Patients take omega-3 fish oil supplements in the belief that these oils are heart healthy. But emerging evidence suggests a possible downside and that the benefits are dependent on an individual’s current state of health.

In the 1970’s, Danish researchers first observed how Greenland Eskimos had a fatty acid plasma profile with a higher concentration of long-chain polyunsaturated fatty acids – known as omega-3 fatty acids. At the time, the indigenous Eskimos had a lower incidence of cardiovascular disease mortality compared to the wider Danish population. The reasons for these were unclear but researchers speculated that it has something to do with the higher intake of these omega-3 fatty acids.

Over the next few years, it became clear that there were two fatty acids in particular, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) which conferred a cardiovascular benefit. In fact, as early as 1985, it was shown that men who were initially free of coronary heart disease had a 50% lower cardiovascular mortality if they consumed at least 30 g of fish per day. What’s more, a later randomised, post-myocardial infarction dietary trial in men found that those eating more fatty fish like salmon, sardines and herring had a 29% reduction in two-year all-cause mortality. By 2021, a meta-analysis of 38 randomised controlled trials with nearly 150,000 participants, clearly demonstrated that consumption of omega fatty acids, reduced cardiovascular mortality and improved cardiovascular outcomes. 

But as more data emerged, it also became clear that there was a potential drawback to supplementing with these fatty acids.

Fish oils and the risk of arrhythmias

Some of the earlier fish oil supplement studies had detected that the consumption of fish oils also increased the risk of arrhythmias and in particular, atrial fibrillation (AF). Taking a closer look, in a 2021 meta-analysis, researchers showed that there was a 37% higher risk of incident AF compared with placebo, in randomised trials of fish oil supplementation in patients with cardiovascular disease.

But which one of the two oils were responsible – or was it both of them?

Unfortunately, the available data can’t pinpoint which of the two oils was responsible. Some evidence suggested that DHA was the likely culprit, because those with the highest levels of EPA had a 45% lower risk of AF.

But what about eating fish instead. Does that increase the risk of developing AF?

Thankfully, it seems not. In a 2023 prospective analysis with over 300,000 participants, the authors concluded that ‘ consumption of marine omega-3 fatty acids was not associated with a higher incidence of AF‘. Other work appears to confirm this lack of effect and failed to find an association between blood levels of both EPA and DHA, from eating fish, and a greater risk of incident AF.

So who benefits the most from taking fish oils?

It really depends on someone’s health status. For instance, if someone does not have cardiovascular disease, taking fish oil supplements increased the risk of developing atrial fibrillation by 13%. But in those with existing cardiovascular disease and AF, taking fish oils reduces the risk of progression to a major adverse cardiovascular event by 8%.

Are plant-based omega-3 fatty acids any safer?

In short, yes. Intake of the plant-based omega-3 fatty acid alpha-linolenic acid (ALA) for example, in flaxseed and rapeseed oils (for those who don’t like fish), is not associated with incident AF.

What should clinicians tell their patients?

It seems that while there are possible risks of developing an arrhythmia like AF, from taking fish oil supplements, eating oily fish or plant-based alternatives is not associated with these risks. Nevertheless, if patients do choose to take fish oil supplements, they need to be advised on the risk of developing an arrhythmia and that this ultimately could erode any recognised cardiovascular benefits.