RecruitingMeasuring Protein Turnover in Humans Across the Lifespan by Metabolic Labeling With Deuterium OxideBackground: Proteins are essential to the health and structure of the cells that make up body tissues. Most proteins become damaged over time and are replaced with new ones. This process is called "protein turnover." Stress, disease, and aging can affect this process. Researchers want to better understand how aging affects protein turnover. Objective: To measure rates of protein turnover in healthy adults. Eligibility: Healthy people aged 20 years and older with a body mass index between 20 and 30. Design: Participants will have 6 study visits over 4 to 6 weeks. They will fast 12 hours before each visit. Participants will be screened. They will have a physical exam, with blood and urine tests and tests of their heart function. They will lie down while blood pressure cuffs are used on their arms and legs. Participants will be given bottles of heavy water to drink at home on a schedule for 21 days. Each bottle holds about 3.5 tablespoons. Heavy water is odorless, colorless, and tasteless, like normal drinking water. It is safe to drink and has been used in research for many years. Participants will have tests during study visits, including: Imaging scans of a leg. Exercise on a treadmill. Biopsies of muscle, skin, and fat: Small samples of tissue will be cut from the calf and abdomen. Resting metabolic rate: Participants will lie still and breathe into a mask for 20 minutes. Knee/grip strength: Participants will do strength tests with their legs. They will squeeze a device with their hands. D3-Creatine: Participants will take 1 pill of D3-Creatine, which occurs naturally in muscle.