Nelson-Denny Practice Test 7

Brown Fat: Tissue That Burns to Warm

¹Most of the fat in the human body is white adipose tissue, which stores energy in a single large droplet of lipid inside each cell. A second type, brown adipose tissue, has nearly the opposite job: instead of saving energy, it spends energy to produce heat. Its cells hold many small lipid droplets and are packed with mitochondria, whose iron-containing pigments give the tissue much of its color.

²In most cells, mitochondria use the energy released by breaking down food to make ATP, the molecule that powers cellular work. The mitochondria of brown fat contain a protein called thermogenin, also known as uncoupling protein 1. It lets hydrogen ions leak back across the inner mitochondrial membrane without driving ATP production, so energy that would have been captured in ATP is released as heat instead. Because this warms the body without muscle contractions, the process is called nonshivering thermogenesis.

³Newborn infants depend heavily on brown fat. A baby has a large surface area for its mass, so it loses heat quickly, and it cannot shiver effectively. Deposits of brown fat between the shoulder blades and around the neck and kidneys help keep it warm. For much of the twentieth century, scientists assumed that these deposits largely disappeared in adulthood. Imaging studies have since shown that many adults keep active brown fat, mostly in the neck and upper chest, and that it becomes more active after exposure to cold. Whether this tissue plays a meaningful part in regulating body weight remains an open question.