A study published in Archives of Neurology found that people who eat fish rich in omega-3 fatty acids can lower the decline for a person with Alzheimer's disease.
Those who ate fish once a week reduced their mental decline by 10% while those who ate fish more than once a week reduced their decline by 13%.
The lead researcher reported that this reduction in mental decline is the equivalent of being three or four years younger in age.
So...if you can remember to get it and cook it, eat fish at least once a week. (Especially Salmon!)