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Context Cold exposure mobilizes lipids to feed thermogenic processes in organs, including brown adipose tissue (BAT). In rodents, BAT metabolic activity exhibits a diurnal rhythm, which is highest at the start of the wakeful period. Objective We investigated whether cold-induced thermogenesis displays diurnal variation in humans and differs between the sexes. Methods This randomized crossover study included 24 young and lean male (n = 12) and female (n = 12) participants who underwent 2.5-hour personalized cooling using water-perfused mattresses in the morning (7:45 am) and evening (7:45 pm), with 1 day in between. We measured energy expenditure (EE) and supraclavicular skin temperature in response to cold exposure. Results In males, cold-induced EE was higher in the morning than in the evening (+54% ± 10% vs +30% ± 7%; P = 0.05) but did not differ between morning and evening in females (+37% ± 9% vs +30% ± 10%; P = 0.42). Only in males, supraclavicular skin temperature upon cold increased more in morning than evening (+0.2 ± 0.1 °C vs −0.2 ± 0.2 °C; P = 0.05). In males, circulating free fatty acid (FFA) levels were increased after morning cold exposure, but not evening (+90% ± 18% vs +9% ± 8%; P &amp;lt; 0.001). In females, circulating FFA (+94% ± 21% vs +20% ± 5%; P = 0.006), but also triglycerides (+42% ± 5% vs +29% ± 4%, P = 0.01) and cholesterol levels (+17% ± 2% vs 11% ± 2%; P = 0.05) were more increased after cold exposure in morning than in evening. Conclusion Cold-induced thermogenesis is higher in morning than evening in males; however, lipid metabolism is more modulated in the morning than the evening in females.

Cold-Induced Thermogenesis Shows a Diurnal Variation That Unfolds Differently in Males and Females