SECTION V 

 THE SIGNIFICANCE OF THE FOOD-STUFFS 



WHEN the proteins are taken as food they are rapidly and almost completely 

 metabolised, so that the energy output of the body increases pari passu 

 with the increased protein food. With a large excess of protein, a certain 

 limited storage of this material is possible, but the stored-up protein rapidly 

 disappears on deprivation of food. On this account the course of the meta- 

 bolism during starvation is the same after the first two or three days, whether 

 the animal has previously received large or small amounts of protein in its 

 diet. In man, even this limited power of storing nitrogen is apparently 

 absent. Under no circumstances can we produce a laying on of fat in the 

 body, even in carnivora, however much the protein income is increased. 



The metabolism of fats and carbohydrates, on the other hand, is deter- 

 mined, not by the amount of these substances in the food, but by the energy 

 requirements, i.e. the functional activity of the living tissues. Any excess * 

 of either of these foods simply gives rise to their storage in the body almost 

 entirely in the form of fat. How are we to regard the particular position 

 taken up by the proteins in metabolism ? Voit, whose laborious observa- 

 tions form the foundation of all our present knowledge of metabolism, drew 

 a sharp contrast between the proteins which were built up to form parts of 

 the living cells, the tissue or morphotic protein, and those which underwent 

 rapid oxidation in the tissue juices without ever forming an integral con- 

 stituent of the living protoplasm. The latter he designated circulating 

 protein. The rapid fall in the nitrogenous excretion during the first two 

 days of starvation he ascribed to the using up of the circulating protein. 

 As soon as this was exhausted the animal was reduced to living on its own 

 tissues, so bringing into the metabolic cycle the tissue-proteins themselves. 

 This theory has been energetically attacked of late years by Pfluger, according 

 to whom the whole of the protein, which is broken down and oxidised to urea, 

 must at one time have formed an integral part of a living cell, so that tissue- 

 protein would be the sole source of the urea. He explains the rapid excretion 

 of nitrogen which follows the ingestion of a protein meal by the special 

 avidity of the animal cell for protein. When enough of this is presented to it 

 it feeds upon nothing else, and only when there is a comparative lack of pro- 

 tein will it make use of carbohydrate or fat for its needs. Thus while a dog 



* That is, assuming that the animal is able to digest and absorb the excess. 

 On, this factor probably depsnds the possibility of fattening an animal. 



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