Nonshivering

Nonshivering thermogenesis is a physiological process by which heat is produced without rhythmic muscle contractions (shivering). In mammals, nonshivering thermogenesis is mainly mediated by brown adipose tissue (BAT), which contains mitochondria rich in the thermogenic protein UCP1 (uncoupling protein 1). Activation of BAT occurs in response to cold exposure or sympathetic stimulation, which releases norepinephrine. Norepinephrine binds beta-adrenergic receptors on brown adipocytes, stimulating lipolysis and the uptake of fatty acids, which fuel mitochondrial oxidation. UCP1 uncouples oxidative phosphorylation from ATP production, releasing energy as heat instead of storing it as chemical energy.

Beige or brite adipocytes can emerge in white adipose tissue under certain stimuli, contributing to nonshivering thermogenesis as well. Thyroid hormones, particularly triiodothyronine (T3), enhance NST by promoting mitochondrial biogenesis and UCP1 expression.

Nonshivering thermogenesis is especially important in newborns, who have relatively abundant brown fat to maintain body temperature. In adults, brown fat activity persists at low levels and can be augmented by cold exposure or pharmacological agents; its role in daily energy expenditure and obesity is an area of active research.

Distinguishing nonshivering thermogenesis from shivering thermogenesis is straightforward: NST occurs without involuntary muscle contractions, while shivering rapidly increases metabolic heat production through muscle activity. NST has potential therapeutic implications for metabolic disorders, though safe and effective activation in humans remains under study.