When unacclimatized animals are placed in a cold environment, they will acutely defend their body temperature by means of shivering thermogenesis (muscle contractions), which increases heat production (energy expenditure). However, upon prolonged cold exposure, shivering will gradually decrease, but energy expenditure remains elevated, indicating increased NST

Interestingly, an older study in humans already showed that prolonged cold exposure (12°C, 8 h/d, 31 days) in healthy men also resulted in a gradual decrease of shivering, while heat production remained elevated (11).

Before cold acclimation, 94% (16 out of 17) of the subjects showed cold-activated BAT depots, and after cold acclimation, 100% BAT prevalence was observed. Cold acclimation increased upper body BAT activity from 2.4 ± 0.7 to 2.8 ± 0.5 SUV mean (P < 0.01) and detectable BAT volume expanded from 665 ± 451 cc before to 913 ± 458 cc (P < 0.01) afterwards. Analysis of dynamic PET/CT imaging of the supraclavicular region revealed that upon cold acclimation, glucose uptake rate did not increase significantly (6.9 ± 3.0 vs. 7.6 ± 2.5 μmol/min/100 g; P > 0.05).

Or in normal people speak: People who were exposed to the cold got more brown fat.

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