Antioxidants are molecules 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. In the mid-2000s, about 20% of North Americans and Europeans were taking at least one antioxidant dietary supplement.
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. Damage caused by free radicals is thought to cause or contribute to cardiovascular disease, cancer, Alzheimer's disease, agerelated 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 light, and toxins such as cigarette smoke or certain chemicals.
The common feature of free radicals is that their molecular structure contains and 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-genetic material), proteins, and lipids (fats). Antioxidants are molecules that react with free radicals in ways that neutralize them so they no longer are 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 (alphatocopherol), vitamin A (retinol), and beta-carotene are the most important antioxidants the body must obtain 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, although 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 tuna also have high levels of selenium. It is a necessary part of enzymes involved in antioxidant reactions. Glutathione and coenzyme Q (ubiquinone) are the most important antioxidants the body can make for itself.
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. Research has since shown that although there are relationships between antioxidant levels and health, antioxidant dietary supplements are not magic bullets to prevent age-related diseases.
One problem in determining whether there is a cause and effect relationship between oxidative stress and disease is that often it is not possible to tell if oxidative stress causes a disease or if the disease brings about oxidative stress as a result of biochemical changes in diseased cells. Also, everyone develops oxidative stress as they age, but not everyone develops the same diseases. The interactions between 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. Lowdensity 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.
Researchers thought increasing the amount of antioxidants in the blood by taking supplements would decrease the number of free radicals available to interact with LDL cholesterol and thus lower the risk of developing cardiovascular disease. This theory has not been proved. In fact, a paper published in the Journal of the American Medical Association on February 28, 2007, analyzed 68 trials of antioxidant supplements involving about 232,600 patients. The authors concluded that antioxidant supplements did not prolong life. In fact, when only rigorous, wellcontrolled studies were analyzed, the risk of dying increased 5%. This analysis is quite controversial, with some experts questioning the analytical methods used. However, the American Heart Association and similar organizations in other countries advocate cardiovascular disease prevention through consumption of fruits, vegetables, whole grains and nuts high in antioxidants and other heart-protecting nutrients instead of antioxidant supplements.
CANCER. Free radicals damage DNA, and sometimes this damage leads to development of cancer. In laboratory cell cultures and animal studies, antioxidants appear to slow the development of cancer. The results have been mixed 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 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. The position of the American Cancer Society, the National Cancer Institute, and several international health organizations is that antioxidants should come from a healthy diet high in fruits and vegetables and low in fat and not from dietary supplements.
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. However, study participants who took antioxidant supplements over several years did not have a reduced risk of developing these diseases.
The mixed results obtained in human studies of antioxidant supplements suggests that all antioxidants should come from foods and not from dietary supplements. There is also little information on the safety of antioxidant supplements in children and women who are pregnant or breastfeeding.
The interaction among various antioxidants, enzymes, coenzymes, drugs, herbal and dietary supplements is complex and incompletely understood. Specific antioxidants may have known interactions and should be discussed with a physician.
Antioxidants acquired by eating fruits and vegetables promote health. No complications are expected from antioxidants in food. Antioxidant dietary supplements may interact with other supplements, prescription drugs, over-the-counter drugs, and herbal supplements in ways that cause undesirable side effects. Consult a physician prior to taking an antioxidant supplement.
Parents should encourage their children to eat a healthy and varied diet high in fruits, vegetables, and whole grains. There is no need to give children antioxidant dietary supplements. The safety of these supplements in children has not been studied.
Helps maintain healthy hair and nails, enhances immunity, works with vitamin E to protect cells from damage. Reduces the risk of cancer, particularly lung, prostate, and colorectal.
Garlic, seeds, Brazil nuts, meat, eggs, poultry, seafood, whole grains. The amount in plant sources varies according to the content of the soil.
Keeps skin healthy, helps prevent night blindness and infections, promotes growth and bone development.
Red, yellow-orange, and leafy green vegetables and fruits, including carrots, apricots, cantaloupe, peppers, tomatoes, spinach, broccoli, sweet potatoes, and pumpkin.
Acts as the protector of essential fats in cell membranes and red blood cells. Reduces risk of cancer, heart disease, and other age-associated diseases.
Peanut butter, nuts, seeds, vegetable oils and margarine, wheat germ, avocado, whole grains, salad dressings.
Destroys free radicals inside and outside cells. Helps in the formation of connective tissue, the healing of wounds, and iron absorption, and also helps to prevent bruising and keep gums healthy. May reduce risk of cataracts, heart disease, and cancer.
Peppers, tomatoes, citrus fruits and juices, berries, broccoli, spinach, cabbage, potatoes, mango, papaya.