Effects of Aging and Aerobic Exercise Training on Brain Glucose Metabolism

Healthy sedentary adults aged 18-30 or 65-80 years of all ethnicities will be eligible. Pregnant women, children, prisoners or other at risk populations will not be recruited. Inclusion Criteria: * Age 18-30 years or 65-80 years Exclusion Criteria: * Body mass index (BMI) \>31 kg/m2 * Smoking * Pregnancy * Participation in structured exercise (\>2 times per week for 30 minutes or longer) * Cardiovascular, metabolic (type 2 diabetes, fasting plasma glucose at or above 110 mg/dL and untreated hypo- or hyperthyroidism) or renal disease * Orthopedic problems that would keep them from being able to ride an exercise bicycle, lift weights or do a combination of these exercise * Medications that are known to impact on mitochondrial function: Corticosteroids, opiates, benzodiazepines, tricyclic antidepressants, beta blockers, sulfonylureas, insulin, anticoagulants, barbiturates, insulin sensitizers, fibrates (PPAR gamma agonist) * Claustrophobia