Pilot Study of Isocaloric Time Restricted Eating on Ketone Metabolism and Immunoregulation

Background: Time restricted eating (TRE) is a form of fasting in which a person eats only during a set window of time, which is usually between 4 and 10 hours each day. Researchers want to know more about how TRE may affect health. Objective: To learn how TRE affects women with different body sizes. Eligibility: Healthy women aged 18 to 50 years. Design: Participants will have 2 visits: 1 screening visit and one 5-day stay in the clinic. Participants will fast before both visits. They will have a physical exam with blood tests. They will talk to a nutritionist about the foods they eat. They will lay under a clear hood for up to 45 minutes during a test that measures how many calories they burn while resting. Participants will keep a food diary for up to 7 days before their clinic stay. They will apply a continuous glucose monitor the day before they go to the clinic. This is a device that attaches to the skin of the stomach. They will wear this device throughout their clinic stay. All meals will be provided during the clinic stay. Participants will follow TRE on 3 days. They will answer survey questions and have tests during their stay, including: * DXA (dual energy X-ray absorptiometry) scan. Participants will lie on a padded table. Their body will be scanned to measure how much muscle, bone, fat, and other tissues they have. * Stable isotope tracer study. Small amounts of sugar and other substances will be given through a tube attached to a needle inserted into a vein in the arm. Blood samples will be collected.