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Traditional Scandinavian culture promotes sauna bathing, sometimes combined with brief periods of winter-swimming. This exposure to extreme cold promotes the formation of brown adipose tissue, which may enhance metabolic health. Findings of a new report show that winter swimmers have an increased metabolic response to cold temperatures.

Brown adipose tissue is a type of fat involved in thermogenesis – the production of body heat. Cold-induced thermogenesis involves uncoupling of the mitochondrial electron transport chain, reducing the efficiency of ATP synthesis. This less efficient mode of energy production consumes more calories than normal ATP production and gives off heat as a byproduct. Brown fat activity contributes to overall energy expenditure and the regulation of body fat. Previous research has reported an association between increased brown adipose tissue and better whole-body glucose and insulin sensitivity in adults.

The authors recruited eight young, healthy males between ages 18 and 35 who participated in winter swimming two to three times per week. Seven participants also used sauna bathing during their winter swimming practice. The authors recruited an additional eight participants who did not participate in winter swimming or sauna bathing and who were matched for age, body mass index, and maximal metabolic rate. The participants provided a blood sample and completed an oral glucose tolerance test to measure glucose sensitivity. The researchers measured body temperature using infrared thermography, body composition using x-ray absorptiometry, and oxygen consumption during a strenuous cycling exercise, a measure of maximal metabolic rate. Finally, the participants completed a sleep study to determine the effects of circadian rhythm on brown adipose tissue metabolism.

Winter swimmers had a lower core body temperature and reduced uptake of glucose by brown adipose tissue at a comfortable room temperature. They also had lower plasma glucose levels at the end of the glucose tolerance test, suggesting better glucose utilization by skeletal muscle and adipose tissue. When exposed to cold, winter swimmers had a greater increase in thermogenesis compared to participants who did not participate in winter-swimming. Finally, during the sleep study, winter swimmers had increased thermogenesis a few hours prior to waking. Combined with a lower body temperature, these fluctuations in body temperature during the night may improve sleep quality; however, the authors did not measure this variable in the current study.

The authors conclude that winter-swimming combined with sauna bathing has distinct effects on thermoregulation at room temperature and upon cold exposure. These alterations in metabolism may improve health through increased resting metabolic rate and improved sleep quality. The authors noted that because participants participated in both sauna bathing and winter-swimming, it is not possible to attribute these differences to cold exposure alone.

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