Human Brown Adipose Tissue is Active at Thermoneutrality in a Circadian Rhythm and is Highly Regulated in Winter Swimmers

To investigate human brown adipose tissue (BAT) physiology, we recruited young healthy men that were either winter swimmers (WS) or controls. We used 2-deoxy-2-[18F]fluoro-D-glucose PET/MRI-scanning, infrared thermography, iButtons and ingestible thermometer pills to assess BAT glucose uptake, and BAT and core temperature. In a crossover design with interventions performed in randomized order, subjects performed individualized cooling and thermoneutral protocols. We monitored a subset for BAT circadian rhythm. In response to cooling, WS increased more in BAT temperature, BAT glucose uptake and energy expenditure compared to control subjects. At thermoneutrality, WS had higher BAT temperature and lower core temperature, despite BAT glucose uptake in controls only. All subjects demonstrated a nocturnal reduction in BAT temperature, while a peak was observed at 4:30-5:30 am in WS, coinciding with rising plasma cortisol levels. In conclusion, WS have highly regulated BAT, and BAT is active at thermoneutrality in a circadian rhythm in humans.