts of antioxidant-rich foods might help to protect against diseases. Because of these results, there has been a lot of research on antioxidant supplements. Rigorous trials of antioxidant supplements in large numbers of people have not found that high doses of antioxidant supplements prevent disease. This section describes the preliminary research findings, the results of the clinical trials, and possible explanations for the differences in study results.
Observational and Laboratory Studies
Observational studies on the typical eating habits, lifestyles, and health histories of large groups of people have shown that those who ate more vegetables and fruits had lower risks of several diseases, including cardiovascular disease, stroke, cancer, and cataracts. Observational studies can provide ideas about possible relationships between dietary or lifestyle factors and disease risk, but they cannot show that one factor causes another because they cannot account for other factors that may be involved. For example, people who eat more antioxidant-rich foods might also be more likely to exercise and less likely to smoke. It may be that these factors, rather than antioxidants, account for their lower disease risk.

Researchers have also studied antioxidants in laboratory experiments. These experiments showed that antioxidants interacted with free radicals and stabilized them, thus preventing the free radicals from causing cell damage.
Clinical Trials of Antioxidants
Because the results of such research seemed very promising, large, long-term studies—many of which were funded by the National Institutes of Health (NIH)—were conducted to test whether antioxidant supplements, when taken for periods of at least a few years, could help prevent diseases such as cardiovascular diseases and cancer in people. In these studies, volunteers were randomly assigned to take either an antioxidant or a placebo (an identical-looking product that did not contain the antioxidant). The research was conducted in a double-blind manner (neither the study participants nor the investigators knew which product was being taken). Studies of this type—called clinical trials—are designed to provide clear answers to specific questions about how a substance affects people’s health.

Among the earliest of these studies were three large NIH-sponsored trials of high-dose supplements of beta-carotene, alone or in combination with other nutrients. These trials, completed in the mid-1990s, all showed that beta-carotene did not protect against cancer or cardiovascular disease. In one trial, beta-carotene supplements increased the risk of lung cancer in smokers, and in another trial, supplements containing both beta-carotene and vitamin A had the same effect.

More recent studies have also found that in most instances antioxidant supplements did not help to prevent disease. For example:

The Women’s Health Study, which included almost 40,000 healthy women at least 45 years of age, found that vitamin E supplements did not reduce the risk of heart attack, stroke, cancer, age-related macular degeneration, or cataracts. Although vitamin E supplements were associated with fewer deaths from cardiovascular causes, they did not reduce the overall death rate of study participants.
The Women’s Antioxidant Cardiovascular Study found no beneficial effects of vitamin C, vitamin E, or beta-carotene supplements on cardiovascular events (heart attack, stroke, or death from cardiovascular diseases) or the likelihood of developing diabetes or cancer in more than 8,000 female health professionals, aged 40 years or older, who were at high risk for cardiovascular disease. Antioxidant supplements also did not slow changes in cognitive function among women in this study who were aged 65 or older.
The Physicians’ Health Study II, which included more than 14,000 male physicians aged 50 or older, found that neither vitamin E nor vitamin C supplements reduced the risk of major cardiovascular events (heart attack, stroke, or death from cardiovascular disease), cancer, or cataracts. Vitamin E supplements were associated with an increased risk of hemorrhagic stroke in this study.
The Selenium and Vitamin E Cancer Prevention Trial (SELECT)—a study of more than 35,000 men aged 50 or older—found that selenium and vitamin E supplements, taken alone or together, did not prevent prostate cancer. A 2011 updated analysis from this trial, based on a longer follow-up period of study participants, concluded that vitamin E supplements increased the occurrence of prostate cancer by 17 percent in men who received the vitamin E supplement alone compared with those who received a placebo. There was no increase in prostate cancer when vitamin E and selenium were taken together.

Unlike the studies described above, two large studies on age-related macular degeneration (AMD), a leading cause of vision loss in older adults, found a beneficial effect of dietary supplements that include antioxidants. Both studies were sponsored by the National Institutes of Health.

The first study, called AREDS (for Age-Related Eye Disease Study), showed that a combination of antioxidants (vitamin C, vitamin E, and beta-carotene) plus zinc and copper reduced the risk of progression of intermediate AMD to advanced AMD by 25 percent over 5 years. However, during the period when this study was performed, other research showed that taking beta-carotene supplements increases the risk of lung cancer in people who smoke cigarettes.
The second study, called AREDS2 (for Age-Related Eye Disease Study 2), showed that the carotenoids lutein and zeaxanthin are an appropriate replacement for beta-carotene in the supplement formula. Unlike beta-carotene, lutein and zeaxanthin did not increase lung cancer risk, and 10-year follow-up data on the study participants showed that lutein and zeaxanthin were more effective than beta-carotene in reducing the risk of progression of AMD.

Why Don’t Antioxidant Supplements Work?

