Altered brown fat thermoregulation and enhanced cold-induced thermogenesis in young, healthy, winter-swimming men

The Scandinavian winter-swimming culture combines brief dips in cold water with hot sauna sessions, with conceivable effects on body temperature. We study thermogenic brown adipose tissue (BAT) in experienced winter-swimming men performing this activity 2–3 times per week. Our data suggest a lower thermal comfort state in the winter swimmers compared with controls, with a lower core temperature and absence of BAT activity. In response to cold, we observe greater increases in cold-induced thermogenesis and supraclavicular skin temperature in the winter swimmers, whereas BAT glucose uptake and muscle activity increase similarly to those of the controls. All subjects demonstrate nocturnal reduction in supraclavicular skin temperature, whereas a distinct peak occurs at 4:30–5:30 a.m. in the winter swimmers. Our data leverage understanding of BAT in adult human thermoregulation, suggest both heat and cold acclimation in winter swimmers, and propose winter swimming as a potential strategy for increasing energy expenditure. [Display omitted] Winter swimmers have a lower core temperature at a thermal comfort state than controlsWinter swimmers had no BAT glucose uptake at a thermal comfort stateWinter swimmers have higher cold-induced thermogenesis than control subjectsHuman supraclavicular skin temperature varies with a diurnal rhythm Søberg et al. find that winter-swimming men, combining cold-water immersion with hot sauna, burn more calories during cooling than controls, despite similar activation of brown fat. The authors propose a lower thermal set point in winter swimmers, reflected by lower core temperature and inactive brown fat at a thermal comfort state.