Brown fat
Adipose tissue specialised in producing heat, rich in mitochondria, which burns energy rather than storing it, and remains active in small amounts in adults.
Brown fat, or brown adipose tissue, is functionally the opposite of white fat: where white fat stores energy, brown fat dissipates it as heat. Its colour comes from its density of mitochondria and its rich blood supply.
The mechanism relies on a protein, UCP1, which uncouples mitochondrial respiration from ATP production: the energy is lost as heat instead of being stored. This is the principle of non-shivering thermogenesis, essential in newborns, who have proportionally a lot of it.
It was long thought to disappear in adulthood. Medical imaging showed the opposite in the early 2000s: active deposits persist, notably around the neck and along the spine, and switch on when exposed to cold. This discovery revived research on obesity.
Caution is still needed on applications, however. The amounts involved in adults are small and the extra energy spent is modest over a whole day. Cold exposure protocols, cold showers and so-called thermogenic compounds have not shown a significant effect on weight in humans: this is a line of research, not a weight-loss method.