Once a hypothesis possesses overwhelming evidence and strong predictive power, scientists start calling it a “theory” (see our sidebar, “Theory or hypothesis?”).

One well-known hypothesis is that diets high in fish or omega-3 fish oil drop the risk of heart disease and its adverse outcomes.

Why do omega-3/heart studies conflict?
Headlines trumpeting news of seemingly contrary studies suggest – wrongly – that this idea remains a mere hypothesis.

That misperception can be traced to the media's frequent failure to put research reports into perspective or describe them accurately.

Biomedical studies on a single subject – like the heart-health effects of omega-3s – often yield differing results.

This is largely because of two factors:

• Variations in the experimental methods used.
• The myriad lifestyle and genetic variables affecting study participants, whose effects can be hard to account for accurately.

We've reported on several meta-analyses that have produced conflicting conclusions about the effects of fish or omega-3 fish oil on people's risk for heart disease and its adverse outcomes.

For example, see “Does Fish Fight Stroke?”, “New Evidence Analysis Supports Omega-3 Heart Benefits”, “Media Misreports Omega-3 Heart Study”, “Experts Find New Fish-and-Health Review Deeply Distorting” “Omega-3s Cut Heart Risks & Death Rates in Clinical Trial”

In every case, researchers' inadvertent omissions or poor decisions account for the cases when a meta-analysis came to negative conclusions about the ability of fish or omega-3 fish oil to reduce people's risk for heart problems.

And this applies to a new meta-analysis that sought to determine the ability of fish and omega-3 fish oils to reduce stroke risk.

Although the authors concluded that only fish showed clear evidence for cutting stroke-risk, they inexplicably left out a large clinical study that showed strong anti-stroke benefits from fish oil.

Anglo-Dutch evidence review flawed with regard to fish oil

The Anglo-Dutch team analyzed data from 38 studies in almost 800,000 people, including people diagnosed with cardiovascular disease, and heart-healthy people at lower risk for stroke.

Their analysis showed that – compared with eating one or fewer servings of fish a week – eating two to four servings of fish a week cut the risk of stroke by six percent.

Specifically, that finding applied to a six percent drop in the risk of any “cerebro-vascular” (brain blood-system) problem … but stroke was the dominant manifestation, by far.

And people who reported eating five or more servings a week had a 12 percent lower risk of stroke.

On the other hand, neither higher blood levels of omega 3s, nor fish oil supplements were significantly associated with a reduced risk.

As the Anglo-Dutch team wrote (Chowdhury R et al. 2012):

• “Our findings […] reinforce a potentially modest beneficial role of fish intake in the cause of cerebrovascular disease.”
• “Such an advantage was less evident for long chain omega-3 fatty acids in both observational studies and interventions [clinical trials using omega-3 fish oil] targeting primary and secondary stroke prevention.”

And the authors omitted a 2008 paper, in which researchers who analyzed a very large, well-regarded clinical trial – the Japan EPA Lipid Intervention Study (JELIS) – found that those in the fish oil group were 20 percent less likely to suffer a stroke (Tanaka K et al. 2008).

Therefore, the findings of the new the Anglo-Dutch meta-analysis don't reflect all of the relevant scientific evidence on this subject … and they say little about the anti-stroke effects of fish and fish oil in healthy people.

They noted the lack of evidence from fish-oil trials in healthy people:

The American Heart Association – and public health authorities worldwide – says that fish and omega-3s help guard against stroke.

Given that fact, and the limitations and flaws of the new analysis, it makes sense to rely on fish and/or omega-3 fish oils as serious allies.

• Chowdhury R, Stevens S, Gorman D, Pan A, Warnakula S, Chowdhury S, Ward H, Johnson L, Crowe F, Hu FB, Franco OH. Association between fish consumption, long chain omega 3 fatty acids, and risk of cerebrovascular disease: systematic review and meta-analysis. BMJ. 2012 Oct 30;345:e6698. doi: 10.1136/bmj.e6698.
• Hooper L, Thompson RL, Harrison RA, Summerbell CD, Moore H, Worthington HV, Durrington PN, Ness AR, Capps NE, Davey Smith G, Riemersma RA, Ebrahim SB. Omega 3 fatty acids for prevention and treatment of cardiovascular disease. Cochrane Database Syst Rev. 2004 Oct 18;(4):CD003177. Review.
• Marik PE, Varon J. Omega-3 dietary supplements and the risk of cardiovascular events: a systematic review. Clin Cardiol. 2009 Jul;32(7):365-72. Review
• Mozaffarian D, Wu JH. Omega-3 fatty acids and cardiovascular disease: effects on risk factors, molecular pathways, and clinical events. J Am Coll Cardiol. 2011 Nov 8;58(20):2047-67. Review.
• Rizos EC, Ntzani EE, Bika E, Kostapanos MS, Elisaf MS. Association between omega-3 fatty acid supplementation and risk of major cardiovascular disease events: a systematic review and meta-analysis. JAMA. 2012 Sep 12;308(10):1024-33. Review.
• Tanaka K, Ishikawa Y, Yokoyama M, Origasa H, Matsuzaki M, Saito Y, Matsuzawa Y, Sasaki J, Oikawa S, Hishida H, Itakura H, Kita T, Kitabatake A, Nakaya N, Sakata T, Shimada K, Shirato K; JELIS Investigators, Japan. Reduction in the recurrence of stroke by eicosapentaenoic acid for hypercholesterolemic patients: subanalysis of the JELIS trial. Stroke. 2008 Jul;39(7):2052-8. Epub 2008 May 1. Erratum in: Stroke. 2008 Sep;39(9): e149.