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890 METABOLISM.
and which permits the estimation of the nitrogen of the urine and the carbon
dioxide expired, as well as the inspired oxygen and the quantity of heat produced.
If we start from the theoretically calculated formulae for the various possible
transformations of the proteins, fats, and carbohydrates in the body, it is clear
that other values must be obtained for the heat, carbon dioxide, oxygen, and
nitrogen of the urine, when one, for example, admits of a complete combustion
of proteins to urea, carbon dioxide, and water, or of a partial splitting off of fat.
Another relation between heat, carbon dioxide, and oxygen is also to be expected
when the fat is completely burnt or when it is decomposed into sugar, carbon
dioxide, and water. In this way, by a comparison of the values found in special
cases with the figures calculated for the various transformations, Kaufmann
attempts to explain the various decomposition processes in the body under dif-
ferent nutritive conditions.
The organic foodstuffs serve in part to replace the necessary losses
of the organs and in part as sources of energy. Under all circumstances
a restitution of the protein-like constituents of the organs is necessary.
This replacement is, according to Rubner, represented by the so-called
wear-and-tear quota (see below) which amounts to about 4-6 per cent of
the total energy transformed and which can be supplied by proteins only.
For the supply of the remaining exchange, which according to Rubner
serves as source of energy, all three groups of organic foodstuffs can be
used, and investigations carried out by Rubner have taught that these
foodstuffs can act as sources of energy in the animal body in a proportion
which corresponds with the respective figures of their heat value. This
is apparent from the following table. In this is found the weight of
the various foods equal to 100 grams of fat, a part determined from
experiments on animals and a part calculated from figures of the heat
values:
From Experiments From the Difference,
on Animals. Heat Value. per cent.
Syntonin 225 213 +5.6
Muscle-flesh (dried) .... 243 235 +4.3
Starch 232 229 +1.3
Cane-sugar 234 235 -0
Glucose 256 255 -0
From the given isodynamic value of the various foods it follows that
these substances replace one another in the body almost in exact ratio
to the energy contained in them. Thus in round numbers 227 grams of
protein and carbohydrate are equal to or isodynamic with 100 grams of
fat in regard to source of energy, because each yields 930 calories on com-
bustion in the oody.
By means of recent very important calorimetric investigations, Rub-
ner !
has shown that the heat produced in an animal in several series of
experiments extending over forty-five days corresponded to within 0.47
per cent of the physiological heat of combustion calculated from the decom-
Zeitschr. f. Biologie, 30.
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