Overview

This update to a previous review included 79 randomised controlled trials (112 059 participants) that compared diet advice or dietary supplementation to promote omega-3 fatty acid intake with placebo, no supplementation, usual diet or lower dose omega-3.1 The population studied was adults aged over 18 years with or without CVD and trial duration was between 12 and 72 months. The primary objective was to assess whether long-chain omega-3 (LCn3) or alpha-linolenic acid (ALA) fats affect a range of outcomes including all-cause mortality, cardiovascular deaths, cardiovascular events, coronary heart disease (CHD) deaths and CHD events.

High-quality evidence showed little or no effect of increasing LCn3 fatty acids on all-cause mortality (relative risk [RR] 0.98, 95% CI 0.90 to 1.03; 92 653 participants in 39 trials; high‐quality evidence). There was also no effect on cardiovascular mortality (RR 0.95, 95% CI 0.87 to 1.03; 67 772 participants in 25 trials) or on cardiovascular events (RR 0.99, 95% CI 0.94 to 1.04; 90 378 participants in 38 trials). Moderate-quality evidence also suggested that LCn3 made little or no difference to CHD deaths, coronary heart events, stroke or arrhythmia.

Similar results were seen with ALAs (either ALA-rich [eg, walnuts] or enriched foods [eg, margarine]. Increased ALA levels had little or no effect on all-cause mortality (RR 1.01, 95% CI 0.84 to 1.20; 19 327 participants in four trials) or on cardiovascular mortality (RR 0.96, 95% CI 0.74 to 1.25; 18 619 participants in four trials). There was low-quality to moderate-quality evidence that increased ALA may slightly reduce the risk of CHD mortality, arrhythmia and CVD events, but the effects were small and few ALA trials addressed these outcomes. The authors estimated that 143 people would need to increase their ALA intake to prevent one person developing an arrhythmia, and 1000 would need to take more ALA to prevent one person experiencing a CVD event or dying from CHD.

Omega-3 may slightly affect serum lipid markers but the evidence is conflicting; LCn3 slightly reduced triglycerides and increased HDL, whereas ALA increased HDL. There was no difference in body weight between treatment groups. Data were collected on adverse effects including nausea, abdominal discomfort, diarrhoea, skin itching, headache and reflux but a meta-analysis found no suggestion that omega-3 treatment increased or decreased these effects (although there was significant variation between trials).

Authors’ conclusions

Supplemental LCn3 fatty acids are probably not useful for preventing or treating CVD, but may have a small effect on reducing serum triglycerides and increasing HDL. Fish and seafoods that contain omega-3 fats are nutrient-rich useful foods even without cardiovascular benefits. However, the evidence in this review suggested that it would be appropriate to review official recommendations supporting supplemental LCn3 fatty acid intake. ALA is an essential fatty acid and an important part of a mixed diet. Increasing intake may be slightly beneficial for the prevention or treatment of CVD.

(This project was supported by the National Institute for Health Research, via Cochrane Infrastructure funding to the Cochrane Heart Group.)

Context

This updated review was commissioned by the WHO to inform its updated guidance on polyunsaturated fatty acid intake in adults and children. It affirms the current guidance from the National Institute for Health and Care Excellence, which encourages consumption of two portions of fish per week but advises that omega-3 fatty acid compounds should not be offered for primary or secondary prevention of CVD and that there is no evidence that omega-3 fatty acid compounds help to prevent CVD.2