Systemic b-Adrenergic Stimulation of Thermogenesis Is Not Accompanied by Brown Adipose Tissue Activity in Humans

Brown adipose tissue (BAT) is currently considered as a target to combat obesity and diabetes in humans. BAT is densely in-nervated by the sympathetic nervous system (SNS) and can be stimulated by b-adrenergic agonists, at least in animals. However, the exact role of the b-adrenergic part of the SNS in BAT activa-tion in humans is not known yet. In this study, we measured BAT activity by 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) positron emission tomography/computed tomography imaging in 10 lean men during systemic infusion of the nonselective b-agonist iso-prenaline (ISO) and compared this with cold-activated BAT ac-tivity. ISO successfully mimicked sympathetic stimulation as shown by increased cardiovascular and metabolic activity. En-ergy expenditure increased to similar levels as during cold expo-sure. Surprisingly, BAT was not activated during b-adrenergic stimulation. We next examined whether the high plasma fre