Antioxidants are substances that prevent oxygen molecules from interacting with other molecules in a process called oxidation. In the body, antioxidants combine with potentially damaging molecules called free radicals to prevent the free radicals from causing damage to cell membranes, DNA, and proteins in the cell. Common antioxidants important to human health are vitamins A, C, E; beta–carotene; and selenium. By some estimates, up to 10% of North Americans and Europeans were taking at least one antioxidant dietary supplement in 2012.
The role of antioxidants in the body is complex and not completely understood. Antioxidants combine with free radicals so that the free radicals cannot react with, or oxidize, other molecules. In this way, antioxidants help slow or prevent damage to cells. Antioxidant enzymes present in the body also work to prevent oxidation. Damage caused by free radicals is thought to cause or contribute to cardiovascular disease, cancer, Alzheimer’s disease, age–related changes in vision, and other signs of aging.
However, no direct cause–and–effect relationship between antioxidant intake and disease prevention has been proven. Antioxidants unrelated to those of importance in the body have commercial uses in the preservation of processed food and in many industrial processes.
Oxygen is essential to many reactions that occur within cells. Free radicals form mainly as a result of normal cellular metabolism involving oxygen. They can also form in abnormally large amounts when the body is exposed to radiation, ultraviolet (UV) light, and toxins such as cigarette smoke or certain chemicals. The common feature of free radicals is that their molecular structure contains an unpaired electron. Free radical molecules with an unpaired electron are unstable and have a strong tendency to react with other molecules by ‘‘stealing’’ an electron from them to form a more stable electron pair. This reaction is called oxidation (even when it happens with molecules other than oxygen).
In the body, free radicals cause damage when they react with deoxyribonucleic acid (DNA), proteins, and lipids (fats). Antioxidants are molecules that react with free radicals in ways that neutralize them so they are no longer able to ‘‘steal’’ electrons and cause damage.
Some important human antioxidants must be acquired through diet, while others can be made by the body. Vitamin C (ascorbic acid), vitamin E (alpha–tocopherol), vitamin A (retinol), and beta–carotene are the most important antioxidants obtained from food sources.
Flavonoids found in tea, chocolate, grapes, berries, onions, and wine also appear to have antioxidant activity, although their role in health is unclear.
Selenium is sometimes classified as an antioxidant due to its activities in the body, but strictly, it is not. Selenium is a mineral that must be acquired through diet. Plants grown in geographic locations with selenium–rich soil provide a rich source of this mineral. Brazil nuts and fish (e.g., tuna, halibut, sardines, salmon) are considered the best sources of selenium. Shellfish, meat, poultry, eggs, mushrooms, and grains (wheat, barley, rice, oats) also contain selenium, although the amounts in plant–based foods depend on the concentration of selenium in the soil in which they were grown. Selenium is necessary in the formation of the enzymes involved in antioxidant reactions.
Glutathione and coenzyme Q (ubiquinone) are the most important antioxidants produced by the body.
Antioxidants and disease
When free radicals build up faster than antioxidants can neutralize them, the body develops a condition called oxidative stress. Oxidative stress reduces the body’s ability to deal with damage to cells and is thought to play a role in the development of chronic diseases such as cardiovascular disease, cancer, and Alzheimer’s disease. Researchers know that a diet high in fruits and vegetables containing antioxidants promotes health and decreases the risk of developing some chronic diseases such as atherosclerosis (hardening of the arteries).
In the early 2000s, dietary supplements containing antioxidants were popularized as a way to reduce oxidative stress; prevent health problems such as cancer, stroke, heart attack, and dementia; and live longer.
We recommend that you take the powerful antioxidant Resveratrol:
Research has since shown that although there are relationships between antioxidant levels and health, antioxidant dietary supplements are not adept at preventing age–related diseases. In fact, some evidence now suggests that high intakes of antioxidants may actually have harmful effects in promoting the development of cancers and preventing the prophylactic effects of certain cholesterol–reducing medications.
One problem in determining whether there is a cause–and–effect relationship between oxidative stress and disease is that it is often unclear whether oxidative stress causes a disease or if the disease brings about oxidative stress.
Also, everyone develops oxidative stress as they age, but not everyone develops the same diseases. The interactions among an individual’s diet, environment, genetic make–up, and health are complex and still not well understood.
Antioxidants remain of great interest to researchers seeking ways to prevent and cure chronic disease. Many clinical trials are underway to determine safety and effectiveness of different antioxidants, both alone and in combination with other drugs and supplements.
- CARDIOVASCULAR DISEASE
The strongest link between antioxidant levels and health is related to the development of cardiovascular disease. Low–density lipoprotein cholesterol (LDL or ‘‘bad’’ cholesterol) appears to react with free radicals. This changes the LDL cholesterol in a way that allows it to accumulate in cells lining the blood vessels. These cholesterol– loaded cells are precursors to the development of plaque, hard deposits that line blood vessels and cause cardiovascular disease, heart attack, and stroke. High dietary intakes and blood levels of antioxidant nutrients (e.g., beta–carotene) have been associated with lower risk of cardiovascular disease.
In some laboratory cell cultures and animal studies, antioxidants have appeared to slow the development of cancer. The results have been mixed, however, in studies where humans took antioxidant dietary supplements.
A large study of 29,000 men showed that when a beta–carotene dietary supplement was taken by men who smoked, they developed lung cancer at a rate 18% higher and died at a rate 8% higher than men who did not receive the supplement.
Another study that gave men dietary supplements of beta–carotene and vitamin A was stopped when researchers found that the men receiving the beta–carotene had a 46% greater chance of dying from lung cancer than those who did not receive the supplement.
Other large studies have shown either no or only slight protective effects against cancer.
- AGE–RELATED VISION IMPAIRMENT
Cataracts and age–related macular degeneration are two types of vision impairment common in older individuals. Cataracts develop because of changes in the protein in the lens of the eye. These changes cause the lens to become cloudy and limit vision. The changes may be due to damage by free radicals. Age–related macular degeneration is an irreversible disease of the retina that causes blindness. Two carotenoid antioxidants, zeaxanthin and lutein, are found in the retina and are essential to vision.
- ALZHEIMER’S DISEASE
Antioxidants have been associated with slowing the progression of memory loss and other cognitive disorders, including Alzheimer’s disease, but results have been inconclusive.