Questions about omega-3 fatty acid supplements come up often in the atrial fibrillation (AFib) clinic.
The story begins with the simple observation that populations who eat lots of oily fish have fewer coronary events. This correlation provoked great interest in concentrating fish oils in pill form and studying their use to promote health.
OMENI secondary analysis
Peder Myhre, MD, and colleagues recently published a secondary analysis of the OMENI trial looking at both the risk and possible causes of AFib in the omega-3 group.
The OMENI trial randomly assigned slightly more than 1,000 older patients (mean age, 75 years) post–myocardial infarction to either 1.8 g/d of fish oil supplements versus placebo for 2 years. The supplements comprised 930 mg of eicosapentaenoic acid (EPA) and 660 mg of docosahexaenoic acid (DHA). The main trial reported no difference in a composite primary endpoint of MI, revascularization, stroke, death, or hospitalization for heart failure.
The secondary analysis explored the 75% of patients in the main trial who had no history of AFib. It looked at how many in each group developed either true clinical AFib or what the authors called micro-AFib, defined as short bursts of irregular atrial activity lasting seconds.
The sub-analysis had three main findings: Patients in the supplement arm had a 90% higher rate of AFib or micro-AFib, compared with patients on placebo, EPA had the strongest effect on the association, and there was a graded risk for AFib with increasing serum EPA levels.
The authors raised the possibility that more micro-AFib might be a possible mediator of AFib risk.
Trials of low-dose EPA and DHA
First, the low-dose trials. In the ASCEND trial from 2018, more than 15,000 patients with diabetes were randomly assigned to either 1 g of omega-3 fatty acids (460-mg EPA and 380-mg DHA) or mineral oil.
The trial was neutral. After 7.4 years, the primary endpoint of MI, stroke, transient ischemic attack, or cardiovascular death occurred in 8.9% of the supplement group versus 9.2% of the placebo arm.The incidence of AFib was higher in the omega-3 group but did not reach statistical significance (2.1% vs. 1.7% for placebo; hazard ratio, 1.23; 95% confidence interval, 0.98-1.54).
Another neutral CV trial, VITAL, specifically studied the effects of marine omega-3 pills (460-mg EPA and 380-mg DHA) in older adults without heart disease, cancer, or AFib. After slightly more than 5 years, AFib occurred at a similar rate in the active arm and placebo arms (3.7% vs. 3.4% for placebo; HR, 1.09; 95% CI, 0.96-1.24; P = .19)
Trials of very high-dose marine omega-3s
Next came trials of higher doses in higher-risk populations.
In 2020, JAMA published the STRENGTH trial, which compared 4 g/d of a carboxylic acid formulation of EPA and DHA with a corn oil placebo in more than 13,000 patients who either had established atherosclerotic CV disease (ASCVD) or were at high risk for ASCVD.
The trial was terminated early because of futility and a signal of increased AFib risk in the supplement arm.
Nearly the same number of patients in the supplement versus placebo arm experienced a primary composite endpoint of major adverse cardiac events: 12.0% versus 12.2%, respectively.
AFib was a tertiary endpoint in this trial. An increase in investigator-reported new-onset AFib was observed in the omega-3 group: 2.2% vs. 1.3% for corn oil (HR, 1.69; 95% CI, 1.29-2.21; nominal P < .001).
The REDUCE-IT trial randomly assigned more than 8,000 patients who had ASCVD or diabetes and high ASCVD risk and elevated triglyceride levels to either 4 g of icosapent ethyl daily, a concentrated form of EPA, or a mineral oil placebo.
After nearly 5 years, there was a 4.8% absolute risk reduction in the primary endpoint of CV death, MI, stroke, revascularization, or unstable angina with icosapent ethyl. An increase in atherogenic biomarkers in the mineral oil placebo complicated interpretation of this trial.
Hospitalization for AFib or flutter occurred in 3.1% of the active arm versus 2.1% of the mineral oil group (P = .004).