In brief
When the body makes more reactive oxygen-containing substances than it can handle. Cells make these substances during normal activities, and too many can damage cells.
- Antioxidants taken as purified chemicals might act differently from those eaten in foods.
- Antioxidant supplements have not been shown to prevent age-related macular degeneration in people who do not already have it.
- Smoking increases the amount of vitamin C the body needs to repair damage caused by free radicals.
How it works
During their normal activities, your cells produce highly reactive substances that contain oxygen, including free radicals formed when the body turns food into energy. Overproduction of these substances can lead to a condition called oxidative stress, which may damage cells. Vitamin C acts as an antioxidant in the body.
People who eat more vegetables and fruit have lower risks of several chronic diseases, but rigorous trials have not shown that antioxidant supplements prevent cancer or heart disease. High-dose beta-carotene supplements increased the risk of lung cancer and heart disease in people at high risk, such as smokers, and high doses of vitamin E may increase the risk of bleeding.
Connections
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Oxidative stress can be raised by
Smoking increases the amount of vitamin C the body needs to repair damage caused by free radicals.
Source: NIH Office of Dietary Supplements, Vitamin C fact sheet for consumers
Oxidative stress can be lowered by
Vitamin C acts as an antioxidant in the body.
Source: NIH Office of Dietary Supplements, Vitamin C fact sheet for consumers
Sources
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Summarised from the NIH National Center for Complementary and Integrative Health and the NIH Office of Dietary Supplements. Sources read on 13 October 2026. General education only, not medical advice. Spotted a mistake? Please tell us through the HuBu-Intel contact page.