Brief exposure to cold temperatures has been found to activate brown fat, which has become a prominent target for promoting cardiometabolic health by burning calories. However, recent research suggests that the time of day may determine the effectiveness of cold exposure in calorie burning.
A study revealed that cold exposure in the morning may enhance metabolism and fat burning more effectively in men compared to cold exposure in the evening. However, the same effectiveness was not observed in women.
Brown fat, also known as brown adipose tissue (BAT), is a unique type of fat that is activated in response to cold temperatures. Its primary role is to generate heat, which helps maintain body temperature and burn calories, particularly from fat.
In rodents, a brown fat activity follows a daily fluctuation pattern and is highest before waking up. During the night, heat production from food digestion and activity decreases, and the body needs to increase its core temperature upon waking. Similarly, humans also exhibit a circadian rhythm in brown fat activity, which differs between men and women when exposed to cold.
For the study, participants underwent a 2.5-hour personalized cooling protocol using water-filled mattresses, both in the morning and evening, with a day between each session. The water temperature was gradually lowered until shivering occurred or the temperature reached 9°C. Following that, the participants were exposed to stable cold for another 90 minutes. Energy expenditure was measured four times during the experiment: at the beginning under thermoneutral conditions, during the cooling down phase, during the stable cold phase, and at the end of cooling.
The analysis of the study found that in men, cold-induced energy expenditure and skin temperature (which serves as a proxy for brown fat activity) were higher in the morning compared to the evening. On the other hand, in women, cold-induced energy expenditure and skin temperature did not significantly differ between morning and evening. Interestingly, women were more tolerant to cold in the morning than in the evening. Furthermore, after cold exposure, women exhibited higher concentrations of free fatty acids, triglycerides, and cholesterol levels in the morning compared to the evening.
Overall, these findings suggest that the timing of cold exposure may play a role in its effectiveness for calorie burning and metabolic health. While men seem to benefit more from morning cold exposure, women exhibit similar responses regardless of the time of day. Further research is necessary to fully understand the underlying mechanisms and potential implications for promoting cardiometabolic health in both men and women.
