FISH OIL may not quite be the elixir of life that we have been led to believe.

Analysis of all the best trials on the subject has found little evidence that eating fish, or taking fish oil capsules, cuts the risk of dying of heart disease, stroke or cancer.

The finding may come as a shock to those who believe that the benefits of omega 3 fatty acids, which are found not just in fish oils but also, in short chain form, in some plant oils such as linseed oil.

The analysis indicates that, despite a lot of work and a multiplicity of trials, it is difficult to show clear benefits. The better the quality of the trial, the lower the apparent benefit.

The findings, published in the British Medical Journal online by a team led by Lee Hooper, of the University of East Anglia, are unlikely to go uncontested.

Other analyses, including one published as recently as 2002, have shown benefits.

Dr Hooper’s team, which included Professor George Davey-Smith, of Bristol University, searched the medical literature for studies of fish oil. They found 48 randomised control trials and 41 “cohort” studies.

Randomised control trials (RCTs) compare the effects of fish oil capsules with a placebo, while cohort studies compare groups with high intakes of fish oil against those with low intakes. RCTs are considered the best source of evidence available in medicine, while cohort studies rank lower.

The team pooled all the data from the studies to increase statistical power. When this was done there was no strong evidence of a reduced risk of dying among those taking supplements.

When only the better studies, with a lower risk of bias, were chosen, the evidence of benefit became even weaker.

These studies were more consistent but showed no evidence of benefit. Overall, they suggested that a 2 per cent reduction in risk, but the error margin was wide.

The benefit could have been as great as 30 per cent, or the disbenefit as much as 36 per cent. So fish oil could reduce deaths, or increase them. The only conclusion possible is that it has no significant effect.

Why does the finding differ from earlier studies? The authors suggest some possibilities. In the cohort studies, which have in the past suggested there is a benefit, it is impossible to eliminate bias.

People who take fish oil capsules differ in so many ways from those who do not that it is impossible to correct completely for these differences. Or, as the team says: “The web of lifestyle, interest in health and social factors seen in the cohort studies included in our review provides an advantage to people taking most omega 3 fats and this makes adequate adjustment for confounding difficult, if not impossible.”

So reliance has to be placed on RCTs, where the bias is smaller. Here they acknowledge that one recent study, by Michal Burr, of the University of Cardiff, had a significant influence on the final result.

Dr Burr found no benefit of fish oil, in a study that included 525 deaths, the second largest conducted, and with the longest follow-up.

Attempting to explain why Dr Burr should have reached the conclusions he did, the team suggest that fish oil may have a short-term benefit, but a long-term disbenefit, because it contains traces of toxic methyl mercury as a contaminant. Another possibility is that the beneficial effects may be limited to small groups of people — such as those with heart failure or those who have had a heart attack — who were not included in the Burr study.

At present, the team says, British dietary guidelines recommend that people eat more oily fish, especially if they have had a heart attack. This advice should not change, they say, but should be regularly reviewed.

They add that it is probably not appropriate to recommend fish oil to people who have angina but who have not had a heart attack.

Trials show that people given the plant-produced version of omega 3 experienced no greater improvement in health than those taking fish oil.

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