Question

Key Points

Vegetables and fruits are rich sources of antioxidants. There is good evidence that eating a diet that includes plenty of vegetables and fruits is healthy, and official U.S. Government policy urges people to eat more of these foods. Research has shown that people who eat more vegetables and fruits have lower risks of several diseases; however, it is not clear whether these results are related to the number of antioxidants in vegetables and fruits, to other components of these foods, to other factors in people’s diets, or other lifestyle choices.
    Rigorous scientific studies involving more than 100,000 people combined have tested whether antioxidant supplements can help prevent chronic diseases, such as cardiovascular diseases, cancer, and cataracts. In most instances, antioxidants did not reduce the risks of developing these diseases.
    Concerns have not been raised about the safety of antioxidants in food. However, high-dose supplements of antioxidants may be linked to health risks in some cases. Supplementing with high doses of beta-carotene may increase the risk of lung cancer in smokers. Supplementing with high doses of vitamin E may increase the risks of prostate cancer and one type of stroke.
    Antioxidant supplements may interact with some medicines.
    Tell all of your healthcare providers about any complementary and integrative health approaches you use. Give them a full picture of what you do to manage your health. This will help ensure coordinated and safe care.

About Free Radicals, Oxidative Stress, and Antioxidants

Free radicals are highly unstable molecules that are naturally formed when you exercise and when your body converts food into energy. Your body can also be exposed to free radicals from a variety of environmental sources, such as cigarette smoke, air pollution, and sunlight. Free radicals can cause “oxidative stress,” a process that can trigger cell damage. Oxidative stress is thought to play a role in a variety of diseases including cancer, cardiovascular diseases, diabetes, Alzheimer’s disease, Parkinson’s disease, and eye diseases such as cataracts and age-related macular degeneration.

Antioxidant molecules have been shown to counteract oxidative stress in laboratory experiments (for example, in cells or animal studies). However, there is debate as to whether consuming large amounts of antioxidants in supplement form benefits health. There is also some concern that consuming antioxidant supplements in excessive doses may be harmful.

Vegetables and fruits are healthy foods and rich sources of antioxidants. Official U.S. Government policy urges people to eat more vegetables and fruits. Concerns have not been raised about the safety of any amounts of antioxidants in food. For more information on antioxidants in foods, visit the U.S. Department of Agriculture webpage on antioxidants and phytonutrients.

Use of Antioxidant Supplements in the United States

A 2009 analysis using data from the National Health and Nutrition Examination Survey (1999–2000 and 2001–2002) estimated the amounts of antioxidants adults in the United States get from foods and supplements. Supplements accounted for 54 percent of vitamin C, 64 percent of vitamin E, 14 percent of alpha- and beta-carotene, and 11 percent of selenium intake.

Safety

High-dose antioxidant supplements may be harmful in some cases. For example, the results of some studies have linked the use of high-dose beta-carotene supplements to an increased risk of lung cancer in smokers and the use of high-dose vitamin E supplements to increased risks of hemorrhagic stroke (a type of stroke caused by bleeding in the brain) and prostate cancer.
    Like some other dietary supplements, antioxidant supplements may interact with certain medications. For example, vitamin E supplements may increase the risk of bleeding in people who are taking anticoagulant drugs (“blood thinners”). There is conflicting evidence on the effects of taking antioxidant supplements during cancer treatment; some studies suggest that this may be beneficial, but others suggest that it may be harmful. The National Cancer Institute recommends that people who are being treated for cancer talk with their healthcare provider before taking supplements.

For more information about the safety of dietary supplements, see the National Center for Complementary and Integrative Health (NCCIH) fact sheet Using Dietary Supplements Wisely.

What the Science Says

Several decades of dietary research findings suggested that consuming greater amounts of antioxidant-rich foods might help to protect against diseases. Because of these results, there has been a lot of research on antioxidant supplements. Rigorous trials of antioxidant supplements in large numbers of people have not found that high doses of antioxidant supplements prevent disease. This section describes the preliminary research findings, the results of the clinical trials, and possible explanations for the differences in study results.
Observational and Laboratory Studies
Observational studies on the typical eating habits, lifestyles, and health histories of large groups of people have shown that those who ate more vegetables and fruits had lower risks of several diseases, including cardiovascular disease, stroke, cancer, and cataracts. Observational studies can provide ideas about possible relationships between dietary or lifestyle factors and disease risk, but they cannot show that one factor causes another because they cannot account for other factors that may be involved. For example, people who eat more antioxidant-rich foods might also be more likely to exercise and less likely to smoke. It may be that these factors, rather than antioxidants, account for their lower disease risk.

Researchers have also studied antioxidants in laboratory experiments. These experiments showed that antioxidants interacted with free radicals and stabilized them, thus preventing the free radicals from causing cell damage.
Clinical Trials of Antioxidants
Because the results of such research seemed very promising, large, long-term studies—many of which were funded by the National Institutes of Health (NIH)—were conducted to test whether antioxidant supplements, when taken for periods of at least a few years, could help prevent diseases such as cardiovascular diseases and cancer in people. In these studies, volunteers were randomly assigned to take either an antioxidant or a placebo (an identical-looking product that did not contain the antioxidant). The research was conducted in a double-blind manner (neither the study participants nor the investigators knew which product was being taken). Studies of this type—called clinical trials—are designed to provide clear answers to specific questions about how a substance affects people’s health.

