Researchers at University of Pittsburgh have carried out the study and found that eating fish, at least once in a week, could protect against the Alzheimer's disease and memory loss, particularly in elderly people.
The study found that elderly people who eat fish at least once a week are three to five times less likely to develop the conditions than people who did not. But it is essential that the fish is cooked in a manner that preserves vital Omega-3 fatty acids which help protect the brain, say the researchers.
Grilling or baking the meat provides the maximum levels of Omega-3, which increase blood flow to the brain, reduce inflammation and limit the build-up of harmful plaques which precedes Alzheimer's.
In contrast fried fish has very low amounts of Omega-3 and consequently offers no protection against dementia and age-related memory loss, known as Mild Cognitive Impairment, 'The Daily Telegraph' reported.
For their study, the researchers recruited a group of 260 healthy volunteers with an average age of 76. They questioned the participants about how regularly they ate fish.
Brain scans carried out ten years later showed that those who did not eat fish regularly had suffered much more shrinkage in key areas of the brain linked to working memory.
A further five years on, they found that 31 per cent of non-regular fish eaters had gone on to develop Alzheimer's or MCI, compared with between three and eight per cent of those who ate fish at least once a week.
Dr Cyrus A Raji, who led the study, said further studies could help identify whether Omega-3 supplements yield similar effects and whether some types of fish offer better protection than others.
He said: "We know from other studies that salmon gives the maximum amount of Omega-3 fatty acids so it is very possible, but we did not look at which fish people were eating in the study.
"Studies like this definitely justify trials that will look at Omega-3 fatty acid supplements. Having said that, I would speculate that taking supplements is no substitute for a lifetime of eating fish."
Experts are, however, not fully convinced.
Dr Simon Ridley, head of research at Alzheimer's Research UK, said: "This study suggests that eating fish on a weekly basis may reduce the risk of cognitive decline, but it is not clear whether other underlying factors may have contributed to the lower risk in people who eat fish."