Oxidative Stress and Antioxidant Defenses

What oxidative stress is

The NIH's National Center for Complementary and Integrative Health (NCCIH) explains that cells produce highly reactive substances that contain oxygen during normal activity, and overproduction can lead to "oxidative stress," which may damage cells. [1] The body has preventive and repair mechanisms, and antioxidant enzymes are described as the most important part of this defense system. [1] A 2020 review in the Journal of Biological Chemistry describes oxidative stress as two-sided: "oxidative eustress" is part of normal signaling, while higher levels cause damage ("oxidative distress"). [2] So "oxidative stress" is not simply bad, and zero is not the goal.

Antioxidants in food versus pills

MedlinePlus says eating plenty of vegetables and fruits is linked with lower risks of certain diseases, but it is not clear whether that is because of the antioxidants, something else in the foods, or other factors. [3] Food-based patterns are covered under dietary patterns and healthy aging.

What trials of antioxidant supplements show

• Mortality. A Cochrane review of 78 randomized trials with 296,707 participants (mean age 63) tested beta-carotene, vitamin A, vitamin C, vitamin E and selenium against placebo or no treatment. Overall, supplements had no significant effect on mortality in the main analysis. [4] In the trials with low risk of bias, beta-carotene and vitamin E were associated with higher mortality, and vitamin C and selenium were not. [4] The reviewers concluded there was no evidence to support antioxidant supplements for primary or secondary prevention. [4] Label: no good evidence of benefit, with possible evidence of harm for some products.
• Cancer. NCCIH says current evidence does not support the idea that antioxidant supplements can prevent cancer, and high doses of beta-carotene may increase lung cancer risk. [1] It reports that the US Preventive Services Task Force in 2022 recommended against beta-carotene or vitamin E supplements for cancer prevention. [1]
• Other harms. High doses of vitamin E may raise bleeding risk and may interact with anticoagulant or antiplatelet medicines. [1] MedlinePlus likewise notes antioxidant supplements may interact with some medicines. [3]
• Exercise. In 39 healthy young men, a 4-week exercise program improved insulin sensitivity only in the absence of vitamin C and vitamin E supplements (study amounts: 1000 mg and 400 IU daily). [5] The authors interpret this as consistent with "mitohormesis", the idea that mild oxidative stress triggers helpful adaptation. [5] This was a small, short study in young men, so it does not show the same in older adults. Label: promising but limited.

Why the "antioxidants slow aging" idea has stalled

The mechanisms in cells and animals are real, but reactive molecules also act as signals, so removing them is not simply protective. [2] The trials above tested clinical outcomes in people and did not find a longevity benefit. A change in a lab measure of oxidation would be a biomarker, not an outcome. If a product claims to "reverse aging" by neutralizing "free radicals", treat it as a marketing claim unless trial evidence is shown. See the claim check on supplements that reverse aging.