This study investigates repetitive cold-water exposure on brown fat activity assessed by PET/CT scanning. Furthermore we will assess glucose control upon winter-swimming. Obese prediabetic men and women will be randomized to winter-swimming or control conditions for 4 months.

Brown fat (BAT) is an energy-consuming tissue. Long-term activation of BAT in adults increases clearance of blood glucose and non-esterified fatty acids (NEFA), insulin sensitivity and metabolic rate. Moreover, adipokines released from BAT, known as batokines, represent a promising but currently unexplored area in terms of metabolic regulation. Cooling activates BAT; however, current cooling protocols fail to activate BAT in a large part of the population. Whether this failure in activation is related to biology, inefficient detection or cooling protocols is unknown. With our initial acute cooling study (protocol number: H-16038581), these protocols have successfully been optimized whereupon novel batokines will be identified. Having established a methodology for assessing BAT activity and batokine profiles, a longer-term cooling intervention (winter swimming) will be performed to examine the effect of repetitive cold-water exposure on BAT activity. Obese pre-diabetic subjects will be included in a 4-month supervised winter swimming (WS) intervention.

It is hypothesized, that repetitive cold-water exposure constituted by WS, increase BAT activation and volume, followed by improved glycemic control in obese pre-diabetic subjects.

Method The study is a randomized controlled, parallel-group intervention study. The investigator will recruit 30 obese pre-diabetic non-winter-swimmers to participate in a WS intervention group (n=15) or control group (n=15) with no WS activities. Subjects will either be randomized to the WS group, who will winter-swim 4 months, 2-3 times/week including sauna-visits if desired, or control conditions with no WS during the 4 months (October to May).

Main aims

• To assess whether WS will have an immediate effect on BAT recruitment or function.
• To assess whether WS will have an effect on glycemic control.

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.