Intended for healthcare professionals


Risks and benefits of omega 3 fats for mortality, cardiovascular disease, and cancer: systematic review

BMJ 2006; 332 doi: (Published 30 March 2006) Cite this as: BMJ 2006;332:752

Just a few concerns on this trial....

I have a big issue when the effects of fish consumption are linked to
the effects of the long-chain omega-3 fatty acids.
Sure, fish is the best natural source of the long-chain omega-3 fatty
acids, but our fish supply is also tainted with mercury, lead, pesticide
residues, and other harmful compounds. Mercury has been known to increase
the risk of cardiovascular disease. While fish oils may protect against
heart disease, is the benefit of eating fish counteracted by a higher
intake of mercury? Apparently not as results from another study published
in the New England Journal of Medicine show that while higher body levels
EPA and DHA were associated with a decreased risk for heart attacks, the
higher the body mercury level the greater risk of a heart attack (Gualler
et al, 2002). Researchers concluded that the high mercury content of fish
may diminish the protective effect of fish intake against heart disease.
So, it is entirely inappropriate to lump fish consumption into the
analysis of the health benefits of the long-chain omega-3 fatty acids.

Another mistake, surely, is pooling the data with both the long-chain
omega-3 fatty acids from fish oils with the short-chain omega-3 fatty
acids alpha- linolenic acid. While the data on the beneficial effects of
the long-chain omega-3 fatty acids is quite solid, for alpha- linolenic
acid the evidence is less convincing and randomized controlled trials are
lacking. One of the studies included in the analysis that should not have
been was conducted not on fish oil, but rather a margarine containing
alpha-linolenic acid (ALA) - that's the omega 3 found in flax – versus a
margarine with linoleic acid (an omega-6 fatty acid) (Bemelmans et al,
2002). Again, including this study appears inappropriate and its exclusion
may have changed the picture entirely.

Finally, it has been stated that “conducting a meta-analysis study on
the effectiveness of omega-3 fats for mortality, cardiovascular disease
and cancer, without considering the impact of excess omega-6 fat in the
diet, is akin to reviewing the efficacy of a healthy diet without
factoring the effects of smoking (Tribole EF, 2006). In other words, a
high omega-6 to omega-3 fatty acid ratio would counteract the impact of an
increased omega-3 fatty acid intake and make the results difficult to
interpret. The reason omega-6 fatty acids counteract the effects of the
omega-3 fatty acids relates to the production of eicosanoids
(prostaglandins, thromboxanes, and leukotrienes) from omega-6 fatty acids.
Chronic excessive production eicosanoids derived from omega-6 fatty acids
is associated with an increased risk heart attacks, thrombotic stroke,
arrhythmia, arthritis, osteoporosis, inflammation and cancer. The overall
benefits of a higher intake of omega-3 fatty acids appears to be related
to reducing the omega-6 to omega-3 fatty acid ratio and availability of
omega-6 fatty acids for eicosanoid synthesis.


1. Guallar E, Sanz-Gallardo MI, van't Veer P, Bode P, et al. Mercury,
fish oils, and the risk of myocardial infarction. N Engl J Med

2. Bemelmans WJ, Broer J, Feskens EJ, et al. Effect of an increased
intake of alpha-linolenic acid and group nutritional education on
cardiovascular risk factors: the Mediterranean Alpha-linolenic Enriched
Groningen Dietary Intervention (MARGARIN) study. Am J Clin Nutr. 2002

3. Tribole EF. Excess Omega-6 Fats Thwart Health Benefits from Omega-
3 Fats. March 27, 2006

Competing interests:
None declared

Competing interests: No competing interests

18 April 2006
stuart lloyd
UCLAN, Preston