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on the excretion of nitrogen. On the other hand, increased muscular exertion
does entail a marked increase in the consumption of oxygen and in the excretion
of carbon dioxide. Non-nitrogenous matter had, therefore, to be considered as a
source or as the main source of the energy of muscular contraction.

       To-day an hypothesis⁽¹⁾almost diametrically the opposite of the theories of
Liebig and Pflüger is held in much favour. According to this view the
nitrogen of the proteid food-stuffs is rapidly eliminated from the products of
digestion, and excreted in the form of urea, whilst the carbohydrate moiety of
the proteid molecule forms, on oxidation, the main source of the energy-
supply of the organism. The tissues themselves undergo degradation under
exceptional circumstances only, as, for example, when the supply of food is
insufficient to produce the energy required by the body. According to those
who hold this view, as the main source of energy is the oxidation of the non-
nitrogenous part of the proteid molecule,⁽²⁾ nitrogenous food should play a com-
paratively subordinate role in the general economy of nutrition, and its function
as a source of energy should be capable of being entirely replaced by carbohy-
drates and fats.

       The researches of Chittenden, Siven and others have all been directed to
the establishment of this conception; Chittenden's, works in particular being
an earnest plea for moderation in the proteid intake. This view clearly
recognises the all-importance of nitrogenous food in the nutrition of the body.
It is the only source of nitrogen available in the system and the only source from
which the proteid-containing tissues of the body can be maintained. It is, there-
fore, obvious that "there must be a certain amount of true proteid tissue broken
down each day, independent of that larger metabolism coincident with the intake
of proteid food." We have therefore to distinguish between two forms of
metabolism—the metabolism of energy and the metabolism of tissue—which are
quite distinct from each other. The tissue changes should be fairly constant
under ordinary conditions for any given individual; the one factor determining
their extent being the weight of the true tissue elements of the body.

       In connection with this view Folin⁽³⁾ has recently made a further advance in
our knowledge. He has shown that the change from a nitrogen-rich to a
nitrogen-poor diet is followed by significant changes in the composition of the
urine. As would be expected, the output of urea diminishes in amount, but,
more important than this, it diminishes relatively in comparison with the other
nitrogenous constituents. On the other hand the kreatinin and sulphur output
remains constant, whether the diet be highly nitrogenous or not.

             ⁽¹⁾ Speck. Ascher and Spiro's Ergeb. der Physiol. Bio-Chem., 1903.

             ⁽²⁾ Schryver ;Bio-chemical. Journal, 1906.

             ⁽³⁾ Folin; American Journal of Physiology, 1905.