Interesting, short post by Kevin Drum about fish oil, sample size, and publication bias:

The Sad Story of Fish Oil and Small Sample Sizes

Excerpt:

“Small studies do a poor job of reliably estimating the effects of medical interventions. For a small study [snip] to get published, it needs to show a big effect — no one is interested in a small study that found nothing.”

And:

“It’s still possible that fish oil has a slight beneficial effect, but it’s unlikely.”

Even if fish oil does not reduce all-cause mortality, it does not change my recommendations regarding DHA supplementation in vegans because it’s one thing to say that supplementing with DHA to increase DHA blood levels above average does not improve health, and another to say that there are no problems with having much lower than average DHA levels.

Thanks, Matt.

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Epidemiologists have observed a low risk of coronary heart disease mortality among native Alaskan and Greenland Eskimos and Japanese who consumed a large amount of fish (3). This led researchers to take an interest in fish and fish oil supplementation in the prevention and treatment of heart disease. At the same time, Eskimos have been found to have a high rate of hemorrhage (4), a risk factor for hemorrhagic stroke, which is also likely due to eating large amounts of fish.

In April, PCRM sent out a news release, Fish Oil Does Not Prevent Heart Disease, reporting on a meta-analysis of fish oil supplement trials. I decided to look into it more, and here is what I found.

Secondary Prevention

In April 2012, a meta-analysis of clinical trails using fish oil supplements to treat heart disease was reported in the Archives of Internal Medicine (conducted by the Korean Meta-analysis Study Group (1)). It combined data from 14 randomized, double-blinded, placebo-controlled trials and found that supplementing with fish oil did not reduce the risk of overall cardiovascular events, all-cause mortality, sudden cardiac death, myocardial infarction, congestive heart failure, transient ischemic stroke, or overall stroke in people with a history of cardiovascular disease.

The results of this study contradicted two large clinical trials with fish oil supplements, the Gruppo Italiano per lo Studio della Sopravvivenza nell’Infarto Miocardico (GISSI) and the Japan EPA Lipid Intervention Study. Both of these studies found a protective effect of fish oil supplementation, but they were not included in the meta-analysis because they did not contain a placebo group. In an accompanying editorial (2), Drs. Frank Hu and JoAnn Manson of the Harvard School of Public Health argued that due to the studies in the meta-analysis having relatively small numbers of people, the evidence from these two larger clinical trials should not have been completely ignored. But, Drs. Hu and Manson also conclude that “To date, there is no conclusive evidence to recommend fish oil supplementation for primary or secondary prevention of [cardiovascular disease].”

Primary Prevention

The above discussion is about secondary prevention, or preventing cardiovascular events in people who have already been diagnosed with cardiovascular disease. According to the Korean Meta-analysis Study Group, there have not been any primary prevention studies of fish oil supplementation in a population free of cardiovascular disease at baseline.

As distinct from fish oil supplementation, there have been two meta-analyses on fish consumption and primary prevention of heart disease (3) and stroke (4). Both were conducted by the Northwestern University Feinberg School of Medicine.

The meta-analysis on fish intake and heart disease combined data from 13 cohort groups and found that eating fish once per week could significantly reduce death from coronary heart disease by 15% in comparison to eating fish less than once per month. The trend for non-fatal heart attacks was not strong nor statistically significant.

The meta-analysis on fish and stroke combined data from nine cohort groups and found that eating fish one to three times per month was protective against ischemic stroke (in which a blood clot prevents the brain from receiving blood), which makes up 87% of strokes. Fish intake was not protective against hemorrhagic stroke (in which bleeding in the brain causes pressure on the brain tissue). See the American Stroke Association’s website for some interesting illustrations of the different types of stroke.

Conclusion

Based on the available evidence, moderate amounts of fish appear to be protective against heart disease and ischemic stroke, but fish oil supplementation probably does not prevent heart disease events or death in people already diagnosed with heart disease.

I am not suggesting people eat fish and it’s worth noting that a vegetarian diet has been associated with a 24% lower risk of heart disease mortality (link). I recommend that most vegans take a DHA supplement to make sure they are on par with fish eaters (link).

References

1. Kwak SM, Myung SK, Lee YJ, Seo HG; for the Korean Meta-analysis Study Group. Efficacy of Omega-3 Fatty Acid Supplements (Eicosapentaenoic Acid and Docosahexaenoic Acid) in the Secondary Prevention of Cardiovascular Disease: A Meta-analysis of Randomized, Double-blind, Placebo-Controlled Trials. Arch Intern Med. 2012 Apr 9. [Epub ahead of print] | link

2. Hu FB, Manson JE. Omega-3 Fatty Acids and Secondary Prevention of Cardiovascular Disease–Is It Just a Fish Tale?: Comment on “Efficacy of Omega-3 Fatty Acid Supplements (Eicosapentaenoic Acid and Docosahexaenoic Acid) in the Secondary Prevention of Cardiovascular Disease”. Arch Intern Med. 2012 Apr 9.
[Epub ahead of print] | link

3. He K, Song Y, Daviglus ML, Liu K, Van Horn L, Dyer AR, Greenland P. Accumulated evidence on fish consumption and coronary heart disease mortality: a meta-analysis of cohort studies. Circulation. 2004 Jun 8;109(22):2705-11. | link

4. He K, Song Y, Daviglus ML, Liu K, Van Horn L, Dyer AR, Goldbourt U, Greenland P. Fish consumption and incidence of stroke: a meta-analysis of cohort studies. Stroke. 2004 Jul;35(7):1538-42. Epub 2004 May 20. | link

This New Scientist article, Omega-3: Fishy claims for fish oil, is good review and casts doubt on the need for omega-3 fatty acid supplementation. That said, I haven’t changed my recommendations for vegans who normally get way too much omega-6s and have lower blood levels of EPA and DHA.

Thanks, Matt!

Over the years, we’ve had to read a number of stories about non-B12-supplementing vegans coming down with neurological problems. And while it’s unfortunate that this man got mercury poisoning, it was a relief to see for once a story implicating animal products instead.

The January 24th issue of The Lancet has a case study of a 59-year old Singapore man who developed severe neurological problems due to eating tuna every day. He originally went to the hospital for lower back pain where the staff noticed that he was suffering from poor memory – he later couldn’t remember why he had gone to the hospital. His speech was slow, he stooped, and had problems walking.

Although his urine did not show mercury, his blood level was 28 μg/L; normal is less than 15 μg/L.

Sources other than the diet were ruled out (he did not have mercury fillings). He could not remember what he typically eats, but his cousin reported that he eats tuna every day and also bought Chinese herbal tea (which can contain mercury) from unlicensed traditional practitioners.

With treatment using the drug penicillamine, and eliminating tuna and the herbal tea, his mercury level got down to 19 μg/L. After five months, his symptoms had improved, but had not completely resolved.

I don’t suppose the Center for Consumer Freedom will be covering this story…

Source: Ho RCM et al. Amnesia, political ambition, and canned tuna. Lancet 2009; 373: 352.