340 A TEXTBOOK OF PHYSIOLOGY 



in the body, and this store is not again lost until the amount of protein 

 in the diet is reduced. 



When there is a large amount of protein in a mixed diet, the fat 

 katabolism in the body is reduced, so that some of the fats, or carbo- 

 hydrates, in the diet are not used, and may be stored up as fat. 

 The proteins are not all of equal value, for some do not contain 

 essential " building stones." Thus, gelatin does not suffice as a tissue- 

 lormer, but can act as a source of energy. When fed with an ade- 

 quate protein diet, g?latin acts as a protein-sparer. Fed during 

 starvation, gslatin spares protein to the extent of about 35 per cent. 

 Generally speaking, about one-fifth of the true protein nitrogen can 

 be replaced by gelatin nitrogen. Gelatin also exerts a sparing action 

 on fat katabolism. Gelatin, therefore, although insufficient for body- 

 building, from the point of view of energy is quite a useful article of 

 diet. Gelatin can be made a much more sufficient tissue-builder by 

 feeding with it those important amino-acids, tyrosin and tryptophane. 

 which are lacking in its constitution. The same holds good for zein, 

 a protein obtained from maize. The ammo-acid content of a diet 

 is important in regard to growth (see Fig. 189, A). 



The Effect of Fat. Fat, when added to the diet, exerts a sparing 

 effect upon the katabolism of protein. Thus, when a dog was fed 

 with 1,500 grammes of protein, its nitrogen excretion was equivalent 

 to the katabolism of 1,512 grammes. On the addition of 150 grammes 

 of fat, the nitrogen excretion equalled 1,474 grammes of protein. 

 Carbohydrates exert a like or even greater sparing effect on proteins. 

 The addition of fats or carbohydrates to a diet of protein, insufficient 

 by itself, will enable an animal to attain nitrogenous equilibrium and 

 even to store protein in the body. But the protein-supply cannot be 

 taken below a certain minimum, a minimum which seems to vary 

 with different foodstuffs. Thus 30 grms. of protein suffice on a diet 

 of potatoes, and 80 grms. on a diet of bread. On a diet largely consist- 

 ing of potatoes the body can be run on a very low plane both of energy 

 and protein value. For times of scarcity, then, the potato is invaluable. 

 The most virile races of the world occupy cool climates and eat food- 

 stuffs yielding high energy, protein, and fat values e.g., meat and 

 cereals. Few people requiring a diet of 3,500 colories can digest with 

 comfort more than 500 grms. of carbohydrates, while the use of more 

 than 120 grms. of protein is wasteful. About 1,000 calories have then 

 to be made up from fat, and this means eating a little over 100 grms. 

 Fat is easily digested, and does not cause the rise in heat production 

 which protein, and to a lesser degree sugar, does. 



