THE SIGNIFICANCE OF THE FOOD-STUFFS 643 



is fed on a rich mixed diet it lives practically on protein alone, storing up the 

 fats and carbohydrates of the food as fat. If food be now withdrawn the 

 animal must live either at the expense of its own living tissue (proteins) 

 or must attack the stored-up fats in its body. The latter alternative, as a 

 matter of fact, takes place. The animal spares the precious protein and lives 

 on the fat of its own body. Hence comes the great fall in the excretion of 

 urea that is observed in starvation, the consumption of proteins sinking to the 

 indispensable minimum. If now a protein meal be given, the cells of the 

 body return to their former way of living, and satisfy as much of their needs 

 as possible at the expense of protein, so that the urea excretion rises almost 

 in proportion to the food given. In order to attain nitrogenous equilibrium 

 on a purely protein diet, it is necessary to give the cells enough protein 

 for their total requirements, i.e. three to five times as much as would corre- 

 spond to the nitrogenous excretion during hunger. If a larger amount of 

 protein be given than is necessary for the maintenance of nitrogenous 

 equilibrium, a certain amount of nitrogen is retained in the body, probably 

 as protein, giving rise to an increase in the total living material of the body, 

 and the animal increases in weight. The amount of urea excreted by an 

 animal is proportional not only to the quantity of protein taken in with the 

 food but also to the weight of the animal ; so the animal which has grown 

 heavier in consequence of increased supply of nitrogenous food will need a 

 larger amount of protein to maintain its nitrogenous equilibrium, which will 

 be produced with the same amount of protein as soen as the animal has 

 increased in weight to a certain extent. In order therefore to maintain the 

 increase in weight, it is necessary to give ever-increasing quantities of protein, 

 and the stuffing process is finally put an end to by the refusal of the digestive 

 organs to digest any more. 



There can be little doubt, however, that Voit was substantially correct 

 in assigning a twofold fate to the ingested protein, and that, as Speck has 

 pointed out, we can consider protein metabolism under two headings, viz. 

 tissue or nutritional metabolism and energy metabolism. Since the proteins 

 form the main constituent of living protoplasm, the death and destruction 

 of cells, which are constantly going on in the body, must result occasionally 

 in the production of substances which are not available for resynthesis, and 

 are therefore turned out of the body with the urine.* A certain proportion 

 of the proteins of the food must therefore be applied to replacing this waste of 

 tissue. The proportion will be larger in cases where a growth of the nitrogen- 

 ous tissues is occurring, as in the young animal or during convalescence from 

 a wasting disorder. On the other hand, a certain proportion of the nitrogen 

 in the urine will be derived from the breakdown of tissues, and this in its turn 



* Tiger&tedt suggests that the irreducible minimal protein consumption may be 

 due, not to a special metabolic cycle of the protein on its way into and out of the living 

 cell, but to the fact that the circulating fluids of the body contain large amounts of 

 protein as essential and constant ingredients, and the living cells therefore, which are 

 bathed by these fluids, cannot refrain, even in times of protein scarcity, from feeding 

 to a certain extent on these the predominant constituents of their nutrient medium. 



