Inclusion Criteria:  Aged between 60 - 85 years of age Good general health with no concomitant diseases Cognitively normal as defined as Mini-mental state examination (MMSE) score ≥ 25 BMI range: 20 - 40 kg/m2  Exclusion Criteria:  Presence of central neurological disease (e.g., brain tumor, stroke, epilepsy, motor neuron disease, multiple sclerosis, Alzheimer's, Parkinson's) Prior major head trauma with loss of consciousness Presence of active unstable and life-threatening illness (e.g., cancer) Presence of major psychiatric disorder within the past 3 years including depression, anxiety, and alcohol (over 3 drinks per day or total of 18 per week) or drug abuse (DSM-IV criteria; Geriatric Depression Score > 6) Presence of diabetes mellitus or use of anti-diabetic agents Use of psychoactive and investigational medications Use of hormone replacement therapy Tobacco smokers Subjects with MRI contraindications