Vitamins Don't Prevent Stomach, Colon Cancers
Certain Antioxidants May Even Increase Cancer Risk, Say Researchers
Sept. 30, 2004 -- Antioxidant supplements may be of no benefit when it comes to preventing stomach and colorectal cancers.
In fact, researchers say that based on a small number of well-designed studies a few antioxidants supplements might increase the risk of certain gastrointestinal cancers or even death.
It's important to keep in mind that the review is a work in progress, not the final word on supplements, cancer, and death.
Goran Bjelakovic, MD, of the University of Nis in Serbia and Montenegro, and colleagues from Copenhagen University Hospital in Denmark, conducted a review of 14 studies of more than 170,000 people.
Some of the trials were of higher quality than others. Several had high participant drop out rates. Some included healthy participants, while others focused on people with a higher risk of developing cancer.
In addition, the studies all used supplements, not naturally occurring antioxidants found in fruits or vegetables. Some used high doses, not the amounts most people would normally consume.
What's more, participants may not have taken the supplements for long enough to reap cancer protection benefits.
That said, the findings showed little support for taking antioxidant supplements to ward off gastrointestinal cancer -- which included cancer of the stomach, pancreas, liver, esophagus, and large intestine.
Little Prevention Seen
Antioxidant supplements covered in the trials included beta-carotene, vitamins A, C, and E, and the mineral selenium. In some trials, only one supplement was taken; other studies looked at the effects of combinations of supplements.
The researchers found that rates of gastrointestinal cancer were the same in those taking supplements and those who didn't.
Only selenium supplements appeared to offer some protection against gastrointestinal cancer. However, only one of the four studies with that finding was of "high methodological quality," say Bjelakovic and colleagues.
Further tests could shed more light on selenium supplements, but overall, the researchers found that "antioxidant supplements might not be beneficial for cancer prevention."
For selenium, it appeared that prevention of cancer was confined to the liver only.
Beta-carotene "increased mortality," say the researchers, but combining beta-carotene and vitamins A and E "significantly increased mortality."
A mix of vitamin A and beta-carotene had "significantly higher" cancer incidence and deaths, compared with a placebo.
Death rates were also significantly higher for beta-carotene and vitamin E.
Verdict Still Out
The review "does not offer convincing proof of hazard," say David Forman of England's University of Leeds, and Douglas Altman of the Centre for Statistics in Medicine in Oxford, England.
Forman and Altman commented on the review for The Lancet.
Bjelakovic and colleagues are currently working on identifying all trials on death after exposure to antioxidants.
The study and commentary appear in the journal's Oct. 2 issue.
SOURCES: Bjelakovic, G. The Lancet, Oct. 2, 2004; vol 364: p 1219-1228. Forman, D. The Lancet, Oct. 2, 2004; vol 364: pp 1193-1994. News release, The Lancet.