This study investigates cold-induced brown fat activation in winter swimmers and not-winter swimmers by skin temperature measures assessed with infra red thermography imaging and skin temperatures. Winter swimmers and not-winter swimmers will participate in an acute cooling intervention and thermoneutral intervention for comparison of energy expenditure and skin temperatures at the supraclavicular area.

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.

Cold induced brown fat activity will be measured by skin temperature measures by means of infra-red thermography imaging and skin temperature.

15 healthy male winter swimmers and 8 healthy male non-winter swimmers (controls) will be included in the study.

Inclusion Criteria:

Age 18-35 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. Winter swimmers: Min. 2 swim/ week. Swimming in winter-season in open water outdoor in Denmark from September/October until study start in March.

Not-winter swimmers: Not winter-swimmers or taken cold showers on a regular basis.

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, in the preliminary investigation.

Use of daily medications, except for seasonal use of antihistamines. Diagnosed sleep disturbances.