Fish oil supplements may increase the risk of someone developing a heart condition or stroke, but could reduce the risk for those who already have cardiovascular disease, according to research.

Fish oil is a rich source of omega-3 fatty acids. The NHS recommends at least one portion of oily fish a week to help prevent the development of cardiovascular disease.

In order to find out how much protection it affords, a team of researchers in China, the US, the UK and Denmark monitored the health of more than 400,000 participants in the UK Biobank for an average of 12 years to estimate the associations between fish oil supplements and new cases of atrial fibrillation (irregular heartbeat); heart attack, stroke, and heart failure; and death in people with no known cardiovascular disease.

They also assessed whether these supplements affected the progression of heart conditions.

Published in the journal BMJ Medicine, the study found that for those with no known cardiovascular disease at the start of the monitoring period, regular use of fish oil supplements was associated with a 13% heightened risk of developing atrial fibrillation and a 5% heightened risk of having a stroke.

But for patients with heart disease at the beginning of the study period, fish oil supplements were associated with a 15% lower risk of progressing from atrial fibrillation to a heart attack, and a 9% lower risk of progressing from heart failure to death.

The benefits and risk of omega-3 supplementation were not uniformly observed, the study noted. The risk of healthy patients going on to have a heart attack, stroke or heart failure was 6% higher in women and 6% higher in non-smokers.

There was also a greater beneficial effect for older people and men with existing heart conditions, where the risk of transition from good health to death was 11% and 7% lower respectively.

This was an observational study, so no conclusions can be drawn about causal factors, the authors caution. And no information was available on either dose or formulation of the fish oil supplements. Given that most of the participants were white, the findings might not be applicable to people of other ethnicities, they add.

Tracy Parker, a senior dietitian at the British Heart Foundation, said: “This research shouldn’t be concerning to people who regularly take fish oil supplements, but it’s also not a green light to start taking them to prevent heart and circulatory diseases.

“In the UK, Nice guidelines don’t recommend taking fish oil supplements to either prevent heart and circulatory diseases or stop another heart attack. Supplements such as omega-3 fatty acids are no substitute for a healthy diet and, instead of focusing on individual nutrients, it’s important to look at your diet as a whole to help lower your risk.

“The traditional Mediterranean diet has been shown time and again to reduce the risk of heart attack or stroke. This includes more fish – white and oily – and less red meat, along with plenty of fruit and vegetables, beans, lentils, nuts, seeds and wholegrains.”