Fish oil protects against deaths from heart problems, but doesn't provide a clear benefit in heart rhythm problems (arrhythmias), according a study published on bmj.com today.

Consuming oily fish at least two to four times a week is recommended for patients after a heart attack. But the evidence for the protective effect of fish oil supplements is based on one large trial from over 10 years ago. More recent trials have showed no beneficial effect of fish oil on patient outcomes.

In an attempt to resolve the uncertainty, Professor Ross Tsuyuki and colleagues from Canada systematically reviewed randomized 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), in more than 30,000 participants in 12 studies.

Fish oil was found to be effective at reducing deaths from heart problems, but showed no strong evidence of a beneficial effect on arrhythmias or deaths from all causes.

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. Six studies of over 31, 000 patients examined the effect of fish oil on sudden cardiac death and showed no benefit. A further 11 studies showed a 20% reduction in deaths from heart problems. 

Interestingly, no evidence was found for a dose-response effect between type of fish oil and reduction in deaths from heart problems, so it was not possible for the researchers to suggest an optimal dose or formulation of fish oil. 

In the accompanying editorial, Dr Eric Brunner and Professor Hiroyasu Iso say that the review emphasises the lack of available high quality evidence and the neglect of this important area of nutrient research. 

They call for increased funding to resolve the uncertainty surrounding the protective effect of fish oil, 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.