American researchers say people with Alzheimer's disease may not benefit from eating fish, even though it is considered to be a "brain food".
Two pieces of research have shown that the chances of getting the disease may be reduced, or its progression prevented, by consuming a fish-based diet, but further work is needed.
Two studies were carried out to determine the effects of DHA, an omega-3 fatty acid found in oily fish.
Funded by the Alzheimer's Disease Co-operative Study (ADCS), the first trial lasted 18 months, during which it compared the effects of DHA and a dummy placebo on 402 volunteers with an average age of 76 who had been diagnosed with mild to moderate Alzheimer's.
The researchers associated with the trial say that, at the end of the study, there was nothing to conclusively show that omega-3 supplements improved participants' memory and mental performance scores.
The second trial ran for six months, during which a DHA manufacturer tested one of its products on a group of 485 healthy people.
It did show some improvement in one test of memory and learning. However, those participating in the trial did not have Alzheimer's disease or any other form of dementia.
The findings of both trials were presented at the International Conference on Alzheimer's Disease (ICAD) in Vienna.
"These trial results do not support the routine use of DHA for patients with Alzheimer's," the Scotsman quoted Dr. Joseph Quinn, from Oregon Health and Sciences University, who led the ADCS study, as saying.
However, the researchers presenting the findings did say that there was some evidence that DHA might help people with a particular genetic make-up.
"These studies show that using omega-3 fatty acids as a treatment late on may not be effective against Alzheimer's," Dr. Simon Ridley, research manager at the Alzheimer's Research Trust, said.
"But with previous population studies suggesting that fish oils could reduce dementia risk, getting oily fish, such as mackerel, herring, salmon and sardines into our weekly menus could still be good advice.
"This shouldn't spell the end of research into omega-3, however. It could be that omega-3 given very early in the disease process could make a difference, but for that to happen we must drive forward studies that improve our methods of diagnosing Alzheimer's disease," he added.
Dr. William Thies, chief medical and scientific officer at the Alzheimer's Association, said: "These two studies, and other recent Alzheimer's therapy trials, raise the possibility that treatments for Alzheimer's must be given very early in the disease for them to be truly effective. For that to happen, we need to get much better at early detection of Alzheimer's."