What is the primary thermoregulatory role of brown adipose tissue in newborns?

• A. It generates heat to maintain body temperature.
• B. It stores excess calories for weeks.
• C. It regulates sleep-wake cycles.
• D. It modulates calcium balance.

Answer: (A)

Brown adipose tissue provides a heat-generating, non-shivering mechanism that helps newborns maintain body temperature when exposed to cold. It contains many mitochondria and expresses thermogenin (uncoupling protein 1), which uncouples oxidative phosphorylation from ATP production and releases energy as heat. When the sympathetic nervous system is activated by cold, norepinephrine stimulates brown fat to mobilize and oxidize fats, producing heat to raise body temperature. This is especially important in newborns, who have a high surface area-to-mass ratio and limited ability to shiver. White fat stores calories long-term, not brown fat, so storing excess calories is not the primary role here. Regulation of sleep-wake cycles and calcium balance involve brain signaling and hormonal/mineral regulation, not brown fat thermogenesis.