Aging is associated with a loss of brain function and conditions such as dementia and Alzheimer's disease. It is likely that decreased brain metabolism is contributing to the progression of age related degenerative diseases. Aerobic exercise training can increase brain volumes and is associated with decreased risk for degenerative brain conditions. However, little is know about the changes that occur to brain metabolism with aerobic training and aging.

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