8 



or feeding stuff, the proportions of the nutrients actually digested and 

 oxidized in the bod}^, and the proportion of the whole latent energy of 

 each which becomes active and useful to the body for warmth and 

 work. However, the fuel value may be and often is calculated from 

 the composition of the food material supplied, on the assumption that 

 1 gram of protein furnishes 4.1 calories, 1 gram fat 9.3 calories, and 1 

 gram carbohydrates 4.1 calories, or 1 pound protein 1,860 calories, 1 

 pound fat 4,220 calories, and 1 pound carbohydrates 1,860 calories. 



The relation between the quantities of nitrogenous and nitrogen-free 

 nutrients in the ration is called the nutritive or nutrient ratio. In cal- 

 culating this ratio 1 pound of fat is taken as equivalent to 2.25 pounds 

 of carboh3^drates — this being approximately the ratio of their fuel 

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

 carbohydrates plus 2.25 times the fat. 



All the organs and tissues of the body contain nitrogen. Protein is 

 the only nutrient which supplies this element, and is therefore essen- 

 tial for building and repairing body tissues. The other elements 

 required, namely, carbon, oxygen, and hydrogen, may be supplied 

 theoretically by protein, fat, or carbohydrates; but a well-balanced 

 diet or ration contains all the nutrients 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 energ3^ 



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. 



The body is often likened to a machine, but it differs from one in a 

 number of important ways; for instance, it is itself built up of the 

 same materials which it utilizes as fuel, and further, if an excess of 

 fuel, i. e., food, is supplied, it may be stored as a reserve material for 

 future use, generally in the form of fat or glycogen, a sugar-like body. 



The amount of work performed by a horse, for convenience in meas- 

 urement, may bo resolved into several factors, as follows: (1) The 

 energy expended in chewing, swallowing, and digesting food, keeping 



170 



