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Do Fish Beat Fish Oil for Stroke?
Review affirms that fishy diets cut stroke risk and sees no benefit from fish oil, but authors excluded key evidence
11/14/2012
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By Craig Weatherby

Science is not a belief system, and does not depend on viewing researchers as secular priests.

Instead, the term “science” means a set of objective methods used to test an idea or “hypothesis”.

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.
These variables make it hard to analyze a group of studies in order to arrive at a general conclusion about a nutrient’s effect on a health condition … the kind of “paper” study known as a meta-analysis.

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.


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 meta-analysis was led by researchers from Britain’s Cambridge University and Holland’s Erasmus MC Rotterdam (Chowdhury R et al. 2012).

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.”
However, only two of the 12 fish-oil clinical trials included in the meta-analysis were conducted in healthy people, with the rest involving people diagnosed with cardiovascular disease.

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:
“Our findings […] also underscore scientific gaps in the experimental evidence, specifically the lack of studies involving healthy populations and interventions [clinical trials] targeting fish intake rather than using supplements, which may have different mechanistic effects.” (Chowdhury R et al. 2012)

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.


Sources
  • 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.

Theory or hypothesis? Confusion reigns
Unfortunately, the term “theory” has become confused with the term “hypothesis”.

In science, a theory is an idea that’s supported by overwhelming evidence and possesses very strong predictive power.

For example, some people dismiss Darwin’s idea (hypothesis) about natural selection and the evolution of species as “just a theory”.

But the evidence supporting his hypothesis became persuasive early in the 20th century … and it became overwhelming with the discovery of DNA and its roles in the mechanisms of evolution.

That overwhelming, virtually un-contradicted evidence has elevated evolution from unproven hypothesis to proven theory.

No hypothesis about human health should be accepted as a virtual fact until the evidence supporting it becomes overwhelming.

For that transition to occur, a hypothesis must be affirmed by repeated, scientifically sound studies … at which point it becomes a “theory” worthy of respect until disproven by compelling new evidence.
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