Abstract
Background
It has been suggested that omega 3 (W3, n-3 or omega-3) fats from oily fish and plants are beneficial to health.
Objectives
To assess whether dietary or supplemental omega 3 fatty acids alter total mortality, cardiovascular events or cancers using both RCT and cohort studies.
Search methods
Five databases including CENTRAL, MEDLINE and EMBASE were searched to February 2002. No language restrictions were applied. Bibliographies were checked and authors contacted.
Selection criteria
RCTs were included where omega 3 intake or advice was randomly allocated and unconfounded, and study duration was at least six months. Cohorts were included where a cohort was followed up for at least six months and omega 3 intake estimated.
Data collection and analysis
Studies were assessed for inclusion, data extracted and quality assessed independently in duplicate. Random effects meta-analysis was performed separately for RCT and cohort data.
Main results
Forty eight randomised controlled trials (36,913 participants) and 41 cohort analyses were included. Pooled trial results did not show a reduction in the risk of total mortality or combined cardiovascular events in those taking additional omega 3 fats (with significant statistical heterogeneity). Sensitivity analysis, retaining only studies at low risk of bias, reduced heterogeneity and again suggested no significant effect of omega 3 fats.
Restricting analysis to trials increasing fish-based omega 3 fats, or those increasing short chain omega 3s, did not suggest significant effects on mortality or cardiovascular events in either group. Subgroup analysis by dietary advice or supplementation, baseline risk of CVD or omega 3 dose suggested no clear effects of these factors on primary outcomes.
Neither RCTs nor cohorts suggested increased relative risk of cancers with higher omega 3 intake but estimates were imprecise so a clinically important effect could not be excluded.
Authors’ conclusions
It is not clear that dietary or supplemental omega 3 fats alter total mortality, combined cardiovascular events or cancers in people with, or at high risk of, cardiovascular disease or in the general population. There is no evidence we should advise people to stop taking rich sources of omega 3 fats, but further high quality trials are needed to confirm suggestions of a protective effect of omega 3 fats on cardiovascular health.
There is no clear evidence that omega 3 fats differ in effectiveness according to fish or plant sources, dietary or supplemental sources, dose or presence of placebo.
Plain language summary
There is not enough evidence to say that people should stop taking rich sources of omega 3 fats, but further high quality trials are needed to confirm the previously suggested protective effect of omega 3 fats for those at increased cardiovascular risk
The review shows that it is not clear whether dietary or supplemental omega 3 fats (found in oily fish and some vegetable oils) alter total deaths, cardiovascular events (such as heart attacks and strokes) or cancers in the general population, or in people at risk of, or with, cardiovascular disease. When the analysis was limited to fish-based or plant-based, dietary or supplemental omega 3 fats there was still no evidence of reduction in deaths or cardiovascular events in any group.
