In addition to the energy-storing white adipose tissue (WAT), mammals possess brown adipose tissue (BAT) that burns fat to release heat for thermogenesis. BAT is abundant in mammals with high thermoregulatory demands, such as small mammals and the neonates of large mammals1. BAT was previously believed to be present only in small mammals and human infants. However, active BAT was also demonstrated in adult humans in the early 1990s2,3. Interestingly, in adult humans, BAT activity shows an inverse correlation with body mass index (BMI) and the percentage of body fat4,5. These findings indicate that BAT may play an important role in whole-body energy metabolism, although direct evidence is still lacking. In the current study, we found that BAT transplantation improved whole-body energy metabolism and increased insulin sensitivity. In addition, BAT transplantation not only prevented high-fat diet (HFD)-induced weight gain but also reversed preexisting obesity. Furthermore, we showed that these effects were BAT-transplantation specific, as transplantation of other tissues did not produce similar effects.

BAT is a major organ that can generate large amounts of heat; it is responsible for at least 60% of non-shivering thermogenesis in cold-acclimated animals6. Therefore, we investigated whether BAT transplantation produced any effect on thermogenesis. We demonstrated that BAT transplantation not only significantly increased the core body temperature of animals under thermoneutral conditions (Figure 1C), but also greatly increased the core body temperature of animals that were challenged by exposure to cold conditions (4 °C, 6 h) (Figure 1C-1D). This elevation in body temperature was linked to an increase in energy metabolism, as evidenced by a large increase in oxygen consumption (Figure 1E) that was not accompanied by a significant change in the respiratory quotient (RQ) (Supplementary information, Figure S1E).

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