CHEMISTRY OF FOODS AND FEEDING. 1173 



The greater portion of the food consumed by the animal is 

 used for the last two purposes named, the production of heat 

 and energy. The food, after being digested, is taken into the 

 blood, and carried through the system ; the blood, while passing 

 through the lungs, absorbs oxygen from the air, and this com- 

 bines with part of the food in the blood. This combination is 

 exactly the same in character with that which takes place when 

 a substance is burned, and consequently it is often said that the 

 food is "burned in the blood." 



By this process the heat of the body is maintained, and the 

 energy required to keep up the vital process and enable the 

 animal to move about is supplied. For this purpose all the 

 principal constituents of food albuminoids, carbohydrates, and 

 fat can be used, but are not of equal value, fat being worth 

 about two and one-half times as much as carbohydrates, and 

 more than twice as much as albuminoids. 



Use of Different Food Constituents. Albuminoids are 

 capable of meeting all the requirements of the animul system. 

 They can be changed into muscle, and so provide for repair of 

 waste and for growth; they can also be converted into fat, and 

 can be burned in the system for production of heat and energy. 

 When used for other purposes than the production of muscle, 

 the nitrogen they contain is separated in the form of a com- 

 pound called urea, and removed from the system through the 

 kidneys. 



Carbohydrates and fats are capable of being converted into 

 fat, and of being burned in the system for production of heat 

 and energy, but they can not be used for repair of waste or 

 for growth. 



From what has already been stated, it will be seen that it 

 is impossible to make any absolute standard of valuation for 

 the different constituents of food. The albuminoids possess a 

 special value, because for a certain purpose no other food con- 

 stituent can replace them. If an animal is not supplied with 

 sufficient albuminoids the deficiency can not be made up by in- 

 creasing the supply of carbohydrates and fats. But if a food 

 contains sufficient albuminoids to meet the needs of the system 



