Non-shivering thermogenesis and which tissue mediates it in humans?

• A. Skeletal muscle producing heat via shivering mechanisms.
• B. White adipose tissue releasing heat through lipolysis.
• C. Brown adipose tissue via uncoupling protein 1 (UCP1).
• D. The brainstem controlling autonomic heat loss.

Answer: (C)

Non-shivering thermogenesis is heat production without muscle contractions. In humans, this role is fulfilled primarily by brown adipose tissue, which contains mitochondria rich in the protein UCP1. UCP1 uncouples oxidative phosphorylation from ATP production, so the energy from fuel oxidation is released as heat instead of being stored as ATP. Cold exposure stimulates the sympathetic nervous system to release norepinephrine, which activates β-adrenergic signaling, increases lipolysis to supply fatty acids, and upregulates UCP1 activity to boost heat production.

This differs from shivering, where heat comes from rapid muscle contractions; white adipose tissue mainly stores energy and releasing fatty acids rather than producing heat via UCP1; and the brainstem regulates heat loss and overall thermoregulation rather than directly generating heat.