Older people who take omega-3 fish oil supplements are probably not reducing their chances of losing cognitive function, according to a new Cochrane systematic review. Based on the available data from studies lasting up to 3.5 years, the researchers concluded that the supplements offered no benefits for cognitive health over placebo capsules or margarines, but that longer term effects are worth investigating.

Omega-3 fatty acids are fats responsible for many important jobs in the body. We get these fats through our daily diets and the three major omega-3 fats are: alpha linolenic acid (ALA) from sources such as nuts and seeds and eicosapentoic acid (EPA) and docosahexaenoic acid (DHA) from sources including oily fish such as salmon and mackerel. A number of studies have hinted that omega-3 fatty acids and DHA in particular may be involved in keeping nerve cells in the brain healthy into old age. However, there is limited evidence for the role of these fats in preventing cognitive decline and dementia.

The researchers, led by Emma Sydenham at the London School of Hygiene & Tropical Medicine (LSHTM), London, UK, gathered together evidence from three high quality trials comparing the effects of omega-3 fatty acids taken in capsules or margarine spread to those of sunflower oil, olive oil or regular margarine. A total of 3,536 people over the age of 60 took part in the trials, which lasted between six and 40 months. None of the participants had any signs of poor cognitive health or dementia at the start of the trials.

The researchers found no benefit of taking the omega-3 capsules or spread over placebo capsules or spread. Participants given omega-3 did not score better in standard mental state examinations or in memory and verbal fluency tests than those given placebo.

"From these studies, there doesn't appear to be any benefit for cognitive health for older people of taking omega-3 supplements," said Alan Dangour, a nutritionist at LSHTM and co-author of the report. "However, these were relatively short-term studies, so we saw very little deterioration in cognitive function in either the intervention groups or the control groups. It may take much longer to see any effect of these supplements."

The researchers conclude that the longer term effects of omega-3 fatty acids on cognitive decline and dementia need to be explored in further studies, particularly in people with low intakes of omega-3 fatty acids in their diet. In the meantime, they stress other potential health benefits. "Fish is an important part of a healthy diet and we would still support the recommendation to eat two portions a week, including one portion of oily fish," said Dangour.