11 



values — so that the nutritive ratio is actually that of the protein to the 

 carbohydrates plus 2.25 times the fat. 



The body is necessarily made up of the same chemical elements as 

 occur in food. Nitrogen is the characteristic element of body tissue and 

 fluids. Carbon, oxygen, and hydrogen are also present, as well as the 

 elements making up the various mineral matters of the body. Protein 

 is the only nutrient which contains nitrogen, therefore this nutrient is 

 essential for building and repairing body tissues. The carbon, oxy- 

 gen, and hydrogen may be supplied theoretically by protein, fat, or 

 carbohydrates; but a well-balanced diet or ration contains all in proper 

 proportion. Protein, fat, and carbohydrates may be burned with the 

 formation of carbon dioxid and water, and therefore all may serve as 

 sources of energy. 



The mineral matter in food is required for a number of different 

 purposes, a considerable amount being needed for the formation of 

 the skeleton. Some is also present in the organs and tissues. It can 

 not, however, be regarded as a source of energy, according to com- 

 monly accepted theories, since it can not be burned with the formation 

 of carbon dioxid and water. The water present in food is not a 

 nutrient in the sense that it serves for building tissue or yielding 

 energy, but it is essential, serving to carry the food in the digestive 

 processes, to dilute the blood, and for many other physiological pur- 

 poses. The oxygen of the air is required by all living animals for the 

 combustion, or oxidation, of the fuel constituents of food. 



When foods are burned in the body, i. e., oxidized, they give up the 

 latent energy present in them. In determining the fuel value of pro- 

 tein, due allowance is made for the fact that combustion is not as com- 

 plete in the body as in a furnace. In the latter, practically all organic 

 materials are burned to carbon dioxid, water, and nitrogen; in the 

 bod}^, to carbon dioxid, water, and some cleavage product containing 

 nitrogen, such as urea, uric acid, hippuric acid, and similar bodies 

 which require further combustion before the free nitrogen is liberated. 

 Combustion in a furnace and combustion in the body do not appear to 

 be at all similar, but, generally speaking, they are the same from a 

 chemical standpoint. The former takes place rapidly with the evolu- 

 tion of heat, and usually of light; the latter, more slowly and incon- 

 spicuously. If food is likened to fuel and the body to a furnace, the 

 respiratory products given off from the lungs correspond to the com- 

 bustion products which pass out through the flue. Ashes, in so far as 

 they consist of material which will not burn (sand, bits of rock, etc.), 

 and bits of coal which do not burn because they fall through the grate, 

 or for some similar reason escape combustion, represent the feces (the 

 indigestible and accidentally undigested material derived from the food). 

 The bits of coal found in the ashes which are partially burned, but 

 still contain some material valuable as fuel, correspond to the urea 



