606 METABOLISM, NUTRITION AND DIETETICS 



during starvation. From this lower limit up to 2,500 grammes oi 

 meat a day nitrogenous equilibrium could always be attained, the 

 animal putting on some flesh at each increase of diet, until at length 

 the whole 2,500 grammes was regularly 'used up in the twenty- four 

 hours. A further increase was only checked by digestive troubles. 

 A man, or at least a civilized man, can consume a much smaller 

 amount both absolutely and in proportion to the body- weight. 

 Rubner, with a body-weight of 72 kilos, was able to digest and absorb 

 over 1,400 grammes of lean meat; Ranke, with about the same 

 body-weight, could only use up 1,300 grammes on the first day of 

 his experiment, and less than 1,000 grammes on the third. But 

 whether the surplus of protein food above the necessary minimum 

 is great or small, nitrogen equilibrium is eventually attained, and 

 thereafter all the nitrogen of the food regularly appears in the 

 excreta; the explanation of this fact will be considered a little 

 later (p. 609). 



So much for a purely protein diet. When fat is given in addition 

 to protein, nitrogenous equilibrium is attained with a smaller quantity 

 of the latter. A dog which, with protein food alone, is putting on 

 flesh, will put on more of it before nitrogenous equilibrium is reached 

 if a considerable quantity of fat be added to its diet. Fat, therefore, 

 economizes protein to a certain extent, as we have already recog- 

 nized in the case of the starving animal. On the other hand, when 

 protein is given in large quantities to a fat animal, the consumption 

 of fat is increased ; and if the food contains little or none, the body- 

 fat will diminish, while at the same time ' flesh ' may be put on. 

 The Banting cure for corpulence consists in putting the patient 

 upon a diet containing much protein, but little fat or carbo-hydrate ; 

 and the fact just mentioned throws light upon its action. 



All that we have here said of fat is true of carbo-hydrates. To a 

 great extent these two kinds of food substances are complementary. 

 Carbo-hydrates economize proteins as fat does, but to a greater 

 extent, so that with an abundant supply of carbo-hydrate in the 

 food the minimum protein requirement can be forced down much 

 below what is possible on a diet of protein and fat alone. Carbo- 

 hydrates also economize fat, so that when a sufficient quantity of 

 starch or sugar is given to an otherwise starving animal, all loss of 

 carbon from the body, except that which goes off in the urea, krea- 

 tinin, etc., still excreted, can be prevented. Of course, the animal 

 ultimately dies, because the continuous, though diminished, loss of 

 protein cannot be made good. The fact that carbo-hydrates econo- 

 mize proteins so much more efficiently than fat indicates that sugar 

 is essential in the bodily metabolism, so that when carbo-hydrates 

 are absent from the food some of the protein must be broken down 

 so as to yield eventually the compounds necessary for the formation 

 of carbo-hydrate. It is probable, indeed, that purified proteins, 



