Fish Oil Supplements May Increase Atrial Fibrillation Risk for Some? - Studies Conflict (2024)

Fish oil studies reveal a rift in the science around the pros and cons of omega-3 fatty acids, raising questions about how much a person should take—if any at all.

A 2024 study published in BMJ Medicine suggests that ingesting these oily little pills may contribute to atrial fibrillation and stroke. However, an analysis of the collective research suggests that the seeming contradiction in the science is more likely reflecting the fact that people are different and what may harm some is a help to others.

Fish oil, a rich source of omega-3 fatty acids, is commonly recommended as a dietary intervention to prevent cardiovascular disease. The American Heart Association recommends 0.5 to 1.8 grams per day of combined EPA and DHA, two forms of omega-3s, as either fatty fish or supplements.

The latest fish oil findings published in BMJ Medicine are just another chapter in the ongoing saga of scientific disputes over the health impacts of various foods and supplements, including red meat, saturated fat, and eggs.

To understand this premise, we need to dig into how science works and take a larger look at the research done to date.

Science is complex and contains limitations. Yet, scientific findings are often boiled down into bite-sized pieces that feed over-simplified headlines and leave readers reeling from an onslaught of seemingly contradictory findings.

This article serves as a case study of how the media often reports sanitized sound bites and governing agencies decree sweeping recommendations and health policies based on an oversimplification of complex health issues.

While this article focuses solely on fish oil, the same analysis can be applied to a variety of health issues ranging from salt intake to statins.

Background

In the United States, a person dies from cardiovascular disease (CVD) every 33 seconds, according to the Centers for Disease Control and Prevention.

Many doctors and dietitians recommend omega-3 fatty acids to reduce cardiovascular events, including heart attack or stroke.  Fish oil is one of the most used non-vitamin/non-mineral dietary supplements in the United States, according to the National Institutes of Health’s Office of Dietary Supplements. According to the BMJ Medicine study, despite its popularity, the effect of fish oil on various cardiovascular outcomes has been contentious.

The study noted that previous clinical studies, as well as those analyzing population-wide patterns, have found people were less likely to suffer from a stroke if they supplemented with omega-3 fatty acids. Other studies found that people didn’t benefit at all.

What is already known on this topic

Findings of the effects of omega 3 fatty acids or fish oil on the risk of cardiovascular disease are controversial

Most previous studies focused on one health outcome and did not characterise specific cardiovascular disease outcomes (eg, atrial fibrillation, myocardial infarction, stroke, heart failure, and major adverse cardiovascular events)

Whether fish oil could differentially affect the dynamic course of cardiovascular diseases, from atrial fibrillation to major adverse cardiovascular events, to other specific cardiovascular disease outcomes, or even to death, is unclear

What this study adds

In people with no known cardiovascular disease, regular use of fish oil supplements was associated with an increased relative risk of atrial fibrillation and stroke

In people with known cardiovascular disease, the beneficial effects of fish oil supplements were seen on transitions from atrial fibrillation to major adverse cardiovascular events, atrial fibrillation to myocardial infarction, and heart failure to death

How this study might affect research, practice, or policy

Regular use of fish oil supplements might have different roles in the progression of cardiovascular disease

Further studies are needed to determine the precise mechanisms for the development and prognosis of cardiovascular disease events with regular use of fish oil supplements

Study Objectives: Clarifying the Effects of Fish Oil

The BMJ Medicine study examined the effects of fish oil supplements on different stages of cardiovascular health, including atrial fibrillation (abnormal heart rhythm), major adverse cardiovascular events, and death. Major adverse cardiovascular events were defined as stroke, myocardial infarction, and/or heart failure.

415,737 people, aged 40-69, participated in the study.

The researchers asked two primary questions:

1. Primary Prevention: How does fish oil affect the development of atrial fibrillation, major adverse cardiovascular events, and death in individuals with no known CVD?

2. Secondary Prevention: How does fish oil affect the progression of CVD in people with existing atrial fibrillation and/or major adverse cardiovascular events?

Study Design

Participants were categorized as regular users or non-users of fish oil supplements based on self-reported data collected during baseline surveys. The study followed the participants for a median of 11.9 years, tracking incidences of atrial fibrillation, heart failure, stroke, myocardial infarction, and death through various health records. The study adjusted for numerous covariates, including age, gender, lifestyle factors, and pre-existing health conditions.

Key Findings

The results from the study were divided into two distinct categories—primary prevention and secondary prevention.

