Circadian rhythms – the body’s daily cycles of biological, hormonal, and behavioral patterns – play critical roles in human health. Disturbances in these rhythms may increase susceptibility to metabolic disorders, such as diabetes and obesity. Findings from a new study indicate that diurnal circadian variations in brown adipose tissue activity may contribute to metabolic disorders.

Brown adipose tissue is a type of fat involved in thermogenesis – the production of body heat. There are two types of thermogenesis: diet-induced and cold-induced. Diet-induced thermogenesis involves an increase in the metabolic rate that occurs after consuming a meal. Cold-induced thermogenesis involves uncoupling electron transport from ATP synthesis and repetitive, non-productive transport of ions across the adipose cell membrane. In the past, scientists believed that brown fat was present only in newborns, where it served to protect against heat loss via cold-induced thermogenesis. However, recent research has identified active brown fat in adults, typically following cold exposure. Brown fat activity contributes to overall energy expenditure and the regulation of body fat.

The authors of the study conducted a two-part investigation. In the first part, 21 healthy men (20 to 50 years old) underwent positron emission tomography (PET) scans to detect the presence of brown fat. During the PET scans, the men sat in a cool (66°F, 19°C) room for two hours while wearing lightweight clothes (a T-shirt and shorts) and periodically placing a towel-wrapped block of ice against the soles of their feet. The authors of the study categorized the men as having high or low quantities of brown fat. Then the men ate a standardized meal, and the authors of the study measured several parameters of the men’s metabolic function, including energy expenditure, diet-induced thermogenesis, and fat oxidation.

They found that men with high quantities of brown fat tended to have higher diet-induced thermogenesis and fat oxidation than those with low quantities, especially after breakfast, suggesting that brown fat has a greater influence on diet-induced thermogenesis earlier in the day.

In the second part of the study, the authors categorized 23 healthy men (20 to 29 years old) as having high or low quantities of brown fat using the same procedure used in the first study. Then they used a thermal imaging camera to measure the men’s skin temperature at the supraclavicular region (just above the collar bones, an area where brown fat is typically present). They took measurements in the morning and evening in warm (80°F, 27°C) conditions and after the men had been sitting for 90 minutes in cool (66°F, 19°C) conditions. They also measured the men’s energy expenditure, cold-induced thermogenesis, and fat oxidation.

They found that the men’s energy expenditure, fat oxidation, and supraclavicular temperature were higher in the men with high quantities of brown fat compared to those with low quantities. The men’s energy expenditure in the morning was nearly equal for both high and low brown fat groups in warm conditions, but it was higher in cool conditions among those with high brown fat quantities. Energy expenditure in the evening was the same among both groups regardless of temperature. Cold-induced thermogenesis among the men with high brown fat quantities was higher in the morning than in the evening.

These findings suggest that brown fat activity exhibits diurnal circadian variations that influence metabolic function. These variations may explain associations between meal timing, obesity, and related metabolic disorders. Time-restricted eating resets the circadian clock to promote metabolic health. Learn more about time-restricted eating in our overview article.

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