What Are Antioxidants?
| Types of Antioxidants
| The Link to Cancer, Heart Disease, and Aging
| Other Conditions
| What Does This Mean to You?
In the 1990s, "antioxidants" became famous as the nutritional equivalent of the fountain of youth. Even doctors jumped on the bandwagon. Increasing your intake of
vitamin E, beta-carotene, and
was touted as an easy and painless way to prevent
heart disease, vision problems,
arthritis, and practically every other illness.
Unfortunately, an accumulating body of research has tended to put the damper on this excitement. It now appears that antioxidant proponents had jumped the gun; in fact, these supplements may have little benefit for these purposes. Worse still, some may even
risk of cancer and heart disease.
What Are Antioxidants?
Antioxidants are chemical substances naturally found in foods. They function as a protective "shield" against harmful, unstable molecules known as "free radicals." Free radicals are produced throughout the body as a result of normal body functions and have some important uses, such as fighting infection. However, free radicals can also cause harm to healthy tissues and are believed to play a role in many diseases.
The potential harmful effects of excessive free radicals range from cosmetic to life-threatening. They may break down skin tissue, making it look older than it is. They may injure the lens of the eye, accelerating the development of
cataracts. And they may make it harder for cells to repair themselves, increasing the risk of cancer, heart disease, and other diseases of age.
The body itself has a well-developed system for containing free radicals. High consumption of certain nutrients might tend to augment this system, and thereby improve health.
Types of Antioxidants
There are a variety of antioxidants present in many different foods. The following is a list of the most well known antioxidants and the foods in which they are found. Also listed are selenium and manganese, which are not antioxidants themselves, but are necessary for the body to create its own natural antioxidants.
|Antioxidant or Antioxidant-producing Nutrient||Food Sources|
and other carotenoids (eg, lutein, lycopene, zeaxanthin)
||Yellow/orange/red or dark green fruits and vegetables|
|Vitamin C||Citrus fruit, strawberries, red chili peppers, sweet peppers, dark-green leafy vegetables, broccoli, cauliflower, cabbage, and strawberries|
|Vitamin E||Polyunsaturated vegetable oils, seeds, nuts, and whole grains|
|Manganese||Whole grains, legumes, avocados, grape juice, chocolate, egg yolks, nuts, seeds, boysenberries, blueberries, pineapples, dark green vegetables|
|Selenium||Meat, chicken, seafood, milk, whole grains, nuts, and vegetables|
The Link to Cancer, Heart Disease, and Aging
Scientists became interested in antioxidants several decades ago when they noticed that people who regularly consumed fruits, vegetables, and other foods high in antioxidants had a lower incidence of diseases such as
cancer, heart disease,
arthritis, age-related vision problems,
Parkinson’s disease, and many other conditions. Speculation began to grow that perhaps antioxidants might be key to preventing degenerative diseases and slowing the aging process.
However, in such observational studies it is very difficult to rule out other factors that may play a role. For example, people who take vitamins may also exercise more or take better care of themselves in other ways. Such "confounding factors" make the results of these studies less reliable.
Some scientists back in the early 1990s raised the possibility that other factors in fruits and vegetables besides the major antioxidants were responsible for their beneficial health effects. Or, alternatively, that antioxidant ingredients had to be present in certain combinations, or in the context of a whole food. Nonetheless, based on studies that showed positive physiologic changes when these antioxidants were taken, many scientists remained convinced that isolated antioxidant supplements should be helpful. So, they tested this theory in a large double-blind, placebo-controlled study, enrolling tens of thousands of people who took antioxidant supplements for several years.
Much to everyone’s surprise the majority of these studies did not support a beneficial effect for antioxidant supplementation. Not only did antioxidant supplements fail to prevent illness, under certain circumstances, they appeared to increase it. This was true, not only when isolated antioxidants were used, but with combined antioxidant therapies as well.
Indeed, an analysis of essentially all randomized trials investigating the effects of antioxidant supplements on longevity found no decrease in mortality (with the possible exception of selenium). Moreover, vitamins A and E and beta-carotene appeared to modestly, but significantly, increase mortality. This was true whether the supplements were used singly or in combination.
The convoluted history of the supplement beta-carotene and its potential cancer-preventing benefits provides a prominent example of antioxidants’ disappointing record. The story began in the early 1980s when the cumulative results of many studies suggested that people who eat a lot of fruits and vegetables are significantly less likely to get cancer. A close look at the data pointed to carotenes as the active ingredients in fruits and vegetables. It appeared that a high intake of dietary carotene might significantly reduce the risk of cancers of the
stomach. In addition to these observational studies, evidence from laboratory evaluations showed that beta-carotene helped prevent healthy cells from turning cancerous.
