Wikipedia says this about antioxidants.
An antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions. When the chain reaction occurs in a cell, it can cause damage or death to the cell. Antioxidants terminate these chain reactions by removing free radical intermediates, and inhibit other oxidation reactions. They do this by being oxidized themselves, so antioxidants are often reducing agents such as thiols, ascorbic acid, or polyphenols.
Antioxidants are important additives in gasoline. These antioxidants prevent the formation of gums that interfere with the operation of internal combustion engines.
Although oxidation reactions are crucial for life, they can also be damaging; plants and animals maintain complex systems of multiple types of antioxidants, such as glutathione, vitamin C, and vitamin E as well as enzymes such as catalase, superoxide dismutase and various peroxidases. Low levels of antioxidants, or inhibition of the antioxidant enzymes, cause oxidative stress and may damage or kill cells.
As oxidative stress appears to be an important part of many human diseases, the use of antioxidants in pharmacology is intensively studied, particularly as treatments for stroke and neurodegenerative diseases. Moreover, oxidative stress is both the cause and the consequence of disease.
Antioxidants are widely used in dietary supplements and have been investigated for the prevention of diseases such as cancer, coronary heart disease and even altitude sickness. Although initial studies suggested that antioxidant supplements might promote health, later large clinical trials with a limited number of antioxidants detect no benefit and even suggested that excess supplementation with certain putative antioxidants may be harmful. Antioxidants also have many industrial uses, such as preservatives in food and cosmetics and to prevent the degradation of rubber and gasoline.
Let's see what scientists say about antioxidants.
The major genetic risk factor for Alzheimer's disease (AD), present in about two-thirds of people who develop the disease, is ApoE4, the cholesterol-carrying protein that about a quarter of us are born with. But one of the unsolved mysteries of AD is how ApoE4 causes the risk for the incurable, neurodegenerative disease. In research published this week in The Proceedings of the National Academy of Sciences, researchers at the Buck Institute found a link between ApoE4 and SirT1, an "anti-aging protein" that is targeted by resveratrol, present in red wine.
A chemical found in red wine remains effective at fighting cancer even after the body's metabolism has converted it into other compounds.
New insight into grape seed extract as a therapeutic and preventative measure to fight obesity-induced kidney damage is presented in a new study. Grape seed and skin extract (GSSE) is known to contain powerful antioxidants. This study, published in the journal Applied Physiology, Nutrition, and Metabolism, is the first to make a link between GSSEs and high-fat-diet-induced renal disease.
Contrary to other research, a new study found that the total level of antioxidants in people's diets is not related to their risk of developing stroke or dementia. The study is published in the February 20, 2013, online issue of Neurology®, the medical journal of the American Academy of Neurology. Antioxidants such as lycopene, beta-carotene and vitamins C and E are found in many foods.
Research has suggested that compounds that give colorful fruits their rich hues, especially berries, promote health and might even prevent cancer. But for the first time, scientists have exposed extracts from numerous berries high in those pigments to human saliva to see just what kinds of health-promoting substances are likely to survive and be produced in the mouth.
Women who ate at least three servings of blueberries and strawberries per week had fewer heart attacks. Blueberries and strawberries contain high levels of compounds that have cardiovascular benefits.
"Past studies have shown that having low antioxidant levels and increased reactive oxygen species -- chemical products that bind to body cells and cause damage -- is related to more severe PAD," said Matthew Muller, postdoctoral fellow in Larry Sinoway's lab at Penn State College of Medicine, and lead author of the study.

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