252 ; PRINCIPLES OF ANIMAL NUTRITION. 
Energy from fat............6 4.686 cals. X 0.6939=3.252 cals. 
- “carbohydrates... 5.047 cals. X 0.9765=4.927 
Letalacn gece usa eaeGiateeeaees eked 8.179 “ 
It is obvious that this method of computation affords the means 
of comparing the total energy metabolized during the performance 
of a measured amount of work with the quantity recovered in the 
work itself. It has been extensively used for this purpose by Zuntz 
and his associates, especially in his investigations in conjunction 
with Lehmann and Hagemann* upon work production in the horse, 
which will be considered in a subsequent chapter. The same 
authors ¢ show that the error introduced by the assumption of 
unchanged proteid metabolism is too small to be of any significance. 
Computation from Total Excreta.—The method just described 
naturally leads up to a computation based on the gaseous exchange 
combined with a determination of the urinary products, particu- 
larly nitrogen. The latter shows the total amount of proteids 
metabolized. If we also know, or can compute with sufficient 
accuracy, the carbon, hydrogen, and oxygen of the urinary solids 
we have the data from which to compute the portion of the respira- 
tory exchange due to the protein (see p. 75) and the corresponding 
amount of energy liberated. The residues of carbon dioxide and 
oxygen can then be distributed between the fats and carbohy- 
drates in the manner already described. This method has been 
extensively employed by Kaufmann.{ As already stated, he com- 
putes the gaseous exchange of the proteids on the assumption of an 
oxidation to carbon dioxide, water, and urea only, an assumption 
which, as we have seen, is in some cases considerably wide of the 
truth. 
It is, of course, essential that experiments by this method shall 
cover a sufficient length of time to ensure that the nitrogen excretion 
corresponds with the actual proteid metabolism. It is therefore 
inapplicable to periods of from a few minutes to an hour or so, such 
as have been generally employed in experiments based on the gas- 
eous exchange only. Kaufmann’s experiments extended over five 
* Landw. Jahrb., 18,1; 23, 125; 27, Supp. III. 
Tt Ibd., 27, Supp. IIL, p. 251. 
{ Archives de Physiol., 1896, pp. 329, 342, 757, 
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