Omega-3 supplements do not appear to prevent mental decline in old age, a study review has shown.
Previous research has suggested that omega-3 fatty acids, especially those in fish oil, can help keep brain cells healthy.
But experts now say there is no evidence that they maintain mental function in ageing individuals, at least over timescales of up to three years.
Researchers studied the findings of three trials looking at the effects of omega-3 taken in the form of capsules or added to magarine spread.
In each case, their benefits were compared with those of sunflower oil, olive oil or regular magarine.
A total of 3,536 people over the age of 60 took part in the trials, which lasted between six and 40 months. None had any initial signs of mental decline or dementia.
Participants taking omega-3 scored no better in standard tests of memory and mental performance than those not given the supplements.
The findings are published in the Cochrane Database of Systematic Reviews.
Co-author Dr Alan Dangour, a nutritionist from the London School of Hygiene and Tropical Medicine, said: "From these studies, there doesn't appear to be any benefit for cognitive health for older people of taking omega-3 supplements.
"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."
Further studies were needed to explore the longer term effects of omega-3 on mental function, especially in people with low intakes of the fatty acids in their diet, said the researchers.
They stressed that omega-3 in fish oil might have other 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," Dr Dangour.