Brown adipose tissue is metabolically active fat that dissipates energy as heat via uncoupling protein 1 (UCP1). Distinct from white fat (energy storage) and beige fat (white that browns under stimulus).
Mechanism — UCP1 uncouples electron transport from ATP synthesis — chemical energy becomes heat directly. Activated by cold exposure, sympathetic nervous system tone, and certain compounds (caffeine, capsaicin).
Use case — Cold exposure (cold plunge, cold showers, cool ambient temperature) promotes BAT activation and beige fat conversion. Modest contribution to total energy expenditure but a marker of metabolic health.
Caveats — BAT volume varies enormously between individuals. The energy expenditure case is real but small — don't expect cold plunging to drive weight loss. Better metabolic flexibility is the real prize.