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CALCULATION OF METABOLISM. 883
and Helme, for the elimination of sulphur, as in their experiments the
sulphur elimination from white of egg required about six days and from
casein only two days. These conditions must be considered in metab-
olism experiments.
Besides lecithins and other phosphatides the body takes with its food
pseudonucleins as well as true nucleins, and these are absorbed more or
less completely from the intestinal tract and then assimilated. On the
other hand, the phosphorized protein substances, lecithins and phos-
phatides, are also decomposed within the body, and their phosphorus is
chiefly eliminated as phosphoric acid and also in part as organic phos-
phorus (see page 757). For these reasons the phosphorus is of great
importance in certain investigations on metabolism.
It is found, on comparing the nitrogen of the food with that of the
urine and feces, that there is an excess of the first ; this means that the
body has increased its stock of nitrogenous substances—proteins. If,
on the contrary, the urine and feces contain more nitrogen than the food
taken at the same time, this denotes that the body is giving up part of its
nitrogen—that is, part of its own proteins has been decomposed.
We can, from the quantity of nitrogen, as above stated, calculate the corre-
sponding quantity of proteins by multiplying by 6.25. x
Usually, according to
Voit’s proposition, the nitrogen of the urine is not calculated as decomposed
proteins, but as decomposed muscle-substance or flesh. Lean meat contains on
an average about 3.4 per cent nitrogen; hence each gram of nitrogen of the urine
corresponds in round numbers to about 30 grams of flesh. The assumption that
lean meat contains 3.4 per cent nitrogen is arbitrary, and the relation of N : C
in the proteins of dried meat, which is of great importance in certain experiments
on metabolism, is given differently by various experimenters, namely, 1 : 3.22-
1 : 3.68. Argutinsky found in beef, after complete removal of fat and subtrac-
tion of glycogen, that the relation was 1 : 3.24 (see Chapter X).
The carbon leaves the body chiefly as carbon dioxide, which is elimi-
nated by the lungs and skin. The remainder of the carbon is excreted in
the urine and feces in the form of organic compounds, in which the quan-
tity of carbon can be determined by elementary analysis. It was for-
merly considered sufficient to calculate the quantity of carbon in the urine
from the quantity of nitrogen according to the relation N:C = 1:0.67 to
0.72. This does not seem to be trustworthy, as this relation varies and
depends, according to Tangl, Pfluger, Langstein, and Steinitz,2
upon
the kind of food. Tangl has shown that the richer the food is in car-
bohydrates the more carbon and hence the more heat of combustion per
1
In calculating the protein catabolism from the nitrogen of the urine it must not
be forgotten that the food often contains nitrogenous extractives whose nitrogen cannot
be calculated as protein and for which a special correction must be made, if necessary.
2
Tangl, Arch. f. (Anat. u.) Physiol., 1899, Suppl. Bd.; Pfluger in Pfliiger’s Arch.,
79; Langstein and Steinitz, Centralbl. f. Physiol., 19.
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