This study investigates cold-induced brown fat activation assessed using PET/MR scans. Subjects will participate in an acute cooling intervention day and a thermoneutral intervention day with PET/MR scans on both days. A secondary purpose is to make a validation of an infrared thermography camera by comparison of skin temperatures and SUV of the supraclavicular brown adipose tissue.

The field of human brown adipose tissue (BAT) research is focused on activation of BAT as a means of manipulating energy expenditure and potentially anti-obesity and anti-diabetic properties of the tissue. This is well established in rodent studies and explained as due to the specific brown fat uncoupling protein 1 (UCP1). Despite increasing evidence that indicates a metabolic regulatory role of BAT in humans, BAT activation/recruitment is not fully understood and even less is known about the endocrine capacities of human BAT.

Cold induced brown fat activity will be assessed with PET/MR scans and validate an infra-red thermography camera, as well as perform a proteomic screening using blood samples under these two intervention days.

15 healthy lean male subjects will serve as their own control in a cross-over randomized control study, including a pre-examination day, a thermoneutral-day and a cooling test-day with cold exposure followed by a PET/MR scan to determine BAT activity. An extra cooling day is applied for the external validation of the infrared thermography camera. Blood samples and abdominal fat-biopsies will be taken at thermoneutral condition and under cold stimulation of brown fat.

Inclusion Criteria:

• Age 18-30 years.
• Men
• Caucasian white
• Body Mass Index 19-25 kg/m2.
• Moderate physical activity level (not more than 8-10 hours of training pr. Week)
• Ability to give informed consent.
• Ability to follow verbal and written instructions in Danish.
• Body fat percent under 21% measured at the pre-examination day.

Exclusion Criteria:

• Chronic diseases: Hormonal: Metabolic Diseases mb. Graves, hypothyroidism, myxedema and goiter. Diabetes Melitus, Coronary heart disease (history of myocardial infarction, unstable angina pectoris, or congestive heart failure), cancer treated within last 3 years.
• Tobacco, marijuana or intravenous drug use within 1 year of screening.
• Dieting, recent weight loss (>3 kg within 3 month) or a history of an eating disorder.
• Dietary supplements.
• History of depression, psychosis, or other psychiatric illness requiring hospitalization.
• Alcohol consumption >14 units/week.
• History of alcohol abuse within the past 3 years.
• Known liver disease or elevated liver biomarkers more than 2 times upper normal levels in the preliminary investigation.
• Known kidney disease or elevated kidney biomarkers, eCRF <60 ml/min, in the preliminary investigation.
• Use of daily medications, except for seasonal use of antihistamines.
• Diagnosed sleep disturbances.
• Pacemaker or other electronics implanted in the body.
• Magnetic impressionable metal implants or foreign objects in the body inconsistent with completion of MR.
• Claustrophobia
• Communication problems, including understanding the experimental protocol
• History of contrast allergy.
• Winter swimmers.