A large, five-year study challenges the idea that omega-3 fatty acids can slow age-related cognitive decline. The study, involving 4,000 older adults, was part of the Age-Related Eye Disease Study (AREDS), which established that daily high doses of certain antioxidants and minerals can help slow the progression of age-related macular degeneration. However, a follow-up study found the addition of omega-3 fatty acids to the AREDS formula made no difference.
Omega-3 fatty acids are believed to be responsible for the health benefits associated with regularly eating fish, which is associated with lower rates of AMD, cardiovascular disease, and possibly dementia.
In this study, participants from the AREDS study, all of whom had early or intermediate AMD, were randomly assigned to either omega-3, or lutein and zeaxanthin (nutrients found in large amounts in green leafy vegetables), or both, or a placebo. As they all had AMD, participants also took the AREDS formula, which includes vitamins C, E, beta carotene, and zinc. Cognitive testing took place at the beginning, at 2 years, and at 4 years.
There was no benefit to these supplements: all groups showed a similar rate of cognitive decline over the study period.
The researchers speculate that the failure to find a benefit may lie in the age of the participants — it may be that supplements, to be of benefit, need to be started earlier. The other possibility (and the one I myself give greater weight to, although both factors may well be influential) is that these nutrients need to be taken in food to be effective.
It should be noted that the omega-3 fatty acids taken were those found in fish, not those found in plant foods such as flaxseed, walnuts, soy products, and canola and soybean oils.
 . Effect of omega-3 fatty acids, lutein/zeaxanthin, or other nutrient supplementation on cognitive function: The areds2 randomized clinical trial. JAMA [Internet]. 2015 ;314(8):791 - 801. Available from: http://dx.doi.org/10.1001/jama.2015.9677