In the UK, omega-3 has been a big fad, it seems. You can't seem to get away from products without it. I Although recently it has died down, it mainly stems back to a flawed study on schoolchildren. Is there any peer reviewed and well researched evidence pointing to a conclusion either way?
... analyses revealed positive associations between reported dietary omega-3 intake and gray matter volume in the subgenual ACC, the right hippocampus and the right amygdala, adjusted for total gray matter volume of brain. Unconstrained whole-brain analyses confirmed that higher intake of omega-3 fatty acids was selectively associated with increased greater gray matter volume in these and not other regions.
Higher reported consumption of the long-chain omega-3 fatty acids is associated with greater gray matter volume in nodes of a corticolimbic circuitry supporting emotional arousal and regulation. Such associations may mediate previously observed effects of omega-3 fatty acids on memory, mood and affect regulation.
But don't jump to conclusions. The study doesn't prove that omega-3 fatty acids build gray matter. Perhaps participants with the most gray matter in those brain areas happen to favor foods rich in omega-3 fatty acids.
But if omega-3 fatty acids boost gray matter, that could explain earlier findings (e.g.) linking omega-3 fatty acids to mood regulation.
Previous research has suggested that there is a link between low levels in the brain of the omega-3 fatty acid DHA and Alzheimer’s disease.
Also, the incidence of neurodegenerative diseases such as Alzheimer’s appears to be reduced in populations with a high omega-3 fatty acid diet.
We found that when the level of DHA in neuronal cells drops, the level of zinc rises. The higher levels of zinc can be toxic, resulting in cell death. This type of cell death is a key feature of neurodegenerative diseases, including Alzheimer’s.
And this article suggest omega-3 "might offer a new way of protecting against traumatic brain injury (TBI)"
The tissue damage caused by TBI was significantly reduced in rats taking the highest dose of DHA.
Cellular findings included a significant reduction in expression of a protein (beta amyloid protein) that has been implicated in the development of Alzheimer's disease.
Animals receiving the highest dose of DHA before TBI also had reduced expression of key indicators of brain cell death. The DHA-treated rats also performed better on a test of spatial memory, indicating less behavioural impairment.
While this study suggest that "DHA Improves Memory and Cognitive Function in Older Adults",
The study found that DHA taken for six months improved memory and learning in healthy, older adults with mild memory complaints.
this one says
Supplementation with DHA compared with placebo did not slow the rate of cognitive and functional decline in patients with mild to moderate Alzheimer disease.
both studies were published in 2010.
Omega-3 has been associated with various benefits but many of the studies to measure them have been poor, hence the controversy.
The basic theory that they are important for proper brain function (a much broader idea than just concentration and cognitive ability) was, I think, best articulated in the book The Madness of Adam and Eve by David Horrobin as part of a theory about how the modern human brain developed.
The most interesting study I know of supporting this was conducted as a randomised controlled trial on prison inmates reported here (see also the news comment from the Guardian and the BBC where the prison service seem to express some doubts). But replication trials from other countries are due to report this year. So we might safely say there is some evidence that omega-3s can reduce disfunction.
But there have been badly constructed trials as well, intended to show improvements in children in school. Ben Goldacre demolishes the design of the Durham Experiment thus:
I pointed out, along with several academics, that their experiment was incompetently designed, for no good reason, and so would only produce false positive results.
I have not found a similar demolition of the results on reducing bad behaviour in prisoners. This might be because a lack of omega-3s cause anti-social effects but, when you have an adequate diet, further omega-3s make little difference.