brown adipose tissue

Most of our fat is a storage warehouse, but brown adipose tissue is more like a wood stove: instead of hoarding energy, it burns fat to make heat. Its job is not to store calories but to spend them, keeping the body warm — which is why human babies, who cannot shiver well, are born with generous reserves of it.

Brown fat looks different from white fat under a microscope: its cells are packed with mitochondria (which give it the brown color) and contain a special protein, uncoupling protein 1 (UCP1), that lets mitochondria generate heat instead of the usual cellular energy molecule. It is densely supplied by sympathetic nerves, and noradrenaline released onto it switches on heat production. Thyroid hormone is permissive and supportive of this process.

For decades brown fat was thought to disappear after infancy, but imaging studies showed that adults retain functional deposits, especially around the neck and collarbones, that activate in the cold. This sparked great interest in harnessing brown fat against obesity and diabetes, though its total mass in adults is usually too small to drive large weight changes on its own.