The Jury's Still Out
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
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.
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
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).
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