Primary Prevention in Healthy People

For people with no known CVD at baseline, regular use of fish oil supplements were associated with a significant increased risk of developing atrial fibrillation. No significant effect was found on the transition from a healthy state to major adverse cardiovascular events or death. However, detailed analysis revealed a clearer picture:

• Atrial Fibrillation: 13 percent increased risk
• Stroke: 5 percent increased risk
• Heart Failure: 8 percent reduced risk
• Myocardial Infarction: No significant effect

The increased risk of first-time heart disease and stroke among healthy people led the researchers to caution against the use of fish oil without clear evidence of its benefits.

“Our findings suggest caution in the use of fish oil supplements for primary prevention because of the uncertain cardiovascular benefits and adverse effects,” they wrote.

Secondary Prevention in People With Known Cardiovascular Disease

For participants who already had a diagnosis of atrial fibrillation at baseline, fish oil supplementation showed protective effects, reducing the risk of transitioning to major adverse cardiovascular events and death. Specific benefits were noted as follows:

• Myocardial Infarction: 15 percent risk reduction
• Heart Failure: 5 percent risk reduction
• Stroke: No effect

Fish oil supplements might help manage CVD in people with atrial fibrillation, according to the results. It reduced risks of heart failure, myocardial infarction, and death, although it did not affect the risk of stroke.

Major Cardiovascular Events

For people who experienced major adverse cardiovascular events during the study, fish oil supplementation had no significant effect on overall mortality. However, the effect on specific outcomes was conflicting:

• Heart Failure: 9 percent reduced risk of death
• Myocardial Infarction: 3 percent increased risk of death
• Stroke: 4 percent increased risk of death

Fish oil may have been beneficial for people who had heart failure but could pose risks for others, particularly those who had a stroke or heart attack, according to the study.

Why fish oil affects different stages of health differently was unclear.

Study Strengths and Limitations 

The BMJ Medicine study included a large sample size, extended time frame, and analyzed multiple states of CVD whereas most studies “tended to assess the role of fish oil at a certain stage of cardiovascular disease,” according to the study.

However, because the study simply observed associations in a large population, it could not specifically say what caused those associations.

The researchers stated that genetics, age, sex, smoking status, and the use of statins and antidiabetic medications might influence the effects that supplementing with fish oil has on cardiovascular events.

Additionally, participants self-reported their fish oil consumption without guidance on dosage or quality, leading to a wide range of usage patterns.

Dr. Peter Osborne, a diplomate with the American Clinical Board of Nutrition and founder of Origins Healthcare, took issue with the study’s approach.

“To do a retrospective trial where they are just saying these people have claimed to use omega-3 and then trying to associate that with an increased risk of poor outcomes is ridiculous,” he told The Epoch Times in an interview.

Additionally, most of the study’s participants were from the white ethnic group, which may limit the application of the findings to a broader global population.

Given the study limitations, it is important to compare the findings of the BMJ Medicine study with other studies before drawing conclusions about whether or not someone should supplement.

Comparison With Previous Studies

The results of the BMJ Medicine study were generally in line with several previous randomized controlled trials and meta-analyses.

Previous Findings of Increased Risk of Atrial Fibrillation With Omega-3 Supplement

1. Meta-Analysis: 25 percent increased risk of atrial fibrillation was reported in Circulation in 2021 based on an analysis of seven randomized controlled trials.
2. REDUCE-IT Trial: 48 percent increased risk of atrial fibrillation was reported in the New England Journal of Medicine in 2018 based on a placebo-controlled trial involving 8,179 patients.
3. Meta-Analysis: 26 percent increased risk of atrial fibrillation was reported in The Lancet in 2021 based on an analysis of 38 randomized controlled trials.
4. STRENGTH Trial: 69 percent increased risk of atrial fibrillation was reported in the Journal of the American Medical Association in 2020 based on a randomized clinical trial including 13,078 patients.

The researchers of the STRENGTH trial concluded:

“... there is some uncertainty whether there is net benefit or harm with administration of any omega-3 fatty acid formulation. Given that 2 large clinical trials have now demonstrated a higher incidence rate, albeit small, of atrial fibrillation with high-dose omega-3 fatty acid administration, the mechanisms underscoring this observation require additional investigation.”

Previous Findings of Reduced Risk of Death with Omega-3 Supplement

1. GISSI-Heart Failure Trial: 9 percent reduced risk of all-cause mortality was reported in The Lancet based on a randomized placebo-controlled trial involving 6,975 people with chronic heart failure.
2. Meta-Analysis: 9 percent reduced risk of cardiac death and 17 percent reduced risk of myocardial infarction was reported in Pharmacological Research in 2020 based on an analysis of 16 randomized controlled trials.
3. GISSI-Prevenzione Trial: 31 percent reduced risk of total mortality and 29 percent reduced risk of sudden death was reported in Circulation in 2002 based on a clinical trial involving 11,323 people with recent myocardial infarction.

