The largest ever trial of fish oil supplements has found no evidence that they offer benefits for cognitive function in older people. The British study enrolled 867 participants aged 70-80 years, and lasted two years. After two years, those receiving fish oil capsules had significantly higher levels of omega-3 fatty acids in their blood than those receiving placebo capsules. However, cognitive function did not decline in either group over the period. The researchers caution that it may be that more time is needed for benefits to show.
(2010). Effect of 2-y n-3 long-chain polyunsaturated fatty acid supplementation on cognitive function in older people: a randomized, double-blind, controlled trial.
Am J Clin Nutr. ajcn.2009.29121 - ajcn.2009.29121.