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Taking fish-oil supplements to ward off dementia could be a waste of time, say researchers who have reviewed the best available evidence.

Supplements containing omega-3 offered no greater protection than dummy pills despite suggestions otherwise, the Cochrane Review team found.

The three large studies in the review involved over 3,500 people.

But experts say longer-term studies need to be carried out for more conclusive results.

The current work tracked the health of individuals over a period of three-and-a-half years, so it is still unclear whether there might be some brain protection that kicks in if supplements are taken for much longer than this.

The work looked at randomised controlled trials - the "gold standard" test scientists use to check whether a treatment works.

And it considered different ways of taking omega-3 - in capsules or margarine spread.

Eating plenty of oily fish, such as mackerel, salmon and sardines, will also provide this important fatty acid, which has been linked to a lower risk of heart disease.

Experts already advise that a healthy diet should include at least two portions of fish a week, including one of oily fish.

However, scientific backing for omega-3's use to prevent dementia has been less forthcoming.

This latest review, published by the Cochrane Library, found that participants taking omega-3 scored no better in standard tests of memory and mental performance than those given a placebo.

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.

"So the evidence at the moment is very disappointing. But there's still an open question - if we conducted a longer study, what would that show?"

He added: "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 Marie Janson of Alzheimer's Research UK said: "Cochrane reviews are an excellent way of pulling together high quality scientific evidence.

"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."