Past research had claimed that fish oil can foil dementia even as the hair turns grey with age. But now, a new study, conducted by British researchers, has cast its doubts on these long-believed benefits of fish oil.
Initially, data from a trial of more than 800 older people showed that those who eat plenty of oily fish seem to have better cognitive function.
However, factors such as education and mood explained most of the link and researchers need to explain what, if any, benefits fish oil has on the ageing brain.
In recent years, there has been growing interest in diet as a way of protecting against dementia.
Much focus has been on omega 3 fatty acids found in oily fish, such as salmon and mackerel.
And there are biological reasons, supported by tests in the laboratory, why in theory, these fatty acids would be neuroprotective.
The latest study, conducted by researchers from the London School of Hygiene and Tropical Medicine, found a significant association between eating a couple of portions of fish a week and better scores on tests of cognitive function.
However, when they took into account education and psychological health the link almost disappeared.
Experts recommend eating a couple of portions of fish a week, with at least one being an oily fish, because there are proven benefits on the heart.
Lead author of the study, Dr Alan Dangour, said that claims about the benefits of oily fish in warding of dementia in older people seemed to have been oversold.
"The evidence on this has always been sporadic," the BBC quoted Dangour, as saying.
"What this shows is there is a link between people who eat oily fish and better cognitive function, but if you adjust for education and mood this relationship goes, so it's not at all clear that healthy older people get any benefit from eating fish oil," Dangour added.
The new study has been published in the Journal of Nutrition, Health and Ageing.