FUNCTIONS OF FOOD 113 



animal system to furnish heat and mechanical energy, 

 or it is stored up as reserve matter. Fat has a greater 

 value as a heat and force producer than any other 

 ingredient of food. 



The carbohydrates of the food are chiefly starch, 

 pentosans, sugars, and celluloses; these substances 

 consist of carbon, hydrogen, and oxygen, the last two 

 elements being in the proportion to form water — 

 hence the name. Various other non-nitrogenous con- 

 stituents of food, as pectin, lignin, and vegetable acids, 

 are also generally included under this title, though 

 not, strictly speaking, carbohydrates. Carbohydrates 

 form the largest part of all vegetable foods. They are 

 not permanently stored up in the animal body, but 

 serve, when burnt in the system, for the production 

 of heat and mechanical work. They are also capable, 

 when consumed in excess of immediate requirements, 

 of conversion into fat. 



The carbohydrates and fat are quite incapable of 

 adding to the nitrogenous tissues of the body. They 

 may, however, have this effect indirectly by protect- 

 ing the albuminoids of the food from oxidation. A 

 moderate quantity of albuminoids supplied to a grow- 

 ing animal will thus produce a much larger increase of 

 muscle when accompanied by a liberal supply of carbo- 

 hydrates or fat. In this case the non-nitrogenous 

 ingredients of the food supply the demands for heat 

 and work, and the albuminoids can be devoted to the 

 renovation or increase of tissue. 



If an adult animal receives the small quantity of 



albuminoids and salts necessary to supply the daily 



waste of tissue, we should assume, from what has 



gone before, that the whole of its remaining wants 



8 



