Canadian researchers have suggested that the evidence which supports the protective benefit of fish oil is inconclusive and more research is needed.
According to the researchers while fish oil protects against deaths from heart problems, a clear benefit in heart rhythm problems has not been shown and they say more funding is urgently needed in this neglected area of nutrient research.
Much of the evidence for the protective effect of fish oil supplements is based on one large trial done over 10 years ago and more recent trials have shown no beneficial effect of fish oil on patient outcomes - consuming oily fish at least two to four times a week is recommended for patients after a heart attack.
Professor Ross Tsuyuki and colleagues, in an attempt to resolve the uncertainty, reviewed randomised trials of fish oil as a dietary supplement in the prevention of cardiac deaths and arrhythmias - abnormal electrical activity in the heart that can lead to death.
Their review included more than 30,000 participants taking part in 12 studies and they found that while fish oil was found to be effective at reducing deaths from heart problems, no strong evidence of a beneficial effect on arrhythmias or deaths from all causes was found.
Three of the studies involving over 11,000 participants, analysed the effect of fish oil supplementation on the reduction in implantable cardiac defibrillator interventions and reported a neutral effect - 6 studies of over 31, 000 patients examined the effect of fish oil on sudden cardiac death and showed no benefit - and a further 11 studies showed a 20% reduction in deaths from heart problems.
Because no evidence was found for a dose-response effect between type of fish oil and reduction in deaths from heart problems, it was not possible for the researchers to suggest an optimal dose or formulation of fish oil.
An accompanying editorial by Dr. Eric Brunner and Professor Hiroyasu Iso says the review emphasises the lack of available high quality evidence and the neglect of this important area of nutrient research and they have called for increased funding to resolve the uncertainty surrounding the protective effect of fish oil.
They say this is needed to help the millions of people with heart disease and to protect the world's marine life - which, they say, is facing extinction for commercial gain, partly, and maybe unnecessarily, in the name of public health.
The research is published in the current issue of the British Medical Journal.