Short-term exposure to cold temperatures activates brown fat, which burns calories and is being explored for its potential benefits in promoting cardiometabolic health. Recent research presented at this year’s European Congress on Obesity (ECO) in Dublin, Ireland (17-20 May) suggests that this biological response varies based on the time of day and differs between men and women.
Dr. Mariëtte Boon from Leiden University Medical Center in the Netherlands and colleagues conducted a preliminary study indicating that cold exposure in the morning may be more effective than in the evening at boosting metabolism and burning fat in men, but the same may not hold true for women.
Brown fat, also known as brown adipose tissue (BAT), is a distinct type of fat that is activated in response to cold temperatures. Its primary role is to generate heat to help regulate body temperature by burning calories, particularly from fat.
The study indicates that the optimal time for cold exposure aligns with a specific point in the body’s 24-hour cycle, explains Boon. “It is possible that there are sex differences in how the body responds to cold exposure in terms of enhancing metabolism at certain time points. Furthermore, delivering cold exposure therapies in the morning appears to be more beneficial for men.”
In rodents, the metabolic activity of brown fat fluctuates throughout the day, reaching its peak just before waking up. This biological pattern is logical since heat production from digestion and physical activity declines during the night, and the body needs to raise its core temperature upon waking. However, it remains unknown whether humans exhibit a circadian rhythm in brown fat activity and whether this rhythm differs between men and women when exposed to cold.
To delve deeper into these questions, the researchers conducted a randomized crossover study involving 24 lean adults—12 men (aged 18-31 years; BMI 18-26 kg/m²) and 12 women (aged 18-29 years; BMI 18-26 kg/m²).
The participants underwent a 2.5-hour personalized cooling protocol using water-filled mattresses, with sessions occurring in the morning (7:45 am) and evening (7:45 pm). The order of the sessions was randomized, and a one-day interval was maintained between them.
The water temperature was gradually lowered until shivering occurred or until it reached 9°C. Following this, the participants were exposed to stable cold for an additional 90 minutes.
The researchers measured energy expenditure (using indirect calorimetry) four times during the experiment: at the beginning under thermoneutral conditions (32°C, when the body does not need to generate extra heat to maintain core temperature), during the cooling down phase, the stable cold phase, and at the end of the cooling period (after 90 minutes). The supraclavicular skin temperature was regularly measured using infrared thermography.
The analysis revealed that in men, cold-induced energy expenditure and skin temperature (both indicative of brown fat activity) were higher in the morning compared to the evening.
However, in women, cold-induced energy expenditure and skin temperature did not significantly differ between the morning and the evening. Nonetheless, the females exhibited greater cold tolerance in the morning, meaning they began shivering at a lower temperature during that time.
Moreover, women displayed higher circulating free fatty acid concentrations, triglycerides, and cholesterol levels after morning cold exposure compared to evening exposure.
The authors acknowledge several limitations, including the inability to draw definitive causal conclusions regarding the direct impact of cold exposure on cardiometabolic health. They also note that despite efforts to control diet and sleep, unmeasured lifestyle or genetic factors may have influenced the results.
Nevertheless, this study represents an important initial step in investigating the influence of circadian rhythm on the effects of cold exposure on metabolism, particularly fat metabolism. Currently, the researchers are exploring whether repeated bouts of morning
cold exposure can improve cardiometabolic health in individuals with obesity. At the very least, these findings suggest that considering specific timing when implementing interventions targeting lipid metabolism is warranted, according to Boon.
