522 PRINCIPLES OF ANIMAL NUTRITION. 
RELATIVE VALUES OF NUTRIENTS.—In the foregoing discussion 
it has been tacitly assumed that the stored-up energy of the pro- 
teids, fats, and carbohydrates of the body is all net available energy, 
ready to be utilized directly for the production of mechanical work. 
As we have seen, however, on previous pages, a school of physiolo- 
gists, of which Chauveau may stand as the representative, denies 
this, and holds that the fat in particular must be converted into 
a carbohydrate before it can become directly available. 
In discussing the source of muscular energy in Chapter VI it 
was shown that the recorded results as regards the nature of the 
material metabolized were insufficient to decide the question, since 
the final excretory products are qualitatively and quantitatively 
the same whether the fat is directly metabolized in the muscle or 
undergoes a preliminary cleavage in the liver or elsewhere in the 
body. The results as to energy, however, would be materially 
different in the two cases. The dextrose resulting from the cleavage 
of fat, according to Chauveau’s schematic equation (p. 38), would 
contain but about 64 per cent. of the potential energy of the fat, the 
remainder being liberated as heat. We cannot, however, suppose 
that the energy of this dextrose can be utilized by the muscle any 
more completely than that of dextrose derived directly from the 
food. It follows, then, that the percentage utilization of the total 
energy metabolized during muscular work should be materially 
greater when the metabolized material consists largely or wholly of 
carbohydrates than when it consists chiefly of fat. By supplying 
food consisting largely of one or the other of these materials, it is 
possible to bring about. these conditions, and a determination of 
the respiratory exchange and the nitrogen excretion will then 
afford a check upon the nature of the material metabolized and the 
means of computing the utilization of its potential energy. 
Investigations of this sort have been reported from Zuntz’s 
laboratory. The earliest of these were by Zuntz & Loeb * upon a 
dog, the method being substantially the same as that with which 
the preceding pages have made us familiar. Their final results for 
the energy metabolized per kilogram and meter traveled (including 
the work of ascent) were: 
* Arch. f. (Anat. u.) Physiol., 1894, p. 541. 
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