Overeating Linked to Memory Loss

This CNN article entitled "Overeating May be Linked to Memory Loss" describes a recent study of over 1200 people and reveals that if you eat over 2143 calories/day (versus 1526/day), it raises your risk of Mild Cognitive Impairment (MCI; often seen as a precursor to Alzheimer's disease). What other factors might contribute to memory loss beyond overeating? Could you follow a 1500 calorie/day diet?