Scientists from the University of Niss, Serbia and Montenegro, and the Cochrane Hepato-Biliary Group carried out a review of previously published trials, where antioxidant supplements had been used for the prevention of gastrointestinal cancers.
The researchers looked at 14 randomised trials - totalling over 170,000 participants - before concluding that supplementation with b-carotene, vitamins A, C, E, and selenium (alone or in combination) compared with placebo on oesophageal, gastric, colorectal, pancreatic and liver cancer incidences provided "no protective effect".
They then went a step further and concluded there was "a small but statistically significant increase of 6 per cent relative risk in mortality among people taking antioxidants compared with placebo". And added that two combinations of supplements, namely b-carotene and vitamin A and E, were associated with an even higher relative mortality risk of 30 per cent for b-carotene and vitamin A and 10 per cent for b-carotene and vitamin E.
Four of the trials suggested that selenium was associated with a reduction in gastro- intestinal cancer risk, "but this may be due to bias", said the researchers.
"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," said lead researcher Dr Goran Bjelakovic.
However, David Forman from the University of Leeds, UK and Douglas Altman from Cancer Research UK, were keen to emphasize that the findings of the study should only be viewed as preliminary.
Forman told NutraIngredients.com that he was comfortable with the conclusion that there was no proof that vitamin supplements had protective effects against gastrointestinal cancer. But, he felt the authors of the study had more confidence in the findings that supplementation could be responsible for premature deaths than he did.
"This is somewhat preliminary and there are a number of statistical problems with the way the researchers looked at the data," he said. He noted that a particular problem was the inclusion of one study, which had used an "abnormal" group of patients, including people that were unhealthy and smoked.
"Part of the problem is also that the researchers looked at all available studies dealing with gastro-intestinal cancer and supplementation, but they now need to do another study taking into consideration all papers on mortality and supplementation," he added.
The group is now carrying out such research, but - depending on the results - it could have been beneficial to the supplement industry if this study had been concluded before such potentially damaging research was published.
As Forman and Altman noted: "If their findings are correct, 9000 in every million users of such supplements will die prematurely as a result. 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".
Although, even if this were found to be the case, "these researchers would need to identify which specific interventions were associated with any risk," they said. "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 UK's vitamin and supplement market is one of the largest in Europe with consumers spending £300m on dietary supplements every year.