Health reporter, BBC News
Oily fish is proven to be good for the heart
A UK study has cast doubt on claims that eating oily fish can protect against dementia in old age.
Data from a trial of more than 800 older people initially showed that those who eat plenty of oily fish seem to have better cognitive function.
But factors such as education and mood explained most of the link.
Researchers need to clarify what, if any, benefits fish oil has on the ageing brain, they wrote in the Journal of Nutrition, Health and Ageing.
In recent years, there has been increasing interest in diet as a way of preventing dementia.
Much focus has been on omega 3 fatty acids found in oily fish, such as salmon and mackerel.
And there are biological reasons, backed by tests in the laboratory, why in theory, these fatty acids would be neuroprotective.
The latest study found a significant association between eating a couple of portions of fish a week and better scores on tests of cognitive function.
But when the researchers, from the London School of Hygiene and Tropical Medicine, took into account education and psychological health the association almost disappeared.
Experts advise 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.
Study leader Dr Alan Dangour said claims about the benefits of oily fish in warding off dementia in older people seemed to have been oversold.
"The evidence on this has always been sporadic.
"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."
The evidence collected by Dr Dangour was for a study due to report later this year comparing fish oil supplements with placebo.
He added that this randomised, controlled study should provide clarification.
Neil Hunt, chief executive of the Alzheimer's Society, said: "One of the best ways to reduce your risk of dementia is by eating a Mediterranean diet rich in fruit, vegetables, grains, fish and poultry.
"However, we still do not know which components of this sort of diet help the most.
"Unfortunately this study does not add to our understanding.
"Once age, sex and education are accounted for the research does not show any significant benefit of regularly eating oily fish."
Rebecca Wood, of the Alzheimer's Research Trust, said: "Research into the effects of oily fish and other foodstuffs attracts much interest because it may offer a relatively inexpensive way to fight dementia, a devastatingly costly condition.
"Many scientists believe there is a link between diet and reducing dementia risk. More research is desperately needed to understand the effects of diet, including omega-3 fatty acids, on the brain."