The widespread belief that taking omega-3 capsules will help protect you from a heart attack, stroke or early death is wrong, according to a large and comprehensive review of the evidence.

Thousands of people take omega-3 supplements regularly and for years. The belief that it protects the heart has spread – and is promoted in the marketing of the supplements – because the results from early trials suggested the capsules had cardiovascular benefits.

Small amounts of omega-3 fatty acids, are essential for our health. Omega-3 fats are found in certain foods – most famously in oily fish such as salmon and cod liver oil, which contain the long chain fats called eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). Nuts and seeds, and in particular walnuts and rapeseed oil, contain another sort of omega-3, called alpha­linolenic acid (ALA).

But a major review by the respected Cochrane collaboration of all the well-conducted trials carried out internationally to test the effect of omega-3, involving more than 112,000 people, says there is no evidence that the supplements do any good.

“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 Cochrane lead author, Dr Lee Hooper from 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.”

The researchers examined 79 randomised trials of omega-3 fats, of which 25 were considered highly trustworthy because they were designed and carried out well. The studies recruited men and women, some healthy and others with existing illnesses, from North America, Europe, Australia and Asia. Some of those who took part had been asked to eat their usual diet, while the rest supplemented it with extra omega-3 fats for at least a year.

In most of the trials, people in the supplement group were asked to take long chain omega-3 fats in the form of a daily capsule. Only a few trials looked at the effects of eating extra oily fish. Other trials asked people to consume more ALA – the form derived from plants – which was added to margarine or given to them in walnuts.

Fish oil supplements made no difference to the risk of death or heart attacks or strokes, the Cochrane researchers found. Eating more ALA from supplemented margarine or walnuts did convey a small benefit, but the reduction in cardiovascular events was very small.

The researchers embarked on their systematic review at the request of the World Health Organisation, which is updating its guidance on polyunsaturated fats. The belief that omega-3 supplements could protect against cardiovascular diseases came from a couple of positive results from trials in the late 1980s and early 1990s, said Lee. “We’ve all believed it for quite a long while,” she said. “But none of the trials since have shown these results. We somehow haven’t adjusted to that data.”

Lee said there was not enough trial evidence to show whether or not eating more oily fish is beneficial – although she suspected it probably is. Extra fish replaces something else in the diet, which may be less good for you, she said. “Also iodine, selenium, calcium and vitamin D are at good high levels and much less common in other foods that the fish might replace. And if you take an oily fish capsule you might think you have done the healthy thing and now you can relax,” she said.

Tim Chico, professor of cardiovascular medicine at Sheffield University, said it was unlikely any one element of the diet could alone prevent heart disease. “Previous experience has shown that although some types of diet are linked to lower risk of heart disease, when we try to identify the beneficial element of the diet and give it as a supplement it generally has little or no benefit,” he said. It happened with vitamins and now the Cochrane research had shown the same thing with omega-3.

“Such supplements come with a significant cost,” he said, “so to anyone buying them in the hope that they reduce the risk of heart disease, I’d advise them to spend their money on vegetables instead.”