The Mechanism of Human Non-Shivering Thermogenesis and Basal Metabolic Rate

Detailed Summary

Background: \- Changes in how a person's body burns energy or calories can affect their weight over time. The lowest level of energy the body needs to function is called basal metabolic rate. In the cold, we burn extra energy, even before we start to shiver. This is called non-shivering thermogenesis and it occurs in different types of tissue such as muscle and fat. Researchers want to learn more about this type of energy burning and how it is regulated. They hope this will help treat obesity in the future. Objectives: * Sub-study 1: to better understand how non-shivering thermogenesis works. * Sub-study 2: to measure the effects of anti-obesity drugs on basal metabolic rate. * Sub-study 3: to better understand the effects of mirabegron, a beta-3 adrenergic receptor agonist, on brown fat activity. Eligibility: \- Healthy, lean adult males ages 18 to 35. Design: * Participants will be screened with medical history, physical exam, blood test, and EKG. * For sub-studies 1 and 2: * Participants will receive one X-ray scan. * Each day, all participants will: * Have height and weight measured, and have urine collected. * Spend 4 hours in a temperature-controlled room with furniture, toilet area, phone, and computer. They will wear small non-invasive devices to monitor activity, heart rate, temperature, and shivering. * Walk for 30 minutes. * For sub-study 3: * Participants will receive one DXA scan and up to 4 PET/CT scans and 4 MRIs * Each stay, all participants will: * Have height and weight measured, and have urine collected. * Spend 6 hours in a temperature-controlled room with furniture, toilet area, phone, and computer. They will wear small non-invasive devices to monitor activity, heart rate, temperature, and shivering. * Participants will be compensated for their time and participation at the end of the study

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