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... Show moreContext: 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 degrees C vs -0.2 +/- 0.2 degrees 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 < 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. Show less