Previous Studies Reporting No Benefit with Omega-3 Supplement 

1. The VITAL Rhythm Study: No effect on the incidence of atrial fibrillation, major adverse cardiovascular events, or death was reported in Circulation Research in 2020 based on a placebo-controlled trial involving 25,871 healthy men and women aged 50 and older in the United States.
2. Clinical Trial: No reduction in death was reported in the New England Journal of Medicine in 2013 based on a double-blind, placebo-controlled trial involving 12,513 people with CVD risk factors in Italy.

Why Do Studies Show Conflicting Results?

The inconsistency in results may be attributed to variations in current health status, the dose and quality of omega-3 fatty acids, as well as diet.

Dose-Dependent Effect 

The wide variation in dosages across studies leads to difficulty drawing definitive conclusions. Moreover, studies using the same dosage have sometimes produced conflicting results.

For example, the GISSI-Heart Failure Trial reported a reduced risk of death with 1 gram per day, while a 2013 study in the New England Journal of Medicine reported no reduction in death with 1 gram per day.

Adding to the complexity, the REDUCE-IT trial reported reduced risk of heart attack and stroke with 4 grams per day. But, the STRENGTH trial reported no benefit from 4 grams per day.

These contradictions may be partly due to the composition of the omega-3 supplement.

Composition

The composition of omega-3 fatty acids used in studies varies. Some studies use fish oil, containing both omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Others use only EPA or a combination of EPA and DHA. Some studies use a highly purified form. This variation fuels ongoing debate about which form is most effective.

Purity

According to a 2022 article published in Nutrition Reviews, omega-3 supplements from U.S. stores had about a 50 percent chance of containing less omega-3 than stated on the label.

If products contain less omega-3 fatty acids than advertised, the dose may be insufficient to achieve the intended results in studies where supplement content was not specifically labeled—such as in the BMJ Medicine study.

Rancidity

Omega-3 supplements are highly susceptible to oxidation, according to a 2013 review article in BioMed Research International, yet there is no requirement for studies to use non-rancid oil.

According to Dr. Osborne, in the BMJ Medicine study, “There was no assessment of the quality of omega-3—whether the oils were rancid.”

Rancidity can change the structure of the fat, potentially reducing its effectiveness and leading to negative health impacts. Eating rancid fat has led to inflammation, damaged organs, carcinogenesis, and advanced heart disease in animal studies.

Dr. Chris Knobbe, clinical associate professor emeritus University of Texas Southwestern Medical Center, expressed concern over rancid fish oil supplements leading to heart disease.

“I would avoid omega-3 supplements entirely. Supplementation with omega-3 fish oils, flax oil, etc., is inherently detrimental because these oils are oxidized (rancid) within the pill or bottle and are proven to associate with (or carry) high levels of advanced lipid oxidation end products. These oxidized products are biological poisons that perpetrate pathophysiologic dangers, such as atrial fibrillation, stroke, and increased oxidative stress,” he told The Epoch Times in an interview.

Independent tests by Labdoor found more than 10 percent of fish oil supplements from 60 major retail brands were rancid, and nearly half were close to the maximum recommended limit.

According to the article in Nutrition Reviews, two studies from 2015 revealed that oxidation was common, with 50 percent to 92 percent of store-bought supplements exceeding recommended cutoffs for markers of rancidity.

Rancidity is often masked by added flavorings, making it difficult for consumers to detect, according to a 2023 article from George Washington University School of Medicine & Health Sciences.

Dietary Factors

Diet, particularly the balance between omega-3 and omega-6 fatty acids, is a key variable when analyzing the effect of omega-3 fatty acids on CVD.

While both fatty acids are essential and must be obtained through diet, most people consume too many omega-6 fatty acids, largely from refined vegetable oils, processed foods, and grain-fed meats. Omega-3 fatty acids, which are more prevalent in marine animals like fish and shrimp and in plants like flaxseed and walnuts, are consumed in much lower quantities.

The ideal ratio of omega-6 to omega-3 fatty acids is approximately 2:1, according to a 2021 article in Frontiers for Young Minds. However, many Americans exceed a ratio of 20 to 1, which can contribute to CVD.

When studies examine the effect of omega-3 fatty acids on CVD, such as the BMJ Medicine study, and fail to control for the dietary ratio of omega-3 to omega-6, it can lead to conflicting results.

