Antioxidants Explained: Oxidation, Oxidative Stress, Food vs Supplements

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] Free radicals that contain oxygen are the most common type in living tissue and are called reactive oxygen species (ROS). [1] The NCCIH describes "oxidative stress" as overproduction of reactive oxygen-containing substances, which may damage cells and contribute to chronic disease. [2] Researchers who study the field warn that the term is sometimes used loosely, without a clear link to the underlying chemistry. [4] Details are on what free radicals and ROS are and what oxidative stress means and how it is measured.

Where antioxidants come from

The body makes some antioxidants itself. It gets the rest mainly from the diet, and fruits, vegetables and grains are rich sources. [1] The body's main protection comes from its own enzymes, not from small molecules in food or pills. [4] See the body's own antioxidant defenses and the main categories of antioxidants.

Food and supplements are not the same

1. Do people who eat more fruits and vegetables have lower risk?

   Observational: an association, not proof of cause

   A 2017 review of 95 observational studies with more than 2 million participants linked higher intake with lower risk of cardiovascular disease and cancer. [2]

2. Do antioxidant supplements prevent heart disease or cancer?

   No good evidence of benefit in adequately nourished people

   The NCCIH says current evidence does not support this. [2]

3. Do they change the chance of dying?

   Evidence of harm for some forms

   A Cochrane review of 78 randomized trials (296,707 participants) found no evidence to support antioxidant supplements for prevention. Beta-carotene and vitamin E seemed to increase mortality. [3]

4. Is there a tested use?

   Specific tested formulas only: see evidence by use

   Certain supplements containing antioxidants and zinc may slow progression in people who already have age-related macular degeneration (AREDS and AREDS2 formulas). [2]

The NCCIH lists reasons the results differ: other substances in the food or the lifestyle that goes with it may explain the benefit, purified chemicals might act differently from food mixtures, and supplement doses may have different effects than the smaller amounts in foods. [2] NCI makes a similar point. [1] See antioxidant foods vs supplements.

Why more is not automatically better

The NCCIH says large amounts of antioxidants may interfere with important functions in the cell, including its defense mechanisms and normal signaling, and that different antioxidants are not interchangeable. [2] In one small trial of 39 healthy young men, vitamin C and vitamin E (study amounts, 4 weeks) blocked the rise in insulin sensitivity that exercise otherwise produced. [5] That is a single small study of a biomarker-style outcome, so treat it as a caution, not a rule.

Marketing numbers and "total antioxidant capacity"

Laboratory "capacity" scores, such as ORAC, measure how a food sample behaves in a chemical test. A review of total antioxidant capacity assays found that the values for foods cannot be translated to the body's own antioxidant defenses or to health effects. [6] We could not read an official USDA statement about its withdrawn ORAC database, so we do not cite one here.

Map of the Antioxidants library

Vitamin C and E have their own guides: vitamin C and vitamin E. For curcumin and green tea herbs see turmeric and green tea. The full category page is Antioxidants.