Eating fish can reduce the risk of abnormal heartbeats that can be deadly, say US researchers.
Fish should be baked or broiled to be protective, say the experts
But it must be baked or broiled, not fried, to be protective, Dr Dariush Mozaffarian and colleagues at Harvard Medical School in Boston found.
People who ate "good" oily fish one to four times a week reduced their risk of an abnormal heartbeat called atrial fibrillation by nearly a third.
The findings are reported in the journal Circulation.
Atrial fibrillation (AF) is an abnormal heart rhythm in which the upper two chambers of the heart beat faster than normal.
This can cause unpleasant palpitations, the sensation that the heart is beating fast, and breathlessness.
The blood is not pumped out of the heart as well as it should be and may pool and clot. If the blood clot leaves the heart it can lodge in an artery in the brain, causing a stroke.
Oily fish is known to be good for the heart. It provides the omega-3 polyunsaturated fatty acids, which are believed to help provide protection from coronary heart disease.
These are found mainly in oily fish such as herring, kippers, mackerel, pilchards, sardines, salmon, fresh tuna, trout and anchovies. They are also found in green leafy vegetables.
Omega-3 fatty acids are thought to reduce the stickiness of the blood, making it less likely to clot, and prevent the heart from beating irregularly.
But, according to Dr Mozaffarian, no one had looked directly at whether fish intake affects AF.
He studied nearly 5,000 people over the age of 65.
He looked at their diet over the course of a year and then followed the people for another 12 years, watching for any who developed AF.
There were 980 cases of AF over the 12 years.
When the researchers looked at those who went on to develop AF and those who did not they found marked differences in fish intake.
Baked or broiled
Those who ate more fish that was broiled or baked were less likely to have AF.
People who ate these types of fish one to four times per week had a 28% lower risk of AF compared with those who ate fish less than once a month.
Eating five or more portions per week reduced the risk of AF by 31%.
People who ate lots of fried fish or fish burgers did not enjoy the same protection, however.
Dr Mozaffarian said the cooking process had either destroyed all of the good omega-3 or the fried fish were lean white fish like cod rather than oily fish.
"The results suggest that regular intake of tuna or other broiled or baked fish may be a simple and important deterrent to AF among older men and women," he said.
Belinda Linden, head of medical information at the British Heart Foundation, said: "This review provides further evidence of how omega-3 fats may work by specifically blocking the currents that lead to erratic heart rhythms.
"Seven out of ten people in the UK still don't eat the recommended amount of fish.
"By eating a balanced diet which is low in salt and saturated fats, along with fish and plenty of fruit and vegetables, the chances of developing coronary heart disease, the UK's single biggest killer, are significantly reduced," she said.
The Food Standards Agency recently issued advice on how much oily fish it is safe for people to eat.
As well as containing omega-3 fatty acids which help to prevent heart disease, oily fish also contain pollutants that may also pose a health risk to humans.
It says men, boys and women past childbearing age can eat up to four portions of oily fish such as salmon, tuna, trout and sardines a week.
Women of childbearing age should keep to a maximum of two portions a week.