Brown adipose tissue (BAT)

Brown adipose tissue is a thermogenic organ characterized by high mitochondrial density and the expression of uncoupling protein 1 (UCP1), which dissipates the proton gradient across the inner mitochondrial membrane as heat rather than storing it as ATP. In adult humans, metabolically active BAT depots are found primarily in the supraclavicular, paravertebral, and cervical regions and are activated by cold exposure and sympathetic signaling via β3-adrenergic receptors. BAT activity declines with age and increasing adiposity, and lower activity associates with higher BMI, worse insulin sensitivity, and greater cardiometabolic risk in cross-sectional data. Cold exposure, β3-agonists, and candidate compounds such as mirabegron and capsinoids can augment BAT activity, with ongoing investigation into whether sustained augmentation improves metabolic health outcomes in humans.