Fish oil in murky waters

Omega-3 supplements are widely believed to be good for health, with a wide range of perceived benefits (OMG, it's omega-3!). However, scientific studies do not always provide a clear picture of the benefits and risks of omega-3 fatty acids; many of these studies contradict one another. A paper published recently in BMJ Medicine (bit.ly/bmj-omega3) has added to the literature on the subject. While it does not provide conclusive evidence, it does sound a cautionary note about the use of fish oil supplements among the general population.

In healthy people, the risk of fish oil supplements outweighs the benefits, it notes. "Regular use of fish oil supplements might be a risk factor for atrial fibrillation and stroke among the general population..."  However, in people with a prior history of cardiovascular disease, fish oil supplements lowered the risk of "major adverse cardiovascular events".

Why are fish oil supplements taken?

Fish oil supplements, made from tissues of oily fish, are a popular source of omega-3 fatty acids, and are sold over the counter. People take them to maintain heart health, minimise inflammation, and promote brain function.

Consumer demand for fish oil supplements is growing globally. According to Vantage Market Research, the global omega-3 market was valued at $2.15 billion in 2022 and is expected to reach $3.86 billion by 2030, at a CAGR of 7.6% (bit.ly/vantage-omega3).

What did the latest study establish?

In a study group of 4,15,737 participants, 1,30,365 (31%) used fish oil supplements regularly. Both users and non-users were observed for a mean follow-up period of 12 years. Researchers found that in people with no prior history of cardiovascular disease (CVD), using fish oil supplements increased the risk of atrial fibrillation (irregular heart rhythm) by 13% and that of a stroke by 5%. The risk of transition from healthy states to major adverse cardiovascular events was greater in women and non-smoking participants.

In people with a prior history of CVD, fish oil supplements had a protective effect. In people who had atrial fibrillation at the beginning of the study, regular fish oil supplements lowered the risk of developing a heart attack by 15% and of other major adverse cardiac events by 8%.

What were the profiles of the study subjects using fish oil supplements?

Supplement takers had more women, more elderly, more alcohol consumers, and fewer current smokers. Regular users had a higher proportion of elderly people, white people, women, alcohol consumers, oily-fish eaters, and non-oily-fish eaters. Dietary data were obtained from a dietary questionnaire completed by
the patient.

What do some of the other studies say?
Fish oil supplements are widely considered to be good for everybody’s heart, but many studies have shown otherwise. In a 2021 meta-analysis of seven randomised controlled trials (bit.ly/meta-analysis-omega3), researchers found users of marine omega-3 fatty acid supplements had a 25% higher risk of atrial fibrillation events.

The findings from other studies, however, vary on the margins. A large-scale randomised trial from 2020 (bit.ly/ncbi-cvd) found that omega-3 supplements do not significantly reduce the risk of major CVD events and CVD mortality, and had no effect on the risk of atrial fibrillation.

The authors of the most recent study, published in BMJ Medicine, explain these variances. “One possible explanation for the inconsistent results in these studies is that adverse effects might be related to dose and composition (of the supplement),” they record.

Some unanswered questions
The latest study did not ask the participants about the dosage or the quality of fish oil supplements. It is not clear if the study participants were taking the supplement in the right dosage.

Also, it is not clear if any participants were vegetarians. As the body cannot produce omega-3 on its own, the only way to get it is from the diet. In vegetarians, the lack of fish in the diet can cause a deficiency of omega-3 – even though there are vegetarian sources of omega-3. The study does not record how those with diets deficient in omega-3 respond to fish oil supplements.