An international review has suggested taking long-chain omega 3 supplements like fish oil will do little to protect you from a heart attack or stroke.
The just-published Cochrane systematic review analysed results from nearly 80 randomised trials involving more than 100,000 people, to investigate what effect consuming extra omega 3 fat had on diseases of the heart and circulation.
Small amounts of omega 3 fats were considered essential for good health and can be found in the food we eat.
The main types were alphalinolenic acid (ALA), normally found in fats from plant foods such as nuts and seeds, and eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), which were naturally found in fatty fish, such as salmon and fish oils including cod liver oil.
Consuming extra omega 3 fats was widely promoted because of a common belief that it would protect against heart disease, through mechanisms such as reducing blood pressure or reducing cholesterol.
In the studies covered by the Cochrane review, participants were randomly assigned to increase their omega 3 fats or to maintain their usual intake of fat for at least a year.
Most studies investigated the impact of giving a long-chain omega 3 supplement in a capsule form and compared it to a dummy pill, and just a few assessed whole fish intake.
The Cochrane researchers found that increasing long-chain omega 3 provided little if any benefit on most outcomes that they looked at, including reducing risk of cardiovascular events, coronary heart deaths, coronary heart disease events, stroke or heart irregularities.
The risk of death from any cause was 8.8 per cent in people who had increased their intake of omega 3 fats, compared with 9 per cent in people in the control groups.
"We can be confident in the findings of this review which go against the popular belief that long-chain omega 3 supplements protect the heart," said lead author Dr Lee Hooper, of the University of East Anglia.
"This large systematic review included information from many thousands of people over long periods. Despite all this information, we don't see protective effects."
Long-chain omega 3 fats probably did, however, reduce some blood fats - triglycerides and HDL cholesterol.
Reducing triglycerides was likely to be protective of heart diseases, but reducing HDL had the opposite effect.
And while eating more ALA through food or supplements likely had little or no effect on cardiovascular deaths or deaths from any cause, it likely did reduce the risk of heart irregularities - but only from 3.3 per cent to 2.6 per cent.
The review team found that reductions in cardiovascular events with ALA were so small that about 1000 people would need to increase consumption of ALA for one of them to benefit, and similar results were found for cardiovascular death.
"ALA is an essential fatty acid, an important part of a balanced diet, and increasing intakes may be slightly beneficial for prevention or treatment of cardiovascular disease," Hooper said.
Professor Wayne Cutfield, of the University of Auckland-based Liggins Institute, said the review added to a growing number of published studies and systematic review showing that omega three supplements did not benefit heart health or reduce the risk of stroke.
But he added that omega 3 supplements weren't the same as fresh fish, which should be kept in our diets.
"There are many potential roles omega 3 supplements could play in health and well-being. Heart health is not one of these," Cutfield said.
"However other roles are still being explored through randomised controlled trials."
Other researchers have however pointed out what they see as big limitations with the study.
Professor Barbara Meyer of Australia's University of Wollongong, who recently authored a study on the topic, pointed out that the meta-analysis only included studies that had low doses of omega-3 resulting in only a one per cent increase in levels in the blood.
Therefore, Meyer said, it had little or no chance of showing any beneficial effect.
"Given today's health care of people with cardiovascular disease, it is of no surprise that a one per cent increase in omega-3 levels in the blood does not result in reduced death from cardiovascular disease."
Professor Tom Sanders, of King's College London, further noted the review had been unable to allow for the increased intakes of omega-3 fatty acids over the past 20 years, and that most of the trials involved patients with pre-existing cardiovascular disease.
This was important as a substantial proportion of first heart attacks were fatal and previous observational cohort studies - not considered by the Cochrane team - suggested that omega-3 fatty acids may reduce the risk of sudden cardiac death, Sanders said.
"This outcome can only be studied in a primary prevention trial - that is among individuals who have not had a cardiovascular event."