Antioxidant

An antioxidant is a molecule that protects other molecules from oxidation, the loss of electrons. In cells, normal metabolism, especially in mitochondria, produces reactive oxygen species such as superoxide and hydrogen peroxide, along with free radicals, atoms or molecules with an unpaired electron that readily attack lipids, proteins and DNA. When these build up faster than they are removed, the result is oxidative stress, which is linked to aging and to diseases such as atherosclerosis and cancer.

The body defends itself in layers. Enzymes such as superoxide dismutase, catalase and glutathione peroxidase convert reactive molecules into harmless ones and form the most important line of defense. Smaller molecules then mop up what remains: some, such as glutathione and uric acid, are made in the body, while others come from food, including vitamin C, vitamin E, carotenoids such as beta-carotene, and plant compounds called polyphenols. A small-molecule antioxidant typically donates an electron and becomes a weak, stable radical itself. Antioxidants are also added to foods to keep fats from turning rancid.

Diets rich in fruit and vegetables are linked to lower disease risk, but large trials of antioxidant supplements have mostly shown no benefit, and high-dose beta-carotene made lung cancer more likely in smokers.