1 7 HUMAN PHYSIOLOGY 



Gelatin also shields proteids, and even more so than 

 carbohydrates. 



If, in the above assumed case where nutritive equilibrium 

 is maintained with a mixed diet, the amount of fat, carbo- 

 hydrate, or gelatin is increased while the amount of proteid 

 remains constant, the proteid saving is slight. In such 

 cases, the gelatin may shield 30^ of the proteids, the carbo- 

 hydrates i 5$, and the fats still less. But the shielding of 

 proteids is much more effective when, simultaneously with 

 an increase in the other foodstuffs, the amount of proteids is 

 decreased. When the proteid of the food is replaced by fat, 

 carbohydrates, or gelatin, a much smaller quantity of proteid 

 is able to keep the body proteid constant than if the diet 

 consists chiefly of proteids. The minimum amount of pro- 

 teid in a mixed diet, i.e. the absolutely necessary proteid 

 (see page 117), is for a man about 70 g daily; but it has 

 been observed that for a short time he can maintain his 

 body proteid with 40 g of food proteids. But with such 

 small quantities of proteids more fat and carbohydrates must 

 be supplied than would strictly follow from the law of 

 isodynamics. The foodstuffs which can shield the proteids 

 differ from each other in this capacity; the gelatin shields 

 proteids best, then the carbohydrates, and, least of all, the 

 fats. In regard to fats it must be added that the eating of 

 very great quantities may increase the consumption of pro- 

 teids. 



If, in a sufficient, mixed diet, the proteids are increased, 

 the following results : 



1. The extra proteid is, as in pure proteid diet, in part 

 deposited in the body, and part used up. Hence in a 

 mixed diet also, an increase in the supply of proteid causes 

 an increase in proteid consumption. 



2. But by this increase in proteid consumption the oxida- 

 tion of fats and carbohydrates is somewhat lessened, so that 

 fat may be deposited. 



3. In this case also, fats and carbohydrates spare the 

 proteids to such an extent that a greater part of the proteid 



