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.
UK guidelines advising people to eat more oily fish should continue at present but the evidence should be reviewed regularly, say the authors. However, it is probably not appropriate to recommend a high intake of omega 3 fats for people who have angina but have not had a heart attack.
To understand the effects of omega 3 fats on health, we need more high quality randomised controlled trials of long duration that also report the associated harms, they conclude.
We are faced with a paradox, says Eric Brunner in an accompanying editorial. Health recommendations advise increased consumption of oily fish and fish oils. However, industrial fishing has depleted the world's fish stocks by some 90% since 1950, and rising fish prices reduce affordability particularly for people with low incomes.
Global production trends suggest that, although fish farming is expanding rapidly, we probably do not have a sustainable supply of long chain omega 3 fats, he warns.