This study is important because there is no really effective therapy for arrhythmias

Dr Alexander Leaf

They are found in oily fish such as tuna, salmon and mackerel.

Italian researchers investigated the effect of eating the fatty acids in a study of more than 11,000 heart attack survivors.

They found that a one gram daily dose was enough to significantly reduce the risk of death from a sudden heart attack by 42%.

The fatty acid appears to reduce the problem of irregular heartbeats in patients who have a condition called cardiac arrhythmia.

Healthy diet

Chief researcher Dr Roberto Martial, from the Consortia Mario Negroid Sod research institute in Santa Maria Embargo, Italy, said: "The risk of death, and sudden death, is higher in the first months after a heart attack.

"It is exactly in this period that the effect on sudden death was noted."

By the end of the of the three-year study the risk of sudden death was about 2% for people who took the supplements and 2.7% for those who did not.

The study participants all ate a healthy Mediterranean-style diet rich in fruits, vegetables, olive oil and fish.

Yet those who took the fish oil supplements still had fewer deaths than those who did not, the study found.

The exact mechanism by which fatty acids protect the heart is not known.

However, it has been shown that fatty acids play an important role in regulating the electrical activity of heart muscle cells.

One theory is that irregular heartbeat is linked to an imbalance of omega-3 and another fatty acid called omega-6, which is found in plant seed cooking oils.

No current treatments

Dr Alexander Leaf, professor of clinical medicine at Harvard Medical School in Boston, who wrote an accompanying article about the findings, said: "This study is important because there is no really effective therapy for arrhythmias."

The health benefits of omega-3s first became apparent when scientists studied the Inuit people of Greenland in the 1970s.

As a group, the Inuit suffered far less from diseases such as rheumatoid arthritis, diabetes mellitus, psoriasis and heart disease than their European counterparts.

Yet their diet was very high in fat from eating whale, seal, and salmon.

Eventually researchers realised that these foods were all rich in omega-3 fatty acids.

The research is published in Circulation: Journal of the American Heart Association.