transformation into any form of carbohydrate; or, if transformation does occur, under some conditions, it must take place within the muscle and without loss of energy. The practical significance of these facts is at once apparent, for if fat, in order to be available as a source of muscle energy, must first undergo conversion into sugar, it would be far more economical from a physiological standpoint to replace the fat of the diet with carbohydrate in any attempt to provide suitable nourishment for the working muscle. We may safely conclude, however, that fat and carbohydrate, as previously suggested, are in reality both capable of direct metabolism by the muscular tissue, and that each is of value as a source of muscular energy in proportion to its heat of combustion, yielding substantially the same proportion of its potential energy in the form of mechanical work. Regarding the utilization of proteid as a source of energy by the muscle, there are many grounds for believing that here the body has to deal with certain alterations, before the proteid can be made available. We may indeed conjecture the transformation of a non-nitrogenous portion of the proteid molecule into carbohydrate, as a necessary step in its utilization for muscle work. It is certainly true that in the ordinary katabolic processes, through which proteid passes, there is a tendency for the nitrogen-containing portion to be quickly split off and eliminated, leaving a carbonaceous residue which may represent as much as 80 per cent of the total energy of the original proteid. This so-called carbon moiety of the proteid molecule is apparently much less rapidly oxidized than the nitrogenous portion, and may indeed be temporarily stored in the body, in the form of fat or carbohydrate.[49] We have very convincing proof that the carbohydrate glycogen can be formed from proteid. Thus, the feeding of proteid to warm-blooded animals may be accompanied by an accumulation of glycogen in the liver. This is interpreted as meaning that in the cleavage of proteid by digestion the various nitrogenous products formed are somewhere, probably in the liver, still further acted upon; the contained nitrogen with some of the carbon being converted into urea, while the non-nitrogenous residue is transformed into glycogen, or sugar. That some such change takes place, or, more specifically, that carbohydrate does result from proteid is more strikingly shown in human beings suffering with diabetes. In severe forms of this disease, all carbohydrate food consumed is rapidly eliminated through the kidneys in the form of sugar, the body having lost the power of burning sugar. If such a person is placed upon a diet composed exclusively of proteid, sugar still continues to be excreted, and there is observed a certain definite relationship between the nitrogen output and the excretion of sugar, thus implying that they have a common origin.