Antioxidants Explained: Oxidation, Oxidative Stress, Food vs Supplements
What antioxidants, free radicals and oxidative stress are, why food and supplements are not the same thing, and why more antioxidant is not automatically better.
Content last updated . This page has not been medically reviewed.
The short answer
Antioxidants are chemicals that neutralize free radicals, which are unstable molecules that can damage cells. [1] Eating plenty of fruits and vegetables, which are rich in antioxidants, is linked with lower disease risk. Taking antioxidant pills has not shown the same benefit, with one tested exception in eye disease. [2] This page explains the ideas; the pages linked below cover each use and each ingredient.Evidence labels used in this section: established nutritional role, evidence in adequately nourished people, promising but limited, mixed, insufficient evidence, preclinical (lab or animal work only) and marketing claim (a promise with no human evidence behind it).
Oxidation, free radicals and oxidative stress
Free radicals form naturally in the body and play an important role in many normal cell processes. At high concentrations they can damage DNA, proteins and cell membranes. [1] Those that contain oxygen are the most common type and are also called reactive oxygen species. [1] Some exposures, such as ionizing radiation and cigarette smoke, add to the load. [1]"Oxidative stress" is the name for overproduction of these reactive substances. The NCCIH notes that it may damage cells and contribute to chronic diseases such as heart disease and cancer, and that the body has its own repair and prevention systems, with antioxidant enzymes the most important part of that defense. [2]
Where antioxidants come from
The body makes some antioxidants itself and gets the rest mainly from the diet. Fruits, vegetables and grains are rich sources. [1] Examples named by the National Cancer Institute include beta-carotene, lycopene and vitamins A, C and E. [1] Some are also sold as supplements, such as vitamins C and E and beta-carotene. [2]Food and supplements are not the same
| Question | What the sources found | Strength of evidence |
|---|---|---|
| Do people who eat more fruits and vegetables have lower risk? | A 2017 review of 95 observational studies (over 2 million people) linked higher intake with lower risk of cardiovascular disease and cancer. [2] | Observational: shows an association, not proof of cause |
| Do antioxidant supplements prevent heart disease or cancer? | The NCCIH says current evidence does not support this. [2] | Evidence in adequately nourished people: no benefit shown |
| Do they lower the chance of dying? | A Cochrane review of 78 trials (296,707 people) found no evidence to support antioxidant supplements for prevention, and beta-carotene and vitamin E seemed to increase mortality. [3] | Mixed to unfavorable |
| Is there any proven supplement use? | Specific antioxidant-plus-zinc formulas slowed progression in people who already have intermediate age-related macular degeneration. [2] | Tested formulation, see evidence by use |
The NCCIH lists reasons the food and pill results differ: other substances in the food or the lifestyle that goes with it may explain the benefit, purified chemicals may act differently from the mixtures in foods, and supplement doses are higher than amounts in food. [2]
Why more is not automatically better
The NCCIH says large amounts of antioxidants may interfere with important cell functions, including defenses and normal signaling, and that different antioxidants are not interchangeable. [2] In one small trial, a vitamin C and E combination blocked some of the insulin-sensitizing effects of exercise. [4] Details and caveats are on the evidence by use page.Marketing numbers such as ORAC
ORAC is a laboratory score of antioxidant capacity. According to a trade-press report, the USDA removed its ORAC food database in 2012, saying the values were routinely misused to promote products and that laboratory capacity data cannot be extrapolated to effects in the body. [5] See antioxidant safety and marketing claims.Where to go next
Vitamin C and vitamin E have their own guides: vitamin C and vitamin E. For curcumin and green tea see turmeric and green tea; for foods see Functional Foods. Before taking any antioxidant product, check medication interactions and talk with a clinician if you take medicines or are being treated for cancer.Frequently asked questions
Are antioxidants good or bad?
Both words oversimplify. Free radicals have normal roles in cells, and the body has its own antioxidant defenses. [1][2] Foods rich in antioxidants are linked with better health, while high-dose supplements have not prevented disease and some have raised risk. [2]
Should I take an antioxidant supplement?
For most people the evidence does not support it for preventing disease. [2] A person with age-related macular degeneration should ask an eye doctor about the specific formulas tested in the AREDS trials. [2]
Is oxidative stress something I can measure?
This page does not claim a reliable at-home or routine test. Measures that rise or fall in a study (biomarkers) are not the same as better health. See the claims page linked above.
Do antioxidant foods work better than pills?
Observational studies link antioxidant-rich foods with lower disease risk, but these cannot prove the antioxidants are the cause. [2]
References
- National Cancer Institute, Antioxidants and Cancer Prevention. Read 2026-10-22.
- NCCIH, Antioxidant Supplements: What You Need To Know. Read 2026-10-22.
- Bjelakovic et al., Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases (Cochrane Database Syst Rev 2012). Read 2026-10-22.
- Ristow et al., Antioxidants prevent health-promoting effects of physical exercise in humans (PNAS 2009). Read 2026-10-22.
- SupplySide Supplement Journal, USDA Says ORAC Tests Useless, Removes Database for Selected Foods (June 12, 2012; trade press). Read 2026-10-22.