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 metabolism 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 determined, 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 observations 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 constituent 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 Pfliiger, 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 



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

 On this factor probably depends the possibility of fattening an animal. 



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