A study published online by the BMJ doesn't find evidence of a clear benefit of omega 3 fats on health.
These findings do not rule out an important effect of omega 3 fats, but suggest that the evidence should be reviewed regularly, say the researchers.
Consumption of long chain omega 3 fatty acids, found in oily fish and fish oils, and a shorter chain omega 3, found in some plant oils, is thought to protect against heart disease. UK guidelines encourage the general public to eat more oily fish, and higher amounts are advised after a heart attack.
Researchers analysed 89 studies (48 randomised controlled trials and 41 cohort studies) to assess the health effects of long and short chain omega 3 fats on total mortality, cardiovascular events, cancer, and strokes.
Each study involved a treatment group and a control group and investigated the effect of omega 3 intake on health for at least six months. Differences in study quality were taken into account to identify and minimise bias.
Pooling the results showed no strong evidence that omega 3 fats have an effect on total mortality or combined cardiovascular events. The few studies at low risk of bias were more consistent, but they also showed no effect of omega 3 on total mortality or cardiovascular events.
When data on long chain omega 3 fats were analysed separately, total mortality and cardiovascular events were not reduced. No study showed increased risk of cancer or stroke with higher intake of omega 3, but there were too few events to rule out important effects.
Other recent reviews of omega 3 trials found that omega 3 fats decrease mortality, but the publication of a large contradictory trial has changed the overall picture. The authors cannot say exactly why the results of this trial differ from the other large studies in this field.
They therefore conclude that it is not clear whether long chain or short chain omega 3 fats (together or separately) reduce or increase total mortality, cardiovascular events, cancer, or strokes.