Advances in science are often achieved with the development of technology that allows scientists to differentiate and refine concepts which were previously undifferentiated. The field of obesity research has progressed from simple measurement of weight loss to multi-compartment models of fat, fat-free mass and regional distribution of fat. An opportunity for further refinement now presents itself in that magnetic resonance imaging technology can be used to image individual organs and tissues. This presents the opportunity to investigate important unresolved questions. For example, information is lacking on the effects of weight loss on individual organs and tissues. The loss of fat free mass (FFM) cannot fully explain the reported decrease in resting energy expenditure (REE) that accompanies weight loss. Since the FFM compartment consists of numerous tissues and organs, each with a different oxidative metabolic capacity, the possibility exists that individual vital organs may change disproportionately to the change in total FFM with weight loss, thereby explaining reductions in REE post weight loss. Complete organ-tissue volume and mass reconstruction of most major compartments is now possible, using magnetic resonance imaging (MRI).