“Many, if not all, intervention trials” have not controlled for the high omega-6 to omega-3 ratio prevalent in the Western diet, according to a 2010 article in the British Journal of Nutrition. The success or failure of clinical trials using similar doses of omega-3 fatty acids is likely due to not controlling for omega-6 intake, according to a 2006 article in The American Journal of Clinical Nutrition.

The BMJ Medicine study also failed to measure baseline levels of omega-3 and omega-6 fatty acids in the blood, which weakens the study findings, according to Dr. Osborne.

“Because they didn’t start with any kind of baseline, we have no idea that in the people who went on to develop atrial fibrillation, that it was directly associated with the fish oil itself,” Dr. Osborne said.

Baseline testing allows for personalized diet and supplement recommendations, ensuring that additional omega-3 supplementation is beneficial rather than harmful. Without baseline testing, the risk of conflicting study results and adverse health effects increases, Dr. Osborne said.

“Testing should be the number one priority when a doctor is trying to assess whether or not to supplement with omega-3 fatty acids as an adjuvant to their care, in part, because if they are low, it’s going to make a difference as to the quantity of dose the doctor may need to use. But it also creates a baseline by which the doctor can assess over time the outcomes in that patient. Without testing, it’s all guessing,” Dr. Osborne told The Epoch Times.

Unfortunately, research studies often neglect to control for a balance of essential fats in the diet or baseline levels which fuels the contradictory findings. 

People Who Shouldn’t Supplement Fish Oil

Fish oil is not suitable for everyone, including the following people, according to Liu:

• Those allergic to fish.
• Those prone to bleeding. Fish oil has an anticoagulant effect and is not suitable for women with bleeding hemorrhoids or intense menstruation, nor is it suitable for pregnant women or patients preparing for surgery.
• Those who take aspirin. Many people take aspirin to maintain or prevent CVD. Studies have shown that aspirin and fish oil may interact with one another, so be cautious when using them concurrently.

Omega-3s From Natural Foods

Obtaining omega-3 fatty acids from natural foods may be safer and more effective than relying on supplements. Liu recommends the following natural foods:

• Fish and Seafood: Salmon, mackerel, and sardines are all rich in DHA and EPA.
• Plant-Based Oils: Flaxseed and walnut oils contain alpha-linolenic acid (ALA), an essential fatty acid that can be converted into EPA and DHA in the body. Although less efficient than EPA and DHA, it is still a good plant-based option.
• Nuts and Seeds: Walnuts, flaxseeds, and chia seeds are all rich in omega-3 fatty acids.

Safe Supplementation With Fish Oil

For people considering fish oil supplementation, Liu recommends adhering to the following guidelines to minimize risks:

• Control the Dosage: Limit the daily intake of fish oil to under 1 gram to prevent potential risks such as atrial fibrillation.
• Choose Natural Foods: Prioritize getting omega-3 fatty acids from fish and other natural foods to avoid overreliance on supplements.
• Watch Out for Contaminants: Choose fish with low mercury levels, such as salmon and sardines, and avoid those with high mercury levels, such as swordfish and king mackerel.
• Consult Physicians: Use fish oil supplements under the guidance of a doctor, especially if you have CVD or other health conditions.

Conclusion

This article used fish oil as a case study to explore how complex scientific findings are often oversimplified.

The recent BMJ Medicine study on fish oil supplements underscores the complex relationship between omega-3 fatty acids and cardiovascular health. While fish oil is widely recommended for its potential benefits, the study found that regular use might increase the risk of first-time heart disease and stroke in healthy individuals. Conversely, people with existing cardiovascular conditions like atrial fibrillation may experience protective effects.

These mixed outcomes highlight the importance of personalized recommendations based on individual health profiles.

The conflicting findings across various studies highlight the need to consider factors such as supplement dosage and quality, as well as diet. It is crucial for future studies to control for these variables to provide clearer guidance.

Until then, people should consult health care professionals to determine if fish oil and other supplements are appropriate for their specific health needs, ensuring they achieve the right balance for optimal cardiovascular health.

About the Author: Dr. Sina McCullough holds a PhD in Nutrition and a B.S. in Neurobiology, Physiology and Behavior, both from UC Davis. She was the Director of Research and Development for a supplement company and taught Biochemistry and Bioenergetics at UC Davis.

Sources: 

https://www.theepochtimes.com/health/studies-conflict-do-omega-3-supplements-help-or-harm-5665054

https://www.theepochtimes.com/health/fish-oil-supplements-may-increase-atrial-fibrillation-risk-for-some-5781243