Clinical Trials

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: <TAB>- 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. <TAB>- 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: <TAB>- 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

Background: - The way that the body burns calories is known as energy expenditure. Some studies show that when we are cold, we burn more calories to keep our bodies warm. Brown fat is a special kind of fat that can use energy to keep the body warm. Small animals and infants have been known to have brown fat for many years. Recently, it has been suggested that adult humans also have brown fat. If brown fat becomes active (burns calories) in adult humans when exposed to cold, then these people would tend to burn off more calories and might not gain weight easily. Learning more about the relationship between energy expenditure, brown fat, environmental temperature, and body temperature may help explain why some people become obese and other people do not. Objectives: - To better understand how the body burns calories when exposed to different temperatures. - To study brown fat and how it burns calories in cold temperatures. Eligibility: - Healthy men between 18 and 35 or 55 and 75 years of age. - Healthy women between 18 and 35 years of age. - To control for ethnicity, participants must be non-Hispanic whites or African Americans. Design: - Participants will be screened with a physical exam and medical history. Blood and urine samples will be collected. - Participants will stay in the Metabolic Unit of the National Institutes of Health Clinical Center as inpatients for no more than 14 days. The length of the hospital stay will depend on how participants respond to the different study temperatures. - Every afternoon, participants will walk for 30 minutes on a treadmill. All meals will be provided. - Participants will stay up to 5 hours per day in a specialized room with different temperature settings. Temperatures will range from about 61 degrees to 88 degrees Fahrenheit. Body temperature, activity, calorie burning, and cold/hot sensations will be monitored. On the study day of the coldest temperature, participants will have an imaging study to look for brown fat activity. - Participants will be compensated for their time and participation at the end of the study.