Supplementing Fish Oil in Healthy Individuals Can Elevate Heart Risks

Typically, the foundations of a heart-healthy diet consist of vegetables, low-fat dairy, and lean proteins, including fish. Some fish varieties, like salmon, provide the added benefit of being rich in omega-3 fatty acids, which are healthy fats that contribute to raising good cholesterol and lowering triglycerides.

However, this notion has come into question. Studies examining the benefits of fish oil, and fish oil supplements, have shown less conclusive results when it comes to preventing heart disease in individuals who are not considered high-risk. A recent study has revealed that individuals with no history of heart issues who regularly consumed fish oil supplements actually increased their risk of developing atrial fibrillation.

The study included over 415,000 men and women who participated in the UK Biobank, a health-data hub. These participants agreed to share their health records and provide information on their diets and supplement use. Enrolling between 2006 and 2010, they were monitored until 2021 or until their death, with most individuals followed for a median of almost 12 years. Approximately one-third of the participants used fish oil supplements.

Among those with no prior 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 did not take the supplements. The researchers further categorized the participants based on the severity of their heart-related outcomes: women who began the study without any heart problems had a 6% higher chance of experiencing a heart attack, stroke, or heart failure if they took fish oil supplements compared to those who did not.

However, the results were notably different for individuals who had a history of heart disease. As indicated by previous studies, regular fish oil supplementation was associated with a 15% lower risk of progressing to more severe heart problems—for example, from atrial fibrillation to a heart attack or from worsening heart failure to death.

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), notes that the risk of atrial fibrillation associated with fish oil supplementation—especially at high doses—is not a new finding. The risk observed in the current study could be attributed to the fact that some participants may have been taking higher doses, as the researchers only collected information on supplement use without verifying actual use or the doses taken. “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 study’s structure may also contribute to the differing results observed between individuals with and without a history of heart problems. Since the study was conducted as an observational analysis, rather than a randomized controlled trial where participants are assigned specific doses of fish oil supplements and closely monitored, the researchers could not effectively control or adjust for various factors in their study population, including the reasons why people were taking fish oil and their underlying health at the outset of the trial.

In the 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. Notably, in that study, the supplements were not associated with a lower risk of stroke. 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, suggests that those results may more accurately reflect how fish oil affects the heart, as the study utilized quality-controlled sources of supplements and carefully monitored the participants’ dosage.

“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 potential risk may be related to the physiological effects of fish oil. The omega-3 fatty acids in fish oil can have an impact on the electrical rhythms of the heart, particularly the beating of the upper, or atrial, chambers.

Due to this risk, the American Heart Association does 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 suggests that individuals seeking to protect themselves from heart problems should adopt a healthy diet that includes consuming fish about one to two times per week, engaging in regular exercise, minimizing stress, and ensuring adequate sleep.

But for individuals with a higher risk 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 reduce the risk of coronary events. As the study demonstrates, individuals who already have a history of heart disease, and who do not regularly consume fish, tend to derive the most benefit from these effects; such protection does not appear to be transferrable to individuals seeking to reduce their risk of a first-time heart event.

“Correlation does not prove causation,” says Manson regarding 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.”