Why Fish Oil Supplements Can Be Dangerous for the Heart

Most people are familiar with the best things to eat for a healthy heart: vegetables, low-fat dairy, and lean proteins like fish. Some fish like salmon also have the added benefit of being full of omega-3 fatty acids, which are healthy fats that can help to raise good cholesterol and lower triglycerides.

Or so we thought. Studies on the benefits of fish oil, and fish oil supplements, haven’t been as conclusive when it comes to actually preventing heart disease for people who aren’t at higher risk. In the latest, published in BMJ, researchers report that people with no history of heart problems who regularly took fish oil supplements actually increased their risk of atrial fibrillation.

The study involved more than 415,000 men and women in the health-data hub UK Biobank who agreed to share their health records and answer questions about their diets and supplement consumption. The participants joined between 2006 and 2010, and were followed until 2021 or until they died, with most people followed for a median of nearly 12 years. Overall, about a third of them used fish oil supplements.

Among people without a history of heart disease, those who regularly took fish oil supplements had a 13% higher risk of developing atrial fibrillation and a 5% greater risk of stroke compared to those who didn't take the supplements. The researchers also broke down the participants by the severity of heart-related outcomes they experienced: women who started out with no heart problems had a 6% higher chance of having a heart attack, stroke, or heart failure if they took fish oil supplements compared to those who did not take them.

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For people who already had a history of heart disease, however, the results were quite different. For them, as previous studies have shown, regular fish oil supplementation was linked to a 15% lower risk of progressing to more severe heart problems—from having atrial fibrillation to having a heart attack, for example, or from worsening from heart failure to dying.

Dr. Alice Lichtenstein, director and senior scientist at the Cardiovascular Nutrition Laboratory at Tufts University and expert volunteer for the American Heart Association (who was not involved in the study), says the risk of atrial fibrillation associated with fish oil supplementation—especially at high doses—is not new. And the risk noted in the current study could reflect the fact that some of the people were taking higher doses, since the researchers only asked people to report supplement use and could not verify either their actual use or the doses they took. “The only response option for regular fish oil supplement use [in the study] was ‘yes’ or ‘no,’ and rarely is supplement use that consistent,” says Lichtenstein. “The authors [also] have no information about the type of fish oil supplements consumed, which can vary widely, and the dose taken.”

The structure of the study may also explain some of the difference in results between the people who did not have a history of heart problems and those that did. Since the study was conducted as an observational analysis, and not as a randomized controlled trial in which people were assigned to specific doses of fish oil supplements and monitored closely, the researchers could not control or adjust for a number of different factors in their study population, including the reasons why people were taking fish oil and their underlying health at the start of the trial.

In the largest randomized control trial of fish oil in an otherwise healthy population without known heart problems, reported in 2018, fish oil supplements were linked to a 28% lower risk of heart attacks and an overall 17% reduced risk of all heart disease events. In that study, the supplements were not associated with a lower risk of stroke, however. Dr. JoAnn Manson, chief of the division of preventive medicine at Harvard Medical School and physician at the Brigham and Women’s Hospital who co-led the trial, says those results are likely more indicative of how fish oil affects the heart, since the study used quality-controlled sources of supplements and carefully monitored the participants’ dosage.

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“The bottom line is that I think the randomized trials do show differences in atrial fibrillation risk that is dose dependent,” she says. “Doses of 1g per day and lower are not associated with a meaningful increase in atrial fibrillation. But doses of greater than 1g a day show a substantial, close to 50% increased risk of atrial fibrillation.”

The reason for the risk could be related to the physiological effects of fish oil. The omega-3 fatty acids in fish oil can affect the electrical rhythms of the heart, in particular the beating of the upper, or atrial, chambers.

It's because of that risk that the American Heart Association does not recommend fish oil supplements to prevent heart disease. “Fish oil supplements are not routinely recommended for the prevention or treatment of heart disease, particularly considering the potential increased risk of developing atrial fibrillation,” says Lichtenstein. Instead, she says, people interested in protecting themselves from heart problems should follow a healthy diet, which includes eating fish about one to two times a week, exercising regularly, minimizing stress, and getting enough sleep.

But for people with a higher likelihood of heart problems, the risk of atrial fibrillation is counterbalanced by the potential anti-inflammatory, anti-clotting, and triglyceride-lowering benefits of omega-3 fats, which tend to protect the heart and lower the risk of coronary events. As the study shows, people who already have a history of heart disease, and who don't eat much fish regularly, tend to benefit most from those effects; that protection doesn’t appear to transfer to people hoping to reduce their risk of a first-time heart event.

“Correlation does not prove causation,” says Manson about the study results. “I don’t think the public should be alarmed about this study because most organizations currently do not recommend omega-3 supplements for primary prevention of heart disease. They currently recommend one to two servings of fish a week.”