Preventing memory loss by A healthful diet rich in fruits and vegetables as well as healthy fats from fish, nuts, and whole grains

A healthful diet rich in fruits and vegetables as well as healthy fats from fish, nuts, and whole grains is vital in maintaining the health not just of your body but of your brain as well. 

Avoiding saturated fats (in meat and dairy) and trans fats (in commercial products with partially hydrogenated oils) will help keep your arteries clear and cholesterol levels healthy, and that in turn will decrease your chances of stroke, including the small undetectable ones that can damage brain function. 

Avoid excess calories to maintain a normal weight; this lowers your risk for illnesses such as diabetes and hypertension, which can impair your memory.

Eating a lot of fruits and vegetables can be especially beneficial because many are good sources of antioxidants, nutrients that may protect against diseases and age-related deterioration throughout the body.

info source: http://www.health.harvard.edu/newsweek/Preventing_memory_loss.htm