Brown Fat

Brown fat is specialized adipose tissue found in the newborn that allows it to generate heat to maintain its body temperature. Newborns are at a greater risk for hypothermia than older humans. They have a larger surface area-to-volume ratio and cannot warm themselves on their own by seeking a warmer environment, covering themselves, or generating significant heat through muscle contraction or shivering. Moreover, they have less thermal insulation in the form of white adipose tissue to protect them from the cold. To compensate for these deficits, newborns have stores of brown adipose tissue in their necks and backs. While brown fat does not offer the thermal insulation of white fat, it does allow the newborn to generate heat through a process called nonshivering thermogenesis. Human cells have mitochondria that generate chemical energy in the form of adenosine triphosphate (ATP). The mitochondria in brown fat cells, however, have a special mechanism to create heat energy instead of chemical energy to warm the body.

When the newborn is exposed to cold, thyroid-stim-ulating hormone (TSH) and epinephrine are released in the body. These hormones initiate biochemical pathways that activate nonshivering thermogenesis in the mitochondria of brown fat cells, allowing them to produce heat instead of ATP. Brown fat cells are better able to undergo nonshivering thermogenesis than white fat cells because they have a greater number of mitochondria and because they have a greater amount of thermogenin (also called uncoupling protein 1, UCP1), a protein required for mitochondria to switch from making ATP to heat. Brown fat is active at birth and then soon transforms to white fat during normal newborn development. Maternal and fetal malnutrition may decrease the amount of brown fat available in infancy.