THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 107 
from the body. It appears possible, then, that nitrogen equilib- 
rium might have been reached with a less amount than 1500 grams, 
and that with this less amount there might still have been a loss of 
fat-from the body. Whether this possibility is sufficient to explain 
the apparent discrepancy between these and later results must, 
however, remain a matter of conjecture. 
UTILIZATION OF Excess or Prorerps.—We have seen that no 
very considerable or long-continued storage of protein takes place 
in the body of the mature animal. However large the supply of 
food proteids, the body very soon reaches the condition of nitrogen 
equilibrium, the outgo of this element in the excreta equaling the 
supply in the food. This fact, as has been pointed out, does not 
necessarily prove that the elements of the food proteids are com- 
pletely oxidized in the organism. As was shown in Chapter II, 
the abstraction from proteid matter of the elements of urea (or, 
more strictly speaking, of the elements found in the urine) leaves 
a very considerable non-nitrogenous residue available for the pur- 
poses of the organism. It was there stated that this residue could 
serve as a source of energy, and likewise that there was good reason 
to believe that sugar was formed from it, while finally the question 
of its ability to serve as a source of fat was reserved for discussion 
in the present connection. 
Formation of Fat from Proteids. 
Mention has already been made in Chapter IT (p. 29) of the fact, 
first asserted by Liebig,* that the animal body manufactures fat 
from other ingredients of its food. As a result of the investiga- 
tions itcited by the publication of his views regarding the origin 
of animal fat, Liebig’s classification of the nutrients into “ plastic” 
and “respiratory” was generally accepted. The proteids were 
regarded as the material for the growth and repair of the muscles 
and the force exerted by the latter was considered to arise from 
their oxidation, while the non-nitrogenous ingredients of the food, 
especially the carbohydrates, were the source of the animal heat, 
and when present in excess gave rise to a production of fat. 
As time went on, however, observations began to accumulate 
* Compare p. 163. 
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