The possibility that a simple, cheap supplement could reduce cancer rates excited researchers, and in a relatively short time, funds were gathered to conduct several large double-blind, placebo-controlled trials. Most of these studies enrolled people in high-risk groups such as smokers, because it is easier to see results in a study when you look at people who are more likely to develop the disease you are studying.
In 1994, the results of the Alpha-Tocopherol, Beta-Carotene (ATBC) study dashed researchers hopes. The results of this 29,133-person study showed that beta-carotene supplements did not prevent lung cancer, but actually increased the risk of getting it by 18%.
In January 1996, researchers monitoring the Beta-Carotene and Retinol Efficacy Trial (CARET) confirmed the prior bad news with more of their own: the beta-carotene group had 46% more cases of lung cancer deaths. This study involved smokers, former smokers, and workers exposed to asbestos. Alarmed, the National Cancer Institute ended the $42 million CARET trial 21 months sooner than planned. Some scientist attempted to explain these surprising results by suggesting that beta-carotene may simply be ineffective in smokers.
However, similar results were seen in another giant study that did not focus on smokers. In the 12-year Physicians' Health Study of 22,000 male doctors, 50 mg of beta-carotene (about 25 times the amount necessary to provide the daily requirement of vitamin A) taken every other day had no effect—good or bad—on the risk of cancer or heart disease. Another large study of beta-carotene supplements failed to find any effect on the risk of cancer in women.
Undaunted, proponents of antioxidants argued that while beta-carotene alone may have no measurable effect, combining it with other antioxidants—which is the situation found in nature—may still produce beneficial results. To date, however, studies evaluating mixtures of antioxidant supplements have failed to yield conclusive results.
The news is slightly better for two other antioxidant supplements: vitamin E and selenium. Each of these supplements has some evidence from double- blind, placebo-controlled trials indicating that they may reduce risk of certain forms of cancer. However, this evidence is far from definitive, and subsequent research has yet to confirm these preliminary findings. Worse, in some studies, doses of vitamin E above 150 IU per day were associated with increased risk of death. Selenium remains under trial as a possible agent for preventing colon cancer, but it will likely take several additional years to know whether supplementation is of health value.
The evidence for a generalized anticancer effect with other antioxidant supplements, such as vitamin C,
green tea, is not persuasive either, since it is limited almost entirely to observational and test tube studies. Vitamin E or selenium also do not appear to reduce the risk of
There is an ongoing debate now that focuses on antioxidants and their effects on
radiation therapy. It is unclear whether antioxidants interfere with the effects of these therapies by diminishing their effectiveness or by enhancing it. There have been studies published that support each of the opposing ideas. This is a very controversial area and we need more research to be able to clearly answer these questions.
There is no harm in eating foods rich in antioxidants.
As with cancer prevention, the results of many large observational studies suggest that higher consumption of antioxidant supplements may decrease heart disease risk. However, once more, large-scale intervention trials have failed to find benefit with vitamin E, beta-carotene, vitamin C, or mixed antioxidants, and some even suggest that such supplements may modestly increase risk.
Similarly, intervention trials have not consistently found that antioxidants are helpful for macular degeneration. Mixed antioxidants have also failed to be helpful for preventing cataracts, but the antioxidant lutein (which may or may not be acting as an antioxidant) has shown promise for this condition.
Researchers have discovered, though, that antioxidant supplements may help to improve fertility in men who have unexplained infertility. It is believed that antioxidants may help to fight against damage to sperm cells.
What Does This Mean to You?
There are two possible interpretations of these results. One, the more hopeful possibility, is that antioxidants really are helpful, but only when they are taken in the form of naturally occurring food. If this is correct, consuming more fruits and vegetables may be health promoting, while the benefits of taking vitamin supplements still needs to be proven.
However, another possibility is that the apparent correlation between antioxidant intake and reduced rate of disease does not indicate cause and effect, and that consuming extra antioxidants in any form will fail to prevent disease.
So what should an educated consumer do? The best option is probably to eat plenty of fruits and vegetables. If your vegetable or fruit has color to it, it almost certainly contains an antioxidant that might provide a health benefit. Some people may also want to “hedge their bets” and take a multivitamin/multimineral tablet that supplies nutrients at basic nutritional doses. Because of studies showing risk at high doses, vitamin E dosage should probably remain below 150 IU daily, and even this dose does not guarantee safety, as rare adverse effects are difficult to measure. Similarly, we do not know if low doses of beta carotene increase cancer risk.
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Last reviewed April 2011 by Brian Randall, MD
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