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Fish Oil Supplements May Increase Stroke Risk in Healthy Adults

A hand holding a fish oil tablet.
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Fish oil supplements may just do more harm than good in healthy adults, according to a new study published in BMJ Medicine.

After sifting through the medical data of 415,737 participants, researchers found that those who took regular fish oil tablets and had no initial cardiovascular disease went on to have a 13% higher risk of developing atrial fibrillation and a 5% higher risk of having a stroke.

For those with pre-existing cardiac conditions, however, the omega 3-rich fish oil appeared to provide its usual purported benefits; among these participants, the 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 researchers say their findings suggest that fish oil supplements may have different roles in the progression of cardiovascular disease depending on the health of a person.

Other researchers, who were not involved in the study, have cautioned such conclusions.

Something fishy

Fish oil contains high levels of omega-3 fatty acids, which are known to lower the risk of heart disease.

To explore this health effect further, an international group of researchers (from China, Denmark, the UK and the US) accessed health records from the UK Biobank. They drew on information from 415,737 participants (55% of whom were women), all aged 40–69, who were surveyed between 2006 and 2010. All participants’ health was tracked until March 2021 or until their death.

Nearly a third (130,365) of the participants said they regularly consumed fish oil supplements.

Over an average monitoring period of 12 years, 18,367 participants developed atrial fibrillation, 22,636 had a heart attack, stroke or developed heart failure and 22,140 died.

Among those who started with good cardiovascular health but developed atrial fibrillation, 3,085 experienced heart failure, 1,180 had a stroke and 1,415 had a heart attack.

The researchers say this good-to-poor health pattern was tied with the participants’ fish oil habits.

For those starting the study with good cardiovascular health, 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. This phenomenon was 6% more likely to be seen in women and non-smokers.

The expected benefits of fish oil were, however, seen in the participants who already had cardiovascular issues. These supplement swallowers experienced, on average, 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. These apparent protective effects were more likely to be seen in men (7% lower risk) and older participants (11% lower risk).

While the researchers admit that their findings were observational – and no biochemical causation could be proven – they nonetheless conclude that regular use of fish oil supplements may be a risk factor for atrial fibrillation and stroke among the general population but could be beneficial for those who already live with cardiovascular disease.

But other researchers aren’t as confident in the study’s findings. Speaking to Technology Networks, Dr. Duane Mellor, a registered dietician and spokesperson for the British Dietetic Association, questioned whether other factors could be behind the participants’ cardiovascular outcomes.

“Given the limitations in the UK Biobank data in terms of dietary intake and lifestyle measures (which is not unusual given its very large data set), it is quite likely that the association between taking cod liver oil and cardiovascular health outcomes could be a product of other factors.”

“On its own, cod liver oil supplements would have a tiny effect if any on cardiovascular disease risk, so the findings of this study are not that significant. It is the overall lifestyle in terms of dietary pattern (of which only meat and vegetables were considered in this study) and not other foods, known to decrease risk of heart disease,” he said.

“So, this is an interesting study, but it perhaps is just a statistical association, as currently there is no plausible biological mechanism to highlight any increase in risk of cardiovascular disease in individuals who have no previous history.”

About the interviewee

Dr. Duane Mellor is an award-winning registered dietitian and science communicator. They were speaking to Leo Bear-McGuinness, Science Writer at Technology Networks.


Reference: Chen G, Qian ZM, Zhang J, et al. Regular use of fish oil supplements and course of cardiovascular diseases: prospective cohort study. BMJ Med. 2024. doi:10.1136/bmjmed-2022-000451