Updated on November 3, 2025, to include expanded information.

Antioxidants

Antioxidants are compounds that help protect cells from damage caused by free radicals—highly reactive molecules produced during normal metabolism (and from things like pollution, UV light, and cigarette smoke).

Free radicals steal electrons from molecules, creating a chain reaction that damages DNA, fats, and proteins. Antioxidants donate electrons to neutralize these free radicals or help repair the damage.

There are two types of antioxidants: 

1. Enzymatic antioxidants (made by the body): examples include superoxide dismutase (SOD), catalase, and glutathione peroxidase.

2. Non-enzymatic antioxidants (from foods and supplements): examples include vitamins C and E, carotenoids (like beta-carotene and lycopene), flavonoids, and selenium.

Dietary sources: A wide variety of fruits, vegetables, nuts, seeds, whole grains, legumes, and green tea. Colorful produce (berries, citrus, leafy greens, peppers) tends to be rich in antioxidants.

A diet rich in antioxidant-rich foods is associated with various health benefits in observational studies. However, taking high-dose antioxidant supplements does not consistently show the same benefits and can be harmful in some situations. It’s generally best to get antioxidants from a balanced, plant-focused diet unless a healthcare provider advises otherwise.

Common antioxidants and how they work

Direct, Non-enzymatic Antioxidants

These antioxidants rate-limit or stop damage directly. 

Vitamin C (ascorbic acid): It directly scavenges several reactive species (e.g., reactive oxygen species (ROS), like hydroxyl radicals, superoxide, and singlet oxygen). It also helps regenerate vitamin E back to its active form. It particularly operates in watery parts of the cell and in the bloodstream, aiding in iron absorption and helping to maintain redox balance. Typical sources are citrus fruits, berries, kiwi, bell peppers, and broccoli.

Vitamin E (α-tocopherol and other tocopherols): This is a lipid-soluble “chain-breaking” antioxidant that stops lipid peroxidation in cell membranes and lipoprotein particles. The tocopheroxyl radical formed after scavenging is recycled back by vitamin C and other systems.
Typical sources are nuts, seeds, vegetable oils, and leafy greens.

Carotenoids (beta-carotene, lycopene, lutein, zeaxanthin, etc.): These quench singlet oxygen and scavenge free radicals. Some can be converted to vitamin A (in the case of beta-carotene). They act in lipid-rich areas and membranes. Some are in the retina (lutein/zeaxanthin). Typical sources of these antioxidants are carrots, sweet potatoes, tomatoes, leafy greens, and colorful fruits.

Coenzyme Q10 (ubiquinol/ubiquinone): This is a lipid-soluble antioxidant in membranes and mitochondria (energy-generating organelles). It helps regenerate vitamin E and scavenges lipid peroxyl radicals. It acts across cellular membranes, especially in mitochondria.
Typical sources are meats and fatty fish. It is also available as a health supplement.

Astaxanthin: This is a potent lipid-soluble antioxidant that spans the membrane and protects against a broad range of ROS. Sources are seafood (salmon, shrimp) and health supplements.

N-acetylcysteine (NAC): This is a precursor to glutathione (GSH); it has thiol groups that can scavenge certain radicals. It acts throughout cells, aiding replenishment of GSH. It is commonly used as a supplement to boost antioxidant defenses.

Enzymatic (endogenous) Antioxidant Defenses

Superoxide dismutases (SOD): These convert superoxide (O2•−) into hydrogen peroxide (H2O2) and oxygen. The forms and locations of superoxide dismutases are: SOD1 (CuZn-SOD, cytosol), SOD2 (Mn-SOD, mitochondria), and SOD3 (extracellular). Their role is the initial line of defense against ROS produced during metabolism.

Catalase: This breaks down hydrogen peroxide (H2O2) into water and oxygen. It is mostly found in peroxisomes, but is also found in many cells.

Glutathione system (GSH, GPx, GR): Glutathione (GSH): A tripeptide that directly scavenges some ROS and serves as a substrate for glutathione peroxidase. Glutathione peroxidase (GPx): Reduces hydrogen peroxide (H2O2) and lipid hydroperoxides to water and alcohols using GS, and produces oxidized glutathione (GSSG). Glutathione reductase (GR): Regenerates GSH from GSSG using the reduced form of Nicotinamide Adenine Dinucleotide Phosphate (NADPH). A cofactor is selenium, a key component of several GPx enzymes. The Glutathione system acts throughout the cell, especially where peroxide detox is needed.

Thioredoxin system: This maintains protein thiol redox status and helps recycle antioxidants. It works with thioredoxin reductases to maintain a reducing environment. The system is located in the cytosol (the liquid part of the cytoplasm that surrounds organelles within a cell) and mitochondria and provides a broad protective role.

Non-nutrient Cofactors and Other Players

Selenium: An essential cofactor for GPx and some other antioxidant enzymes. Found in Brazil nuts, seafood, grains, and in the body as selenoproteins.

Zinc, Manganese, Copper: These are essential cofactors for SOD enzymes (e.g., CuZn-SOD uses copper and zinc; Mn-SOD uses manganese).
An adequate intake supports the body’s enzymatic defenses.

Alpha-lipoic acid (ALA): Water- and fat-soluble, it scavenges ROS and helps regenerate other antioxidants (glutathione, vitamin C, vitamin E). It supports mitochondrial enzyme function. Present in small amounts in foods. Available as a health supplement.

Flavonoids and polyphenols (e.g., quercetin, EGCG, catechins): These antioxidants direct radical scavenging, metal chelation (reducing Fenton reactions), modulation of cellular antioxidant enzymes, and activation of the Nrf2 pathway, which upregulates endogenous antioxidant defenses. Sources are tea (green/black), apples, onions, berries, citrus, cocoa, and many herbs/spices.

Curcumin and other polyphenols: These have antioxidant activity plus anti-inflammatory effects. They can activate Nrf2 and influence gene expression related to redox balance.

Summary

• Antioxidants work best as a team: direct scavenging, regeneration of other antioxidants, and strengthening the body’s own enzyme systems all contribute.
• The balance matters: whole foods with a mix of antioxidants tend to be more beneficial than high-dose single-nutrient supplements for most people.
• If you’re considering supplements, it’s wise to talk to a healthcare provider, as high doses of some antioxidants can be harmful in certain situations.
• Even antioxidants will struggle to work without a full complement of pure plant-derived minerals in your body. Find out more here.

More about antioxidants can be found here.