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 margarine spread.
In each case, their benefits were compared with 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 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 said.
Dr Marie Janson of Alzheimer's Research UK, the UK's leading dementia research charity, said: "Cochrane reviews are an excellent way of pulling together high quality scientific evidence. This summary has reviewed data from three clinical trials of people taking omega-3 supplements and concluded that they do not have likely benefits in preventing cognitive decline. As these trials had relatively short follow-up periods, there may be longer-term benefits, but further research would be needed to investigate this.
"While taking omega-3 supplements may not be the key to staving off cognitive problems, eating a healthy balanced diet, including fish and other natural sources of omega-3, is important for maintaining good health. We know that what is good for the heart can be good for the head so maintaining a healthy weight, eating a balanced diet, exercising and keeping our blood pressure in check are all ways that we could reduce our risk of cognitive decline and dementia later in life."