Study leader Emma Sydenham at the London School of Hygiene & Tropical Medicine and colleagues gathered 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 age 60 and older 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 beginning of the trials, the researchers said.
The findings, published in the Cochrane Library, said the fish body oil supplements offered no benefits for cognitive health over placebo capsules or margarines, but longer term effects were worth investigating.
"From these studies, there doesn't appear to be any benefit for cognitive health for older people of taking omega-3 supplements," study co-author Alan Dangour said in a statement. "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 concluded that the longer-term effects of omega-3 fatty acids on cognitive decline and dementia need to be explored in further studies.
"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," the researchers said.