Brief exposure to cold temperatures activates brown fat that burn calories and has become a prominent target to promote cardiometabolic health.
According to a study, time of the day determines the effectiveness of cold exposure in burning calories. It has been found that exposure of cold in the morning may enhance metabolism and burn fat more effectively as compared to exposure of cold in the evening in the males but perhaps it is not effective in women.
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Brown fat or brown adipose tissue (BAT) is a distinctive type of fat that gets activated in response to cold temperatures. Its primary role is to produce heat that help to maintain body temperature that burn calories, especially from fat.
In rodents, brown fat activity fluctuates throughout the day and is the highest before waking up. Heat production from food digestion and activity declines during night-time and waking up requires the body to increase its core body temperature. In humans, there is a circadian rhythm in brown fat and it differs in men and women when they are exposed to cold.
Researchers conducted a randomized crossover study in 24 lean adults including 12 men and 12 women.
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The participants underwent a 2.5-hour personalized cooling protocol using water-filled mattresses in the morning and evening, in a random order and with one day in between these study days. The water temperature was lowered gradually until shivering happened or until temperature was reduced to 9°C. Further, the participants were exposed to stable cold for another 90 minutes. The researchers measured energy expenditure four times during the experiment at the start under thermoneutral conditions, cooling down phase, 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 (both a proxy for brown fat activity) were higher in the morning.
However, in females the cold-induced energy expenditure and skin temperature do not differ between morning and evening, while the females were more tolerant to cold in the morning than in the evening
Moreover, free fatty acid concentrations, triglycerides, and cholesterol levels were higher after cold exposure in women in the morning than in the evening.