nonshiveringthermogenesisin

Nonshivering thermogenesis (NST) refers to heat production by metabolic processes that does not involve muscle shivering. In mammals, NST is primarily mediated by brown adipose tissue (BAT), which contains abundant mitochondria and high levels of uncoupling protein 1 (UCP1). UCP1 uncouples oxidative phosphorylation from ATP synthesis, releasing energy as heat instead of storing it as chemical energy. Activation occurs in response to cold exposure or sympathetic nervous system signaling, with norepinephrine binding to beta-adrenergic receptors, stimulating lipolysis and fatty acid oxidation to fuel heat production.

Beige or brite adipocytes can arise within white adipose tissue under chronic cold exposure or certain hormonal cues. These cells acquire UCP1 and thermogenic capacity, contributing to NST in adults.

Other forms of NST involve skeletal muscle, including futile cycles that dissipate energy as heat through SERCA-mediated calcium cycling or through creatine-driven substrate cycles. These muscle-based pathways are less well understood than BAT-associated NST but contribute to whole-body energy expenditure, especially in cold conditions or dietary challenges.

Regulation of NST involves the sympathetic nervous system, thyroid hormones, and signaling molecules such as FGF21 and PGC-1α. Species differences exist; infants have relatively active BAT, and many adults retain inducible BAT that responds to cold, though activity varies.

Studies of NST have implications for obesity and metabolic health, as increasing thermogenic activity could raise energy expenditure. Methods to assess NST include imaging of active BAT using PET or MRI and measuring metabolic rate under cold exposure.