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Antioxidants and Cancer:
The Jury's Still Out

Stroll the aisles of just about any supermarket or drugstore -- not to mention health food store - and you're likely to see row upon row of vitamin supplements, including antioxidants. Millions of Americans use these products in hopes of improving their overall health and preventing diseases like cancer. Many cancer patients -- as many as 60% -- also take some type of vitamin supplement in an effort to boost the effectiveness of their treatment.

But as a recent review in the journal Cancer Research (Vol. 63, No. 5: 4295-4298) points out, cancer patients should be careful about using antioxidant supplements. Medical experts don't really know yet whether antioxidant supplements are helpful or harmful for cancer prevention and treatment.

"...Patients and their physicians should certainly discuss their antioxidant usage," writes lead author Harold E. Seifried, of the Division of Cancer Prevention at the National Cancer Institute (NCI).

Research Results Conflict

Antioxidants are substances that occur naturally in many fruits and vegetables, as well as in nuts, grains and even some meat, poultry and fish. Beta-carotene (found in carrots, cantaloupe and other orange foods), vitamin E (found in nuts, broccoli and corn oil), vitamin A (found in liver, egg yolks and milk), vitamin C (found in citrus fruits and leafy vegetables), and selenium (found in meat and bread) are some of the most commonly known antioxidants.

Animal and cell culture studies have suggested that antioxidants may slow or even prevent the development of cancer, according to the National Cancer Institute (NCI). These compounds may protect cells from damage to their DNA caused by oxygen molecules called free radicals. Although free radicals occur naturally as the body ages, the damage they do can, over the long term, lead to changes in cells that can cause them to grow out of control and eventually cause life-threatening tumors.

It is this apparent benefit of antioxidants that has prompted many people -- including cancer patients - to turn to supplements, perhaps in the hope that larger amounts of these compounds will have an even more protective effect.

But research into the effects of different antioxidants on cancer has had mixed results. At least one study found that antioxidants may help protect against stomach (gastric) cancer, for example. Other research has found no effect on overall cancer risk in people who used antioxidants. And several studies have found that antioxidants may actually increase lung cancer risk in smokers.

Many Unanswered Questions

These conflicting results highlight the problem facing doctors whose patients want to know whether to take antioxidant supplements: too little is known about how antioxidant supplements actually act against disease.

"Antioxidant supplementation may actually cause harm in terms of increased risk of new disease or interference with treatment of existing disease," Seifried writes.

For instance, researchers don't yet know how antioxidant compounds from supplements interact with those found in foods. Do the supplements boost the actions of natural antioxidant compounds, or do they interfere with them? Because different foods contain different amounts of antioxidants, as well as numerous other substances that could influence cancer risk, it is extremely difficult for researchers to answer this question, Seifried notes.

Researchers also don't know whether some forms of a particular antioxidant are more effective against disease than others. Vitamin A comes in three forms, and vitamin E in two, for instance. Could one form be better than the others for fighting cancer?

And is it actually the antioxidant action that prevents disease, or do these compounds act in other ways that may be protective? Vitamin E and selenium, for instance, may have an effect on immune function and apoptosis, a process by which damaged cells self-destruct.

These compounds may also behave differently in people with different genetic characteristics, Seifried writes.

Dosages pose another problem; too much of certain compounds can be harmful. Too much vitamin E, for instance, can cause stomach upset, diarrhea and even bleeding, while too much vitamin A can damage the liver and lead to bone loss.

Antioxidants may also act differently in tumor cells than in normal cells, and doctors don't really know which amount, if any, is the right amount when it comes to antioxidants given along with chemotherapy or radiation therapy. There are even some concerns that antioxidants might be harmful when given with cancer treatment because they may help the cancer cells repair themselves.

Recommendations Tricky to Make

All of these questions make it difficult for doctors to advise people undergoing cancer treatments and those looking to prevent cancer about antioxidant supplements.

"Overall, current knowledge makes it premature to generalize and make specific recommendations about antioxidant usage for those at high risk for cancer or undergoing treatment," Seifried writes.

In fact, a recent report by the US Preventive Services Task Force concluded that there is not enough evidence to either recommend for or against taking vitamins (including antioxidants) for cancer prevention.

There are currently several large clinical trials underway that may help doctors better understand the role of antioxidants in cancer prevention, though. Results are expected in the next few years.

Until more is known, experts say eating a healthy diet is one of the best ways to lower cancer risk. The American Cancer Society recommends eating a variety of fruits and vegetables (at least five servings every day), choosing whole grain products over processed baked goods, and limiting the amount of red meat in the diet (fish, poultry, or beans are better choices).


Reference Source 106

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