|
Vitamin C: How Much Do You Really Need?
Some would have you believe that
vitamin C can prevent cancer, stop heart attacks, do the dishes, and wash
the car. Now government scientists are putting C in its place.
June 19, 2000 -- Feel free to call Rusty Hoge a vitamin C zealot. He
swallows a whopping 14,000 milligrams a day -- 150 times the
government-set Recommended Dietary Allowance (RDA) -- and has quadrupled
the dose when he felt illness coming on.
Since adopting this regimen 10 years ago, the 46-year-old
telecommunications technician from suburban Atlanta claims to have been
sick only once -- and then because he slacked off his normal routine.
"I like to say that taking high doses of vitamin C has one side
effect -- good health," he says.
Like many other devotees, Hoge believes that enormous doses of vitamin
C can fend off cancer and heart disease, slow the aging process, prevent
colds, and extend our lives by up to 35 years. Such claims were
popularized by the late Nobel-prize winning chemist Linus Pauling, who
died in 1994 at the age of 93, and his disciples continue to spread his
gospel.
But hold onto your pill bottles. In recent weeks, some studies have
suggested that far from being a panacea, vitamin C might actually
contribute to heart disease and cancer. What's more, while Americans are
now spending some $724 million per year on vitamin C pills, a government
report released last April suggests that these supplements are simply
unnecessary, whether in large doses or small.
So where does that leave people like Hoge . . . and the rest of us?
Good Cop or Bad Cop?
There's no question that vitamin C is vital for life, especially for
the manufacture of collagen, the rugged protein that holds your body
together. Along with vitamin E and selenium, vitamin C is also a powerful
antioxidant.
Antioxidants act like cops in the body, apprehending the rowdy
molecules called free radicals that form naturally during metabolism and
that can damage cells. One theory holds that free radicals promote
chronic diseases like cancer, heart disease, and Alzheimer's, and that by
increasing our consumption of antioxidants, we can stave off these feared
killers.
The first of two recent studies that called this notion into question
was carried out by David Golde, MD, physician-in-chief at Memorial
Sloan-Kettering Cancer Center in New York City, and described at an
American Cancer Society meeting in March 2000. Golde and his colleagues
transplanted human cancer cells into mice, injected the mice with vitamin
C, and then measured the amount of the vitamin in the cells. They found
that cancer cells seem to soak up large amounts of vitamin C by
converting it into a form that's easier to absorb. The results, Golde
says, raise the possibility that cancer cells may use vitamin C to shield
themselves against radiation and chemotherapy.
The second study, performed by James Dwyer, PhD, an epidemiologist at
the University of Southern California, caused an even bigger stir. Dwyer
told an American Heart Association meeting in March 2000 that middle-aged
men who took 500 milligrams of vitamin C supplements daily showed a rapid
narrowing of their carotid arteries, which supply blood to the brain.
The studies sound alarming, but experts warn against making too much
of them. While Golde says that cancer patients shouldn't take large doses
of the vitamin, other researchers say it's far too early to make that
recommendation. There's no evidence yet that C actually shields cancer
cells from treatment, says Mark Levine, MD, an endocrinologist and
Vitamin C expert at the National Institutes of Health. The cancers tested
in Golde's research, he says, may simply have grown from tissues that
normally take in large amounts of the vitamin.
As for the heart disease finding, Dwyer himself cautioned that it is
preliminary. The study lasted only 18 months and included just 573 men.
And Robert Jacob, PhD, a research chemist with the U.S. Department of
Agriculture, points out that previous studies suggested just the opposite
-- that vitamin C reduces the narrowing of carotid arteries.
No Magic Bullet
While the jury is still out on vitamin C's safety, a comprehensive
review of the evidence suggests that taking supplements doesn't provide
nearly the protection against disease that many people think it does. In
a report released in April 2000, the Institute of Medicine, an
independent scientific organization that advises the federal government,
examined whether vitamin C and other antioxidants prevent chronic
diseases. After weighing the results from more than a thousand studies,
the scientists decided they didn't.
"I think what the data show is that vitamin C is an important
antioxidant," says Jacob, a member of the panel. "But it's not
a magic bullet. Taking more of it is not going to solve those
diseases."
The Institute of Medicine panel did, however, change the recommended
dietary allowance (RDA) for vitamin C. The government now advises that
men get 90 milligrams per day and women 75 milligrams. Smokers should get
an additional 35 milligrams a day, the panel said. That's an increase
from the 60 milligrams of the previous RDA, but it's far lower than the
dose in many multivitamins -- and well below the amounts that people like
Hoge take.
Taking any more than about 200 milligrams per day is likely a waste of
money anyway, according to a landmark study by Mark Levine of the NIH.
Levine's group found that the body's cells can't absorb more than about
100 milligrams per day, and the concentration of vitamin C in the blood
begins to level off at a dose of 200 milligrams per day. The study
appeared in the April 1996 issue of Proceedings of the National
Academy of Sciences.
The message is that large doses simply go to waste, says Balz Frei,
PhD, director of the Linus Pauling Institute. "Above a certain
threshold, you urinate out most of what you take in," says Frei.
The panel found no evidence of serious side effects from large doses
of vitamin C, but recommended a maximum dose of 2,000 milligrams per day
because of the possibility of diarrhea, an inconvenience that usually
passes quickly. As a precaution, anyone with hemochromatosis,
thalassemia, or a history of kidney disease should shun high doses, Jacob
says. So should anyone taking blood thinners or estrogen or who has a
history of renal stones. In such cases, make sure you get your doctor's
advice before considering higher doses of vitamin C.
Contrary to Pauling's claims, the panel concluded that high doses of
vitamin C probably won't protect you from getting a cold, either, though
they may shorten the length of your suffering slightly.
Neither the government's recommendations nor the reversal by Pauling's
own institute has dampened the enthusiasm for vitamin C among true
believers like Hoge.
As for the rest of us, scientists agree that one clear message emerges
from the research: People whose diets are rich in fruits and vegetables
have lower rates of cancer, heart disease, and other chronic illnesses.
That means the ideal way to meet your requirements for vitamin C is by
eating at least five servings of fruits and vegetables a day. You not only
get plenty of vitamin C, but you reap the benefits from all the other
nutrients these foods contain.
Hoge, for his part, plans to stick with his 14,000 milligrams a day
regimen. "I can't imagine anything that would change my mind,"
he says.
Mitchell Leslie is a freelance science
writer who lives in Belmont, Calif.
For More Information From WebMD
Tips
for Getting Vitamin C
Vitamins,
Carotenoids, and Phytochemicals
People
Should Get More Vitamins From Food but Beware of Huge Doses
Fruits
and Vegetables: Eating Your Way to Five a Day
2000 Healtheon/WebMD. All rights reserved.
|
