Novel Methodology to Measure Protein Accumulation

Inclusion Criteria: * Age 18-30 years * Age greater than 65 years Exclusion Criteria: * Active or uncontrolled cardiovascular disease * Chronic kidney disease with serum creatinine ≥ 1.4 mg/dL for women and ≥ 1.5 mg/dL for men * Chronic liver disease (elevation in serum transaminases ≥ 3 times the upper limit of normal) * Any debilitating chronic illness, including malignancy * Significant malabsorptive state, including prior gastric bypass surgery or inflammatory bowel disease * Diabetes mellitus (types 1 or 2) or glucose ≥ 110 mg/dL. * Obesity (BMI ≥ 31 kg/m2) * Anticoagulant therapy (warfarin or heparin) or bleeding disorder that increases risk of bleeding during a muscle biopsy. * Anemia (hemoglobin ≤ 11 g/dL) * Use of medications known to modulate protein synthesis, mitochondrial function, and/or glucose homeostasis (including β-blockers and corticosteroids). * Participation in another study where the 13CPhe was administered during the past 6 months. * Moderate or high level of structured exercise (on average, ≥ 30 minutes per day and ≥ 2 days per week) * Pregnancy * Daily use of tobacco products (smoking or chewing); or smoking ≥7 cigarettes per week, on average. Abstinence from tobacco for ≥3 months is required before enrollment in the study.