Omega-3 fatty acids have been linked to a wide-range of health benefits, including cardiovascular disease (CVD), good development of a baby during pregnancy, joint health, behaviour and mood, and certain cancers.
Most studies have suggested that oil fish and omega 3 supplements reduced mortality, but a large, long-term RCT by Michael Burr and colleagues from the University of Wales College of Medicine, Cardiff (European Journal of Clinical Nutrition, 2003, Vol. 57, pp. 193-200) reported men with angina taking fish oil capsules had a higher risk of heart attack.
The new meta-analysis, published on-line in the British Medical Journal (doi: bmj.38755.366331.2F), reviewed 48 randomised clinical trials (RCT) with 36913 participants and between six months and six years of follow-up, and 41 cohort studies with over half a million participants and follow-up of up to 25 years.
"Long chain and shorter chain omega 3 fats do not have a clear effect on total mortality, combined cardiovascular events, or cancer," wrote lead author Lee Hooper from the University of East Anglia.
In terms of total mortality, the analysis showed that omega 3 fats reduced the risk by 13 per cent, but this was not deemed to be significant.
No significant decreases in risk for either cardiovascular events, including stroke, or cancer were reported.
These results differ from a recent review by Heiner Butcher from the Institute of Clinical Epidemiology, Basel, Switzerland (American Journal of Medicine, 2002, Vol. 112, pp. 298-304), which suggested important benefits of omega-3 fatty acids for morbidity and mortality in coronary heart disease.
However, if the East Anglia researchers omitted the RCT of Burr, similar statistically significant benefits of omega-3 acids were observed.
"It is not clear why the results of Burr et al differ from the other large studies on fish based omega 3," said Hooper.
The Burr study had the longest follow up of all the RCTs analysed and was only focused at men with angina, both of which may cause the contradictory results, said Hooper and colleagues.
Hooper and colleagues stressed that the general public should still be encouraged to consume omega-3 from oily fish or supplements.
"This advice should continue at present but the evidence should be reviewed regularly."
In an accompanying editorial, Eric Brunner from the Royal Free and University College London Medical School agreed: "For the general public some omega 3 fat is good for health. Long chain omega 3 fatty acids are structural components of neuronal and other cell membranes, and they modulate the production of eicosanoids and inflammatory cytokines."
The risk of pollutants from oily fish, such a methyl mercury, dioxins, and polychlorinated biphenols (PCBs) have led to some to claim to reduce fresh fish intake, especially for pregnant women who may damage the development of their babies.
Such advice has seen the number of omega-3 enriched or fortified products on the market increase. Most extracted fish oil are molecularly distilled and steam deodorised to remove contaminants.
But fears about dwindling fish stocks have pushed some companies to start extracting omega-3s from algae. Indeed, companies such as Martek Biosciences and Lonza are already offering algae-derived omega-3 DHA as a dietary supplement.
This week a new Swiss company announced it had found a way to derive omega-3 containing both EPA and DHA from algae.