THURSDAY, Sept. 30, 2004 (HealthDayNews) -- People who take vitamin and antioxidant supplements in the hope they're reducing their risk of gastrointestinal cancer are more likely to die of the disease than those who don't take the supplements, a new study finds.
The research, which reviewed the results of 14 major trials with more than 170,000 participants, found a small but statistically significant increase in gastrointestinal cancer deaths associated with supplements containing beta carotene and vitamins A, C and E.
Four of the trials showed a possible reduction of risk associated with selenium supplements, the report said.
In half the trials, there was a 6 percent increased risk of death from cancers of the esophagus, stomach, pancreas, liver, colon and rectum in persons who took supplements, compared to those who took a placebo, the researchers found.
But certain combinations of supplements seemed to be more dangerous, the researchers said.
A 30 percent higher risk was found for combination supplements containing beta carotene and vitamin A, and a 10 percent risk for supplements combining beta carotene and vitamin E.
"The indication that mortality in supplement-taking patients was higher compared to placebo has to be explored extensively in all randomized trials," said study author Dr. Goran Bjelakovic, a professor of internal medicine at the University of Nis in Serbia and Montenegro.
"The potential protective effect of selenium should be studied in adequate clinical trials," he added.
The study appears in the Oct. 2 issue of The Lancet.
It's not clear why antioxidant supplements might have a harmful effect, Bjelakovic said. One possible explanation is that they might interfere with apoptosis, the process in which the body destroys cells that turn abnormal.
"Someone who takes supplements can suppress apoptosis and thus can influence the growth of different tumors," Bjelakovic said. "But this is only a hypothesis."
Neither the American Cancer Society nor the National Cancer Institute (NCI) recommend vitamin supplements for cancer prevention. A U.S. task force recently reported there is "insufficient evidence" that supplements have any preventive effect. The cancer society recommends getting appropriate amounts of vitamins and minerals by eating a balanced diet.
The NCI is conducting a large-scale trial of selenium and vitamin A for prevention of prostate cancer. The trial was started because two earlier studies suggested a possible protective effect.
In an accompanying editorial in the journal, Drs. David Forman of Leeds University in England and Douglas Altman of Cancer Research United Kingdom, said, "The prospect that vitamin pills might not only do no good but also kill their consumers is a scary proposition given the vast quantities used in certain communities."
If the findings are correct, "9,000 in every million users of such supplements will die prematurely as a result," the editorial said. But it added the review "is a work in progress and does not offer convincing proof of hazard."
Eric Jacobs, a senior epidemiologist at the American Cancer Society, said, "There are other things [that] do work in preventing gastrointestinal cancer. One way to prevent colon cancer is to get screened for it. Quitting smoking helps prevent colon cancer as well as lung cancer, and maintaining proper weight can reduce the risk of gastrointestinal cancer."
More information
The National Cancer Institute has a review of antioxidant supplements and cancer prevention.