Among the earliest of these studies were three large NIH-sponsored trials of high-dose supplements of beta-carotene, alone or in combination with other nutrients. These trials, completed in the mid-1990s, all showed that beta-carotene did not protect against cancer or cardiovascular disease. In one trial, beta-carotene supplements increased the risk of lung cancer in smokers, and in another trial, supplements containing both beta-carotene and vitamin A had the same effect.

More recent studies have also found that in most instances antioxidant supplements did not help to prevent disease. For example:

The Women’s Health Study, which included almost 40,000 healthy women at least 45 years of age, found that vitamin E supplements did not reduce the risk of heart attack, stroke, cancer, age-related macular degeneration, or cataracts. Although vitamin E supplements were associated with fewer deaths from cardiovascular causes, they did not reduce the overall death rate of study participants.
    The Women’s Antioxidant Cardiovascular Study found no beneficial effects of vitamin C, vitamin E, or beta-carotene supplements on cardiovascular events (heart attack, stroke, or death from cardiovascular diseases) or the likelihood of developing diabetes or cancer in more than 8,000 female health professionals, aged 40 years or older, who were at high risk for cardiovascular disease. Antioxidant supplements also did not slow changes in cognitive function among women in this study who were aged 65 or older.
    The Physicians’ Health Study II, which included more than 14,000 male physicians aged 50 or older, found that neither vitamin E nor vitamin C supplements reduced the risk of major cardiovascular events (heart attack, stroke, or death from cardiovascular disease), cancer, or cataracts. Vitamin E supplements were associated with an increased risk of hemorrhagic stroke in this study.
    The Selenium and Vitamin E Cancer Prevention Trial (SELECT)—a study of more than 35,000 men aged 50 or older—found that selenium and vitamin E supplements, taken alone or together, did not prevent prostate cancer. A 2011 updated analysis from this trial, based on a longer follow-up period of study participants, concluded that vitamin E supplements increased the occurrence of prostate cancer by 17 percent in men who received the vitamin E supplement alone compared with those who received a placebo. There was no increase in prostate cancer when vitamin E and selenium were taken together.

Unlike the studies described above, two large studies on age-related macular degeneration (AMD), a leading cause of vision loss in older adults, found a beneficial effect of dietary supplements that include antioxidants. Both studies were sponsored by the National Institutes of Health.

The first study, called AREDS (for Age-Related Eye Disease Study), showed that a combination of antioxidants (vitamin C, vitamin E, and beta-carotene) plus zinc and copper reduced the risk of progression of intermediate AMD to advanced AMD by 25 percent over 5 years. However, during the period when this study was performed, other research showed that taking beta-carotene supplements increases the risk of lung cancer in people who smoke cigarettes.
    The second study, called AREDS2 (for Age-Related Eye Disease Study 2), showed that the carotenoids lutein and zeaxanthin are an appropriate replacement for beta-carotene in the supplement formula. Unlike beta-carotene, lutein and zeaxanthin did not increase lung cancer risk, and 10-year follow-up data on the study participants showed that lutein and zeaxanthin were more effective than beta-carotene in reducing the risk of progression of AMD.

Why Don’t Antioxidant Supplements Work?

Accepted Answer

Most clinical studies of antioxidant supplements have not found them to provide substantial health benefits. Researchers have suggested several reasons for this, including the following: The beneficial health effects of a diet high in vegetables and fruits or other antioxidant-rich foods may be caused by other substances present in the same foods, other dietary factors, or other lifestyle choices rather than antioxidants. The effects of the large doses of antioxidants used in supplementation studies may be different from those of&hellip;

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Article Summary

Key Takeaways

Seek urgent medical care if you notice

Emergency now

See a doctor

Learn safely

Key Points

About Free Radicals, Oxidative Stress, and Antioxidants

Use of Antioxidant Supplements in the United States

Safety

What the Science Says

Observational and Laboratory Studies

Clinical Trials of Antioxidants

Why Don’t Antioxidant Supplements Work?

If You Are Considering Antioxidant Supplements

NCCIH- and NIH-Funded Research

Prepare before seeing a doctor

Which doctor may help?

What to tell the doctor

Questions to ask

Tests to discuss

Avoid these mistakes

Medicine safety and first-aid guide

Safe first steps

OTC medicine safety

Avoid these mistakes

Get urgent help if

Patient health record and symptom diary

Care roadmap for: Meaning of Antioxidants

Check danger signs first

Record the symptom story

Visit a qualified clinician

Do only useful tests

Follow up and return early if worse

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Frequently Asked Questions

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