Antioxidant supplements - not so good for longevity?

Researchers in Denmark (Bjelakovic G et al ) used the Cochrane Collaboration methodology to perform a meta-analysis of all randomised trials comparing antioxidant supplements with placebo for prevention of gastrointestinal cancers (published in the Lancet 2004;364:1219-1228).

They analysed 14 trials involving the use of betacarotene, vitamins A, C and E and the mineral selenium. They were unable to find any effect at all of any of the antioxidant supplements (given either singly or in combination) on any of the gastroinstestinal cancers, with the exception of selenium. Four studies evaluated selenium for different gastrointestinal cancers, and found to have a significant beneficial effect on hepato-cellular carcinoma.

The disturbing and surprising finding from this meta-analysis was that there was a trend towards increased mortality in people taking antioxidant supplements. When trials with adequate methodological quality were analysed separately from other trials there was a significant increase in mortality, especially when one small trial using selenium was excluded. The authors concluded that they could not find evidence that antioxidant supplements can prevent gastrointestinal cancers; on the contrary, they seem to increase overall mortality. However, the potential preventive effect of selenium should be studied in adequate randomised trials.

An editorial in the Lancet has suggested that these negative findings may simply be because sicker people are taking the antioxidants and that it has nothing to do with the antioxidants themselves. However, the authors highlight the fact that there is almost always a bias giving a larger effect to experimental interventions - thus if the bias is operative in this field, the opposite interpretation, that in fact the risk may be larger, cannot be excluded.

The results of this meta-analysis raises many questions about the safety of high dose antioxidant supplements and whether it is possible that these vitamins could in fact stimulate cancer growth. So far the scientific evidence, in balance, has raised questions about the benefits of supplements. For example, vitamin E supplements are no longer recommended for coronary heart disease, beta carotene supplements seem to increase the risk of lung cancer in smokers, the jury is still out on vitamin E and dementia and there is emerging evidence in lab animals that vitamin C can be a cancer promoter in high doses.

Last Updated: Feb 2005