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Antioxidants and Cancer: The Jury's Still Out

Fri Sep 5,11:34 AM ET


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 (news -
web sites) (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 (news - web
sites) 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).

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