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908 METABOLISM.
experiments, as well as the proofs of the formation of fat from proteins,
are also given in Chapter IX.
According to Pfluger’s doctrine, the protein can influence the formation of
fat only in an indirect way, namely, in that it is consumed instead of the non-
nitrogenous bodies and hence the fat and fat-forming carbohydrates are
spared. If sufficient protein is introduced with the food to satisfy the total nu-
tritive requirements, then the decomposition of fat stops; and if non-nitrogenous
food is taken at the same time, this is not consumed, but is stored up in the animal
body, the fats as such, and the carbohydrates at least in great part as fat.
Pfluger defines the " nutritive requirement " as the smallest quantity of
lean meat which produces nitrogenous equilibrium without causing any decom-
position of fat or carbohydrates. At rest and at an average temperature it is
found in dogs to be 2.073 to 2.099 grams of nitrogen 1
(in meat fed) per kilo of
flesh weight (not body weight, as the fat, which often forms a considerable fraction
of the weight of the body, cannot as it were be used as dead measure). Even
when the supply of protein is in excess of the nutritive requirements, Pfluger
found that the protein metabolism increases with an increased supply until
the limit of digestive power is reached, which limit is about 2600 grams of meat
with a dog weighing 30 kilos. In these experiments of Pfluger’s not all of the
excess of protein introduced was completely decomposed, but a part was retained
by the body. Pfluger therefore defends the proposition " that a supply of
proteins only, without fat or carbohydrate does not exclude a protein fattening."
From what has been said on protein metabolism in starvation and
with exclusive protein food, it follows that the protein catabolism in the
animal body never stops, that the extent is dependent in the first place
upon the extent of protein supply, and that the animal body has the prop-
erty, within wide limits, of accommodating the protein catabolism to the
protein supply.
These and certain other peculiarities of protein catabolism have led
Voit to the view that not all proteins in the body are decomposed with
the same ease. Voit differentiates between the proteins fixed in the
tissue-elements, so-called organized proteins, tissue-proteins, from those
proteins which circulate with the fluids in the body and its tissues and
which are taken up by the living cells of the tissues, from the interstitial
fluids washing them, and destroyed. These circulating proteins or supply
proteins are, he claims, more easily and quickly destroyed than the tissue-
proteins. When, therefore, in a fasting animal which has been previously
fed with meat, an abundant and quickly decreasing decomposition of
proteins takes place, while in the further course of starvation this protein
catabolism becomes less in quantity and more uniform, this depends upon
the fact that the supply of circulating proteins is destroyed chiefly in the
first days of starvation and the tissue-proteins in the last days.
The tissue-elements constitute an apparatus of a relatively stable
nature, which has the power of taking proteins from the fluids washing
the tissues and appropriating them, while their own proteins, the tissue-
1
See Schondorff, Pfluger’s Arch., 71.
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