Twenty years of research indicates that fish and n-3 fatty acids (FAs), for example docosahexaenoic acid, may attenuate cognitive decline including Alzheimer's disease in older people. This review concerns reports during 2015-2016 in humans.
One prospective cohort study showed that seafood consumption was related to less neuritic plaques and neurofibrillary tangles in brain autopsies from elderly care residents. In a large 5-year intervention no effects on cognition could be shown either in n-3 FA supplemented or in control patients. Two meta-analyses in community-dwelling patients support preservation of cognition with higher fish intake. Older adults with memory complaints may improve cortical blood flow during memory challenges by n-3 FA supplementation. Recalculations from a report in Alzheimer's disease patients indicated a dose-response pattern between increments of serum n-3 FAs and cognitive improvement. Still, a Cochrane review (using three randomized control trials) concluded that n-3 FAs cannot provide any 6-month benefit in patients with mild/moderate Alzheimer's disease.
The accumulated knowledge indicates that healthy populations may have preventive benefits from fish and docosahexaenoic acid intake, like older adults with memory complaints/mild cognitive impairment, and maybe subgroups of patients with mild/moderate Alzheimer's disease may also show such benefits. Still, more studies are needed.