What Are Antioxidants?

The body naturally produces reactive molecules containing oxygen during normal cellular activity. When produced in excess, these molecules can cause oxidative stress, potentially harming cells and increasing the risk of chronic illnesses such as cancer and heart disease. While the body has systems in place to control this process, antioxidant enzymes play a key role in managing stress levels. In addition, compounds from plants, known as phytochemicals, can also function as antioxidants and influence these cellular pathways. These molecular compounds are the main topic of this overview.

However, increasing our intake of antioxidants beyond typical levels isn't always positive. Large quantities might interfere with normal cellular processes, including communication and defense systems. Furthermore, different antioxidants possess unique traits and are not necessarily interchangeable.

Sources of Antioxidants

Our bodies can produce some antioxidants on their own. Others, however, must be obtained from food—particularly fruits, vegetables, and whole grains. Additionally, antioxidants like vitamin C, vitamin E, and beta-carotene are commonly available in supplement form.

Scientific Research on Antioxidants

Significant studies have evaluated how antioxidant supplements relate to chronic conditions associated with aging. Much attention has been given to vitamins E and C, beta-carotene, and other carotenoids. Research has looked at their potential effects on diseases like cancer, cardiovascular conditions, age-related eye disorders (such as macular degeneration and cataracts), and cognitive decline.

Study Outcomes

Except in the case of macular degeneration, there’s little supporting evidence that antioxidant supplements prevent chronic illnesses.

Conversely, diets rich in fruits and vegetables—foods high in antioxidants—tend to lower the risk of numerous diseases. For instance:

• A wide-ranging review in 2017 of nearly 100 observational studies revealed that those with higher fruit and vegetable intake had a lower risk of cancer and heart disease.
• A 2023 U.K. study linked increased consumption of these foods with fewer cataracts over a nine-year period among more than 72,000 participants.

This contrast may be due to additional beneficial compounds in whole foods, interactions between various nutrients in foods that aren’t replicated in pills, or healthier lifestyle behaviors among individuals who eat more produce.

Do Antioxidants Prevent Cancer?

Current studies suggest antioxidant supplements do not help prevent cancer. In fact, large doses of beta-carotene might increase the risk of developing lung cancer.

Research Findings

• In 2022, expert recommendations advised against the use of beta-carotene or vitamin E to prevent cancer, based on extensive research involving over 90,000 participants.
• These studies showed beta-carotene raised the risk of lung cancer and death from heart disease, particularly in smokers and those exposed to asbestos.
• Vitamin E showed no noticeable benefit in lowering cancer risks across five major studies.
• Due to limited conclusive evidence, no recommendations were made regarding other individual antioxidant supplements such as vitamin C or selenium.

Antioxidants and Heart Disease Prevention

There’s no solid proof that antioxidant supplements can prevent cardiovascular disease. In fact, certain antioxidants like beta-carotene—if taken in high doses—may increase mortality from heart-related conditions.

Key Points

• The U.S. Preventive Services Task Force advised against the use of beta-carotene and vitamin E to prevent cardiovascular problems, citing a lack of benefits in large-scale studies.
• Beta-carotene supplementation was linked to increased cases of lung cancer and cardiovascular deaths, particularly among people with higher risk exposures.
• Vitamin E supplements didn’t reduce heart disease events in the studied population.
• Other antioxidants like vitamin C and selenium couldn’t be assessed due to insufficient research data.

Antioxidants and Cataracts

Evidence suggests that antioxidant supplements do not lower one’s risk of developing cataracts. A review from 2012 examining nearly 120,000 participants found no link between supplement use and reduced cataract risk or progression.

Benefits for Age-Related Macular Degeneration (AMD)

Supplements don’t seem to stop AMD from beginning but may help slow its advancement in those already diagnosed with it.

Detailed Outcomes

• A 2017 analysis of multiple studies found no significant benefit from vitamins E, C, or beta-carotene in preventing AMD among healthy individuals.
• For those with intermediate AMD, a specific supplement mix reduced progression risks by around 25%. This formula included vitamins C and E, beta-carotene, zinc, and copper.
• Later research studied updated versions of this mix, replacing beta-carotene with lutein and zeaxanthin—antioxidants naturally found in the eye—due to associated cancer risks in smokers. These replacements appeared more effective and safer.

Antioxidants and Brain Health

Studies into whether antioxidants help preserve memory and cognitive skills have so far shown inconsistent results.

Review Highlights

• One review suggested long-term use of beta-carotene or vitamin C may offer slight cognitive improvements, though short-term effects and vitamin E showed no benefit.
• Another review investigating antioxidants in people already experiencing mild cognitive issues found no consistent results. In one case, high-dose vitamin E did not prevent the onset of dementia.

Are Antioxidant Supplements Safe?

High doses of antioxidants can carry health risks and should be approached with care.

Potential Side Effects

• Beta-carotene taken in large amounts can raise the likelihood of lung cancer and cardiovascular disease, especially in smokers.
• Vitamin E may increase bleeding risk and interfere with medications meant to manage blood clotting.
• Too much vitamin C could cause digestive upset and alter the effectiveness of certain cancer treatments or worsen iron overload disorders.
• Some antioxidant combinations may reduce the effectiveness of medications such as those used to lower cholesterol levels.

Should Cancer Patients Take Antioxidants?

Cancer patients should speak with their medical team before taking any antioxidant supplements, as these could undermine treatment.

Important Considerations

• Cancer therapies often kill cancer cells by causing oxidative stress. Supplements that neutralize this process might reduce the treatment’s effectiveness.
• In a study involving over 1,100 women with breast cancer, antioxidant supplement use during chemotherapy was associated with higher chances of recurrence and death.

Practical Guidelines

• Rely on a healthy diet rather than supplements to meet your needs whenever possible. Do not delay medical care in favor of supplement use.
• If you have AMD, ask your doctor about supplement options modeled on AREDS or AREDS2 findings.
• Always research supplements thoroughly and seek guidance from medical professionals, especially if pregnant, nursing, or choosing supplements for children.
• Communicate all supplement use with your healthcare providers to make informed, joint decisions.

Sources and References

The information summarized here relies on a range of peer-reviewed studies and government-supported research. See full reports and studies for comprehensive details on study methods and outcomes.

Acknowledgments

We thank the experts at NCCIH for reviewing and updating this material in 2023.

This article is in the public domain. You are free to distribute it. It does not replace professional medical advice. Discuss medical choices with a qualified provider.