Metabolism and Thyroid Hormone Changes During Exposure to Cold Temperatures

This study will examine how body metabolism adjusts to small changes in environmental temperature and how it may be related to weight loss. Healthy, non-obese individuals between 18 and 60 years of age may be eligible for this study. The study consists of one screening visit and one 5-day admission to the NIH Clinical Center for the following procedures: Diet: Participants follow a proscribed diet consisting of 50% carbohydrates, 20% protein and 30% fat. DEXA scan to measure body fat. Subjects lie on a table above a source of X-rays while a very small dose of X-rays is passed through the body. Air displacement plethysmography (Bod-Pod) to study fat composition: Subjects sit in a small booth for 10 minutes wearing a swim suit and breathing normally through a tube. This test measures the person s weight and volume precisely. Metabolic room: Subjects stay 12 hours in a specialized room designed to measure the amount of oxygen breathed in and the amount of carbon dioxide breathed out. The room contains a private toilet and sink, treadmill, bed, desk, window, telephone and computer with television and internet access. While in this room, subjects undergo the following: * Continuous heart rate monitor: Subjects wear an EKG monitor to analyze heart rate variability. * Spontaneous movements: Subjects wear portable accelerometers at the hip and wrist to measure spontaneous physical movements. * Continuous temperature monitor: Subjects swallow a small capsule that transmits internal body temperature via miniature, wireless sensors. Adhesive patches placed on the skin monitor external body temperature. * Blood and urine collections: Subjects collect all of their urine during the 12 hours they are in the metabolic room. Blood samples are taken at specified intervals through a catheter that has been placed in a vein. * Microdialysis: A small needle is placed into fat tissue just under the skin on the abdomen. A solution containing a minimal amount of ethanol is infused and