Which statement about nonshivering thermogenesis is true?

• A. It relies on increased skeletal muscle activity to generate heat.
• B. It does not involve brown adipose tissue.
• C. It is the immediate response at the onset of cold within seconds.
• D. It involves brown adipose tissue activation via norepinephrine signaling and UCP1-mediated thermogenesis.

Answer: (D)

Nonshivering thermogenesis is heat production without muscle contractions, using brown adipose tissue. When cold exposure activates the sympathetic nervous system, norepinephrine is released and acts on brown adipocytes via beta-adrenergic receptors. This signaling triggers processes that mobilize and oxidize fatty acids, and it activates the protein UCP1 in the inner mitochondrial membrane. UCP1 uncouples oxidative phosphorylation from ATP production, causing the energy to be released as heat instead of stored as ATP. This mechanism provides heat without shivering, making brown fat a key player in nonshivering thermogenesis. The other statements describe shivering thermogenesis (muscle activity), deny BAT involvement, or misstate the immediacy of the response, which is why the description of BAT activation with norepinephrine and UCP1-mediated heat production is the accurate one.