INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM. 209 
tissue on a lower level. This fact alone would indicate the need of 
a reasonably liberal supply of protein in the food of working animals. 
If the hypothesis above outlined be approximately correct, it is 
necessary that the food also contain protein which during rest may 
be simply a source of heat, but which during work may be diverted 
to repair the increased waste of nitrogenous tissues caused by ex- 
ertion. This accords with the well-established fact that the dieta- 
ries selected by athletes and others who undertake severe physical 
exertion are almost invariably rich in protein.* It is of course 
difficult to say how far the large amount of proteids in the dietaries 
of athletes represents a real physiological demand and how far it is 
a matter of tradition or of taste, but it hardly seems likely that so 
universal an opinion should be lacking in some considerable basis 
of fact. 
Effects upon the Carbon Metabolism. 
In the foregoing paragraphs we have seen that as a rule the 
total proteid metabolism is not much affected by muscular exertion. 
While proteids undoubtedly have important functions in connection 
with the production of work, it is nevertheless true that normally 
the energy liberated in muscular contraction is derived chiefly or 
wholly from the breaking down of non-nitrogenous material. 
Moreover, even in those cases in which a considerable increase of 
the proteid metabolism has been observed, its amount has been 
entirely insufficient to account for the extra evolution of energy. 
It therefore becomes of especial importance to consider the effects 
of work upon the carbon balance. 
The Gaseous Exchange.—Since the influence of muscular ex- 
ertion upon the proteid metabolism is at most small, it is possible to 
compare the carbon metabolism during work and rest without 
material error upon the basis of the gaseous exchange simply, and 
as a matter of fact a large share of our knowledge of the subject 
rests upon determinations of the respiratory exchange. 
Is Larcety Incrrasep.—The fact that muscular work largely 
increases the evolution of carbon dioxide and water and the con- 
sumption of oxygen by the organism is too familiar from ordinary 
* For a summary of American experiments bearing upon this point see 
Atwater & Benedict, Boston Medical and Surgical Journal, 144, 601 and 629 
