534 A MANUAL Of PHYSIOLOGY , 



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. 



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 recognised 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, and they 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, kreatinin, etc., still excreted, can be prevented. Of course, the 

 animal ultimately dies, because the continuous, though diminished, 

 loss of protein cannot be made good. 



It is only necessary to add that peptone can, while gelatin cannot, 

 replace the ordinary proteins in the food. When only enough protein 

 is taken to prevent loss of nitrogen from the body, one-fifth of the 

 necessary nitrogen can be supplied in the form of gelatin. When the 

 food is much richer than this in ordinary protein, a correspondingly 

 greater proportion of the protein can be replaced by gelatin. The 

 surplus is not used in the endogenous metabolism of the cells (p. 497), 

 but supplies energy to the body after the elimination of its nitrogen 

 as urea, just as the surplus protein would do. Thus gelatin economizes 

 protein in the same way that fat and carbo-hydrates do, but also 

 to some extent in a different way by supplying ' building stones ' 

 for the protoplasm. Gelatin contains most of the amino-acids and 

 other groups which compose the body -proteins, but tyrosin, trypto- 

 phane, and cystin are lacking in the gelatin molecule. It is there- 

 fore an interesting question whether gelatin can fully replace protein 

 when these substances are given in addition. Kaurfmann has 

 shown that his own nitrogen requirement (15*2 grammes) was almost 

 completely covered by a mixture containing 93 per cent, of the 

 nitrogen in the form of gelatin, 4 per cent, as tyrosin, 2 per cent, 

 as cystin, and i per cent, as tryptophane, in addition to the same 

 amounts of carbo-hydrate and fatty food as in the comparison diet, 



