Omega-3s from oily fish and supplements can reduce the risk of cardiac and sudden death, and possibly stroke, says a new review from the US.
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.
But some much publicised studies, and in particular a recent meta-analysis (British Medical Journal, doi: bmj.38755.366331.2F), have claimed that there was no evidence linking omega-3 intake and improvements in heart health.
The new review, published in the American Journal of Clinical Nutrition (Vol. 84, pp. 5-17), takes "a large step forward in helping to resolve the controversies related to the beneficial effects of omega-3 fatty acids on CVD outcomes," according to Richard Dickelbaum and Sharon Akabas from Columbia University, in an independent editorial.
The problems with reviewing and pooling previous studies has come from the fact that many of the studies are not directly comparable, with long- and short-term trials being grouped together, and no separation made between primary and secondary prevention studies.
The new review, authored by Chenchen Wang and associates from Tufts-New England Medical Center, only considered studies that lasted for more than one year, and that reported or estimated omega-3 intakes and CVD outcomes. Care was taken to separate interventions using alpha-linolenic acid (ALA) from interventions using eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), or EPA plus DHA.
Primary prevention of CVD was reported in one randomised clinical trial (RCT), 25 prospective studies and seven case-control studies.
"Most of the large cohort studies reviewed, which involved more than 340,000 participants in total, reported significant reductions after multivariate adjustment in one or more CVD outcomes of interest," reported the reviewers.
For secondary prevention of CVD 11 clinical trials were identified, along with one prospective cohort study. These included six RCTs with supplements, and five diet and dietary advice trials.
The supplement trials considered by the researchers included EPA or EPA plus DHA in doses ranging from 0.27 to 4.8 grams per day. Five of the trials were consider to be of high standard, receiving grades A or B for their methodological quality.
The results from these studies were mixed, but the largest reported that a 0.85 grams per day supplement of EPA plus DHA was reported to reduce the risk of all-cause mortality by 21 per cent, cardiac death by 35 per cent, and sudden death by 45 per cent.
The risk of pollutants from oily fish, such a methyl mercury, dioxins, and polychlorinated biphenols (PCBs) have led to some to advocate a reduction in fresh fish intake, despite others advising that the benefits of fish consumption outweigh the risks.
Such conflicting views on fish intake have seen the number of omega-3 enriched or fortified products on the market increase as consumers seek omega-3s from 'safer' sources. Most extracted fish oil are molecularly distilled and steam deodorised to remove contaminants.
In the accompanying editorial (AJCN, Vol. 84, pp. 1-2), however, Deckelbaum and Akabas said that, despite such concerns, "proper selection and preparation of fish results in a low risk from toxins."
But fears about dwindling fish stocks have pushed some in academia and industry to start producing omega-3s from alternative sources, such as algae extraction. Indeed, companies such as Martek Biosciences and Lonza are already offering algae-derived omega-3 DHA as a dietary supplement.
The reviewers called for future research to address the last lingering issues, with longer well-designed RCTs needed to assess the effects of DHA and EPA on heart health.
Chenchen Wang and associates also called for investigations into how higher fish intake in the diet displaces other foods, such as cheese and meat, which are high in saturated fat.
The editorial said that the study made a significant contribution to resolving the controversies with the subject.
"We believe that the body of existing evidence is strong enough to suggest that in the United States, certainly, and in other countries where omega-3 fatty acid consumption is low, public health initiatives are needed to increase intakes of EPA and DHA," concluded Deckelbaum and Akabas.