Eating fish and other seafood may help prevent Alzheimer’s disease in people at high risk for it, even though seafood is often contaminated with mercury, a known neurotoxin. In a study in the Journal of the American Medical Association, researchers looked at brain autopsies of 286 people (average age 90, dementia-free at the start of the study) and correlated the findings with the subjects’ seafood intake during the 4.5 years before death.
Among the 23 percent of subjects with a key genotype (APOE4) associated with Alzheimer’s risk, those who had eaten seafood at least once a week showed fewer signs of dementia-related brain changes than those who ate little or none; no effect was seen in those without the genotype (possibly because their risk was much lower to begin with). Reassuringly, though mercury levels in the subjects' brains increased with seafood intake, this was not associated with dementia-related signs.
Fish oil supplements had no apparent benefits or risks, though relatively few subjects had reported taking them.
Also see Can Diet Prevent Dementia?