Killer Vitamins

British Study Finds Vitamin Supplements May Increase Cancer Risk

October 1, 2004

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Not only do vitamin supplements not protect against gastro-intestinal cancer, they may slightly increase the risk of cancer, according to a systematic review and meta-analysis of previously published randomised trials in this week’s issue of THE LANCET, a leading British medical journal.

If the findings are correct, 9,000 in every million users of such vitamin supplements will die prematurely as a result of taking something they think is good for them.

"The prospect that vitamin pills may not only do no good but also kill their consumers is a scary speculation given the vast quantities that are used in certain communities,” David Forman of the University of Leeds and Douglas Altman, Cancer Research UK, said in an accompanying commentary.

The researchers cautioned that the findings are preliminary and "(do) not offer convincing proof of hazard," pending further studies.

The mortality analysis in Bjelakovic and colleagues’ review is work in progress, and does not offer convincing proof of hazard. In the event that a hazard is established from a complete review, these researchers will need to identify which specific interventions are associated with any risk. It is unlikely that all supplements will exert a similar effect and it will be vital to establish the safety profile for those with demonstrated benefits.”

"The human diet is a complex mix of oxidants and antioxidants. Excess oxidants can cause cancer by inducing gene mutations," said Goran Bjelakovic, a professor at several European university, the study's lead investigator.

The investigators identified 14 randomised trials totalling over 170,000 participants. Overall, the results did not show any protective effect of supplementation with beta-carotene, vitamins A, C, E, and selenium (alone or in combination) compared with placebo on oesophageal, gastric, colorectal, pancreatic, and liver cancer incidences.

In half the trials, there was a small but statistically significant increase in mortality among people taking antioxidants compared with placebos. The results also showed that two combinations of supplements were associated with increased mortality risk: beta-carotene and vitamin A, and the combination of beta-carotene and vitamin E.

Four of the trials suggested that selenium was associated with a reduction in gastro-intestinal cancer risk.

“We could not find evidence that antioxidant supplements can prevent gastrointestinal cancers; on the contrary, they seem to increase overall mortality. The potential preventive effect of selenium should be studied in adequate randomised trials," Dr. Bjelakovic said.

Previous studies have failed to demonstrate that antioxidant vitamin supplements reduced the risk of heart disease among people at high risk of vascular disease.

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