Brown fat was thought, until recently (c2009), to only exist in babies and hibernating mammals. It is now proven to exist in small doses in adult bodies too. Brown fat differs from white fat in that it contains mitochondria, and is burnt ‘on demand’ to produce heat and warm the blood, as opposed to white fat which is intended for longer term storage and to provide energy to muscles.
Brown fat growth is claimed to be encouraged by exposure to extreme cold, which also encourages the burning of brown fat. This is a theoretical fat burning strategy, explored in the post about cold therapies.