FISH OILS & ATRIAL FIBRILLATION: RED HERRING OR MORE CONTEXT NEEDED?
Regular readers of this blog (both of them) will have noticed that I have frequently looked at dietary supplementation as a health intervention. All too often the claimed benefits of supplements, usually at very large doses, fail to deliver the purported benefits in controlled clinical trials or properly constructed observational studies. With this in mind, I thought it was time to have a look at the benefits of fish oils in all their guises. Now I am a great fan of fish, oily and otherwise and I’m happy to continue consuming my mackerel, sardines and salmon purely on taste grounds. However, I do have some unpleasant memories of, as a child, being force-fed cod liver oil owing to the idea that it would prevent winter illnesses. Despite my protestations that I would prefer repeated winter illnesses to the gag worthy grease, I was dosed each day for several years (and do remember that I was five years old; I suspect that you’d look high and low to find a five year who loves cod liver oil outside of an Inuit settlement).
Population studies show that groups who consumed lots of oily fish had fewer coronary events, and thus people started to look at the prospective use of fish oil-based supplements in primary prevention of cardiovascular disease. A recent study, the OMEMI trial (Omega-3 fatty acids in elderly with Myocardial Infarction) looked at secondary prevention in older patients (70-82 years old) with a recent myocardial infarction (2-8 weeks previously). The supplements were eicosapentaenoic acid (EPA) and docosahexaenoic acid at a total daily dose of 1.8g. The primary end point was a composite of various cardiovascular events, and a secondary endpoint was new episodes of atrial fibrillation (AF). In the initial analysis there was no reduction in clinical events in the two year follow up period. However, a secondary analysis of the 75% of patients with no history of AF found that patients in the supplemented group had a 90% higher rate of AF or micro-AF (defined by the authors as short bursts of irregular atrial activity for seconds). This association seemed to be related to overall levels of serum EPA. Now we come to the interesting part; A recent meta-analysis of 17 cohort studies in the Journal of the American College of Cardiology suggested that regular consumption of foods rich in omega 3 fatty acids did not raise the risk for developing AF. Neither did the authors find any increased risk of AF with “normal” levels of supplementation. They did, however, confirm the potential risk of AF with high dose supplementation. So, the conclusion appears to be that a little of what you fancy does you good, but a lot might not! There are echoes here of Linus Pauling and his advocacy of mega dose vitamin C which has failed to show significant benefit and even the Linus Pauling Institute ironically doesn’t recommend very high dose vitamin C. What I feel is important is the use of meta-analysis of controlled studies to demonstrate true levels of risk that might be either over- or underestimated in smaller studies.
Incidentally this should not be interpreted as criticism of all Linus Pauling’s work. He is one of only two people to be award a Nobel Prize in two different disciplines, chemistry and peace. The award of the Peace prize was for his anti-war (and particularly anti-nuclear) activism and was contentious at the time. The other double winner in two different areas was Marie Curie, in 1903 for physics and in 1911 for chemistry. Those who read my blog about my medical hero Dr James Parkinson will be pleased to know that Marie Curie is my scientific hero: possibly another blog there?