Inclusion Criteria:

• * Adults between the ages of 18-35 OR 65-85.
• * BMI \<28 kg/m2 or BMI between 30 kg/m2 and 40 kg/m2
• * Weight stable (≥ 3 months)
• * Generally healthy

Exclusion Criteria:

• * Participation in ≥ 30 minutes of structured physical activity ≥ 2 days per week.
• * Smoking/tobacco use.
• * Alcohol/substance abuse.
• * Pregnancy and breastfeeding.
• * Anemia.
• * Abnormal renal function.
• * Blood clotting disorders.
• * Coronary artery disease.
• * Uncontrolled thyroid disease.
• * Liver disease.
• * Use of medications known to influence the main outcomes of the study.
• * Orthopedic problems that may be aggravated by exercise.
• * Chronic disease at the discretion of the investigators.