A number of studies have been conducted on the benefits of fish oil on mental decline. None of them have been able to prove that it has any effect.
There have been a number of studies which show that those who eat a lot of fish in their diet tend to have better brain function. The issue is that those people also tend to avoid a high intake of saturated fat and red meat, which is known to affect brain function, and they tend to be more educated. Scientists are now trying to work out whether it is the fish or simply the lifestyle which promotes better brain function.
A number of recent studies have tried to replicate the results of those who eat fish by using fish oil supplements.
The latest looked at the benefits of fish oil supplementation in later life. It included a mix of both DHA and EPA, the two main long-chain omega-3 fatty acids, both thought to have a positive effect on brain function. The test group consisted of approximately 860 healthy adults aged over 70. They were all screened for diabetes, heart disease and cognitive function. All participants performed well on the baseline tests.
Half the participants were asked to take a fish oil supplement for two years and the other half took a placebo. Neither group showed significantly more or less decline than the other, suggesting that the fish oil made no difference.
If you put a number of smaller trials, conducted in the recent past, together, more than 4000 people aged over 65 and supplemented for up to 40 months have been tested. Across these different trials there was no evidence that fish oil has any benefit on brain function or in reducing mental decline.