Many of the 40 million to 50 million Americans who take vitamin C tablets swallowed hard last week upon hearing the news that chemists had found a link between high doses of the supplements and the kind of DNA damage associated with cancer.
But researchers who study the vitamin's effect on health were quick to say that the new findings did not imply that vitamin C supplements cause cancer.
"This was an experiment done in the lab, not in any biological context," said Jeffrey Blumberg, director of antioxidant research at Tufts University in Boston. "And the first thing to point out is that there's no good evidence that vitamin C supplements are actually causing cancer in anyone."
In the new study, which appeared in the June 15 issue of the journal Science, chemists at the University of Pennsylvania extended earlier research demonstrating that, in certain laboratory conditions, vitamin C can cause damage to tissue cells.
"These kinds of studies are very valuable in helping us understand how biology works," said Dr. Tim Byers, a professor of preventive medicine at the University of Colorado Medical School in Denver. "But it's a long, long leap from the laboratory to conclusions about how the nutrient acts in the body."
In fact, no one knows much about how vitamin C acts in the body, and the Pennsylvania study raises broader questions about whether the supplements help protect people's health at all. Americans have been dosing themselves with vitamin C to ward off everything from colds to heart disease and cancer, ever since famed chemist Linus Pauling began promoting the idea more than two decades ago.
Among other vital properties, vitamin C helps the body heal wounds and neutralize molecules called free radicals, which scientists have linked to many diseases associated with aging, including arthritis, atherosclerosis and cancer. In several large studies, researchers have demonstrated that people who get large amounts of the vitamin in foods such as oranges, red peppers, green peppers and other fruits and vegetables are less likely to develop heart disease and cancer than those who eat nutrient-poor diets.
Consumers spend more than $500 million a year on vitamin C, making it one of the top two or three best-selling supplements, according to the Hartman Group, a Bellevue, Wash., research firm. "Certainly some individuals we see in our practice do very well on the supplements," said David Grotto, director of nutrition at the Block Medical Center in Evanston, Ill., which treats many cancer patients.
Yet after more than two decades of research, doctors cannot say whether vitamin C supplements actually deliver any additional benefit to healthy people. Some studies suggest the supplements reduce the risk of cancer and heart disease; others find no protective effect at all.
"Most of the large population studies have been done by investigators trying to find some protective effect" for vitamin C supplements, said Buyers. "But if you look at all the literature, it's pretty much a null result. There's no good evidence that the supplement is protective, there's no good evidence it's causing harm. It's a wash."
As mixed as they are, these results tell researchers several things about vitamin C. If the supplements do affect health, for instance, then their impact is probably small, researchers say. "If vitamin C supplements were acting in a powerful way," says Plumber, "we'd have seen some more consistent findings by now."
And the large population studies performed so far suggest that most Americans are already getting as much of the vitamin as they need in their regular diet, without supplements, doctors say. The government's recommended daily allowance is 75 milligrams a day for women and 90 milligrams for men; smokers are advised to take an additional 35 milligrams.
No large medical society recommends taking supplements to achieve those levels; in heavy vitamin dosing, one man's meat is another's poison.
"There's great individual variability in how people respond not only to vitamin C but all nutrients," says Dr. Ronald Krauss, an endocrinologist at UC Berkeley who studies the relationship between nutrition and disease. The supplements might protect one person from disease but cause harm in another. "As we unravel the action of various genes we'll be able to put this in some overall context. At this time, we just don't know enough to do that."