THE RELATIONS OF METABOLISM TO FOOD-SUPPLY. 133 
after a change in the ration, and can therefore be interpreted as 
showing simply that sufficient time had not elapsed to reach nitro- 
gen equilibrium. : 
If we consider also the somewhat indefinite nature of the term 
mature, and likewise the possibilities of error due to mechanical 
losses of excreta and to escape of nitrogen from the latter by fermen- 
tation and decomposition, we can readily see why the results of a 
short metabolism experiment may not agree with those of a long 
fattening experiment; yet, nevertheless, it must be confessed that 
the impression left by a comparison of the whole mass of evidence 
is that the discrepancy is as yet but partially explained. 
In conclusion, we may anticipate a discussion in Chapter VI, 
and call attention to the fact that muscular exertion may, to a 
limited extent at least, stimulate those constructive processes which 
result in a storage of protein in the body. 
The Minimum of Proteids.—In the preceding section it ap- 
peared that the administration of proteid food to a previously fast- 
ing animal caused a prompt and large increase in the nitrogen 
cleavage and excretion, while but a comparatively small portion 
of the proteids was applied to constructive purposes, the result 
being that two to three times as much proteids must be given as 
are metabolized during fasting before nitrogen equilibrium is 
reached. This effect was there ascribed to the stimulating effect 
of the rapid digestion and resorption of the proteids upon the nitro- 
gen cleavaye, much of the proteids being apparently destroyed 
as such before they can serve for tissue-building. 
We have just seen that the effect of the non-nitrogenous nutri- 
ents is to diminish somewhat the nitrogen cleavage, apparently 
by moderating this stimulating effect. The necessary result is 
that, as the nitrogen supply is increased, it and the nitrogen excretion 
will start more nearly together and approach each other more rapidly 
upon a mixed diet than upon one consisting of proteids only. Conse- 
quently, while the percentage decrease in the proteid metabolism 
is, as we have seen, relatively small, nitrogen equilibrium may be 
reached with a much smaller supply of proteids than is the case in 
the absence of the non-nitrogenous nutrients. Indeed, it is con- 
ceivable that a sufficient supply of carbohydrates or fats in the diet 
should practically destroy the stimulative effects of the proteids. in 
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