Meta-analysis of marine omega-3 supplement trials
In 2021, Baris Gencer and colleagues performed a meta-analysis of these five trials plus 2 more (GISSI-HF and RP) looking specifically at risk for AFib. Their final analysis included more than 81,000 patients followed for nearly 5 years.
Omega 3 fatty acid supplements associated with a 25% increase in the risk for AFib (HR, 1.25; 95% CI, 1.07-1.46P =.013). Exploring further, they noted a dose-dependent relationship. Most of the increased risk occurred in trials that tested greater than 1 g/d.
Summary
When faced with surprise findings, I like to think things through.
First about plausibility. Omega-3 fatty acids clearly exert electrophysiologic effects on cardiac cells, an increase in AFib risk is plausible. The exact underlying mechanism may be unknown, but exact mechanisms are less important than actual clinical effects (see sodium-glucose cotransporter 2 inhibitors).
What about causality? Factors supporting causality include plausibility, consistency of increased AFib risk in multiple studies, and a dose-response relationship.
I see multiple clinical implications of this observation.
The first is the power of the randomized trial to inform practice. If we relied only on observational evidence, we might have assumed that since high fish consumption in populations associated with lower rates of cardiac events, fish oil supplementation would also reduce cardiac events. Other than the outlier trial, REDUCE-IT, with its mineral oil placebo, the preponderance of the randomized controlled trial evidence does not support fish oils for the reduction of CV events.
Randomized controlled trials also exposed the AFib risk. This would have been difficult to sort out in nonrandom observational studies.
Another underappreciated lesson is the notion that drugs, including supplements, can have off-target effects.
Consider the case of statin drugs. It is widely assumed that statins reduce cardiac events by lowering low-density lipoprotein cholesterol (LDL-C). Yet, statins became a mainstay not because of LDL-C lowering but because multiple trials found that this class of drugs reduced cardiac events without increasing adverse effects.
Omega-3 fatty acids reduce triglyceride levels, but this is not enough to adopt the use of these pills. The lack of consistent reduction in CV events and the off-target signal of AFib risk argue against routine use of fish-oil pills.
I will close with uncertainty. Though there is plausibility and multiple reasons to infer causality of marine omega-3s in increasing AFib risk, the effect size remains unknown.
In an editorial accompanying the recent meta-analysis, epidemiologist Michelle Samuel, MPH, PhD, and electrophysiologist Stanley Nattel, MD, cautioned readers on a technical but important point. It concerns the matter of competing risks, such as death, in the analysis of AFib risk, meaning that patients who died may have developed AFib had they lived. They provide a detailed explanation in the open access article, but the take-home is that the exact effect size is difficult to quantify without patient-level original data.
No matter. I find the signal of increased AFib risk an important one to use at the bedside.
Intermittent AFib has an unpredictable natural history. It often resolves as mysteriously as it arises. When patients take fish-oil supplements, I cite these studies, note the lack of CV protection, then I recommend stopping the pills.
This allows for one of the most important interventions in AFib care: time.
Dr. Mandrola is a clinical electrophysiologist with Baptist Medical Associates, Louisville, Ky. He has disclosed no relevant financial relationships.
A version of this article appeared on Medscape.com.