The scientists from the Wageningen Centre for Food Sciences told the European Society of Cardiology meeting in Stockholm that after taking fish oil supplements for a year, patients were no more likely to be protected against the fatal heart rhythm arrhythmia than others who had not taken the supplement.
This contrasts with two studies published last year showing that fish oil may prevent dangerous abnormalities in heart rhythm.
Evidence from earlier studies also indicates that intake of omega-3 fatty acids reduces the risk of sudden death, a leading cause of death in the Western world. Approximately half of all deaths due to cardiovascular disease are sudden deaths and these are usually caused by arrhythmias.
For the Study on Omega-3 Fatty acids and ventricular Arrhythmia (SOFA), the Dutch researchers recruited 546 patients in 26 cardiology centres in eight European countries. All the participants had an Implantable Cardioverter Defibrillator (ICD) inserted into their chests to monitor their heart rate.
The device automatically treats serious rhythm problems by shocking or pacing and it also contains a memory chip that registers all cardiac arrhythmias.
Patients with an ICD were randomly given either 2g of fish oil or placebo oil everyday up to 12 months in a double-blind trial. Patients received the capsules on top of the usual patient care.
At 12 months, 30 per cent of the patients in the fish oil group had experienced either life-threatening arrhythmia or death compared to 33 per cent of the patients in the placebo group. There was no clear difference between the two groups in how long it took before a first life-threatening arrhythmia occurred or patients died after starting treatment.
Among the subgroup of 342 patients who previously had a myocardial infarction there was a trend towards a beneficial effect of fish oil however. In this group, 28 per cent of the patients on fish oil experienced either life-threatening arrhythmia or death compared to 35 per cent of the patients on placebo oil.
"The SOFA trial does not indicate a strong beneficial effect of omega-3 PUFA [polyunsaturated fatty acid] from fish on life-threatening cardiac arrhythmia. However, patients with a previous myocardial infarction might still benefit from fish oil," said lead researcher Dr Brouwer.
"In any case, we found no indications that fish oil would harm patients," he added.