43^ PRINCIPLES OF AGRICULTURAL CHEMISTRY 



grams dry meat) protected 45.2 grams body fat from oxidation, 

 so that 250 grams dry meat would protect 100 grams of fat. But 

 the available energy of water-free meat is 404 calories ; and that 

 of dog fat is 940 calories, so that 100 grams fat has the same 

 heat value as 232 grams of meat, and the meat protected body fat 

 approximately in proportion to its available energy. 



Similar experiments with sugar, starch, and other nutrients 

 have shown that the value of different nutrients to an animal 

 that is fed insufficient food are in proportion to their content of 

 available energy. This is known as the law of isodynamic 

 replacement of nutrients. This law holds only when the thermal 

 energy of the food can be entirely utilized in maintaining the 

 temperature of the body. When the thermal energy is of no 

 value, as when the surrounding temperature is the same as that 

 of the body, nutrients should replace one another only in propor- 

 tion to their content of kinetic energy. (See Chapter XX). 

 When the thermal energy is only partially utilized, the law is 

 only partly true. 



Carnivorous animals may be maintained on a ration consisting 

 of flesh alone. The quantity necessary is between three and four 

 times as much as that oxidized by a starving animal. An addi- 

 tion of fat, sugar, starch, or crude fiber decreases the amount of 

 proteids required. 



Ascertaining the Maintenance Ration. The ration which will 

 keep an animal without loss or gain of fat or flesh is termed the 

 maintenance ration. The maintenance ration is ascertained 

 exactly by feeding an animal on a given ration, and determining 

 the loss or gain of flesh and fat by means of the carbon and 

 nitrogen balance. The protein and non-protein in the ration are 

 decreased, or increased, as appears necessary from the previous 

 experiment, and the carbon and nitrogen balance again 

 determined. That ration which produces only a very slight gain 

 of flesh and fat is considered to be the maintenance ration. It is 

 practically impossible to feed a ration which does not produce 

 either a slight gain or loss. 



