Winter-Swimming and Brown Adipose Tissue Activity in Middel-aged Obese Subjects (WinterBAT).

Inclusion Criteria: * Waist circumference above 80 cm for women and 94 cm for men. * BMI \> 25 but \<40 kg/m2 * Age \> 40 years. * Low to moderate physical activity level (no physical activity or on average 30 minutes of moderate physical activity per day and not more than 3 hours of training pr. Week) Subjects will be included in either one of these cases after the screening: * Fasting plasma glucose above 5.6 millimol per liter (mmoL/L). * HbA1C \>39 millimol per mol (mmol/mol). * 2 hour plasma glucose after a 75 g. oral glucose tolerance test (OGTT) above 7.8 mmol/L. Exclusion Criteria: * Winter swimmer * Any history of Cancer, * Clinically significant liver, kidney, heart, inflammatory or lung-disease. * Pregnancy * Taking medication (not for hay fever), including glucose-lowering treatment. * HbA1c \>55 mmol/mol and/or 2-h plasma glucose in the 75-g OGTT \> 15 mmol/L, fasting plasma glucose above 6.9. * Iron deficiency * Increased International Normalised Ratio for coagulation (INR). * Femoral hernia, vene prostheses (pants prosthesis), thrombs in v. Femoralis. * Abnormal ultrasound echocardiography of the heart found at the health-examination day. * Contraindications to cold exposure including severe Raynauds disease. * History of alcohol abuse or use of more than 14 units pr. Week. * History of drug abuse, including marihuana. * Smoking including occasional smoking. * Pacemaker or other electronics implanted in the body. * Claustrophobia, communication problems, including understanding the experimental protocol. * History of contrast allergy.