Yes, even human brown fat is on fire!
Barbara Cannon and Jan Nedergaard
That adult humans possess brown fat is now accepted — but is the brown fat metabolically active? Does human brown fat actually combust fat to release heat? In this issue of the JCI, Ouellet et al. demonstrate that metabolism in brown fat really is increased when adult humans are exposed to cold. This boosts the possibility that calorie combustion in brown fat may be of significance for our metabolism and, correspondingly, that the absence of brown fat may increase our proneness to obesity — provided that brown fat becomes activated not only by cold but also through food-related stimuli.
Brown adipose tissue is unique in possessing the ability to directly transfer energy from food into heat (1). This is due to the equally unique ability of its characteristic protein uncoupling protein 1 (UCP1) to allow for combustion of food energy in the brown fat mitochondria. Through this, brown fat produces heat for defense against cold — and may prevent obesity by allowing for combustion of energy, instead of storing the excess energy as fat. Brown adipose tissue has long been accepted as a metabolically important organ in small mammals (rats, mice), but only within the last five years has it been brought forward as a possibly metabolically significant tissue in adult humans.
Anatomically, brown adipose tissue in adult humans is found primarily in depots in the neck and around the clavicles (Figure 1). However, accepting the anatomical presence of the tissue is not the same as accepting that it plays an important metabolic role. Acceptance of brown adipose tissue as a significant factor in the metabolism of adult humans will be a stepwise process. Ouellet et al. (2) provide an important further step in this process. They demonstrate that brown adipose tissue in adult humans is actually metabolically highly active when it is stimulated physiologically, that is, even human brown fat is on fire.