Young children have a high resting energy expenditure (REE) relative to their body weight and metabolically active compartment, fat-free mass (FFM). Both body weight and FFM are, however, metabolically heterogeneous and include organs and tissues varying widely in specific metabolic rate (i.e., organ REE/kg/d). One prevailing hypothesis is that most, if not all, of the higher REE observed in young animals and children compared to adults can be accounted for by a larger proportion of high metabolic rate components such as brain, liver, and heart..