Is there any advantage to the body, however, in this carbonaceous residue of the proteid molecule over simple carbohydrate and fat? Can the processes of the body be accomplished more economically, or more advantageously, with a daily diet so constructed that the tissues and organs must depend mainly upon this carbon moiety of the proteid molecule for their energy-yielding material? It has been one of the physiological dogmas of the past, that the tissues and organs of the body, or rather their constituent cells, preferred to use proteid for all their needs whenever it was available. If proteid were wanting, either because of insufficient intake, or because of excessive activity, then the tissue cells would draw upon their store of non-nitrogenous material. Food proteid and tissue proteid, however, were the materials preferred by the organism, so ran the argument, and the large and incessant output of nitrogen which accompanied the intake of proteid was accepted as proof of the general truth of this idea. We might well question wherein lies the great advantage to the body in this continual excretion of nitrogen; whether the loss of energy in handling and removing the nitrogenous portion of the necessarily large proteid intake, in order to render available the non-nitrogenous part of the molecule, might not more than compensate for the supposed gain? But the truly astonishing fact that the output of nitrogen runs parallel with the intake of proteid, that the body cannot store up nitrogen to any large extent, has been taken as conclusive evidence that the organism prefers to use proteid for all of its requirements. Truly, we might just as well argue that this significant rise in the excretion of nitrogen after partaking of a proteid meal is an indication that the body has no need of this excess of nitrogen; that it is indeed a possible source of danger, since the system strives vigorously to rid itself of the surplus, and that the energy-needs of the body can be much more advantageously and economically met from fat and carbohydrate than from the carbonaceous residue resulting from the disruption of the proteid molecule.