In small vertebrates and hibernating animals, a significant proportion of the adipose tissue is brown adipose tissue (BAT), distinguished from white adipose tissue (WAT) by its smaller (diameter 20 to 40 μm), differently shaped (polygonal, not round) adipocytes.

Like white adipocytes, brown adipocytes store TAGs, but in several smaller lipid droplets per cell rather than as a single central droplet. BAT cells have more mitochondria and a richer supply of capillaries and innervation than WAT cells, and it is the cytochromes of mitochondria and the hemoglobin in capillaries that give BAT its characteristic brown color.

In brown adipocytes, fatty acids stored in lipid droplets are released, enter mitochondria, and undergo complete conversion to CO 2 by β oxidation and the citric acid cycle. The reduced FADH 2 and NADH so generated pass their electrons through the respiratory chain to molecular oxygen. In WAT, protons pumped out of the mitochondria during electron transfer reenter the matrix through ATP synthase, with the energy of electron transfer conserved in ATP synthesis.

A unique feature of brown adipocytes is their production of uncoupling protein 1 (UCP1), also called thermogenin . This protein is responsible for one of the principal functions of BAT: thermogenesis. In BAT, UCP1 provides an alternative route for the reentry of protons that bypasses ATP synthase. The energy of the proton gradient is thus dissipated as heat, which can maintain the body (especially the nervous system and viscera) at its optimal temperature when the ambient temperature is relatively low.