NUTRITION 365 



Proteins, unlike carbohydrates and fats, are not completely 

 oxidised in the body, inasmuch as the nitrogen they contain re- 

 appears in the excreta in the form of urea. The complete oxida- 

 tion of 1 gramme of urea yields 2,523 calories (or 2*5 kilo-calories),* 

 which must be subtracted from the value given above for the 

 potential energy of proteins in order to ascertain the energy- 

 value of proteins actually available by the body. Speaking 

 approximately, 1 gramme of protein gives rise to J gramme of 

 urea ; hence the heat of combustion of proteins must be dimin- 

 ished by one-third of 2,523 = 841 calories, before we apply the 

 data to the body. This gives us a heat-value for average proteins 

 of 4,929 calories, or 4-9 kilo-calories, as based on purely physical 

 determinations. This, according to Riibner, is too high. By 

 modern methods of calorimetry he has shown that the actual 

 heat energy obtained from protein in the animal body is about 

 41 kilo-calories. 



It is by the oxidation of food-stuffs in the body that heat is 

 generated, and the table above shows how largely this heat is 

 derived from the carbohydrates and fat. The heat value of 

 food-stuffs is easily calculated on the basis of the figures above 

 given, and the modern method of considering dietary sufficiency 

 is to compare the heat energy in the food with the energy needs 

 of the living body. The law of the conservation of energy 

 applies to the living body. 



There are chemical changes in the body, like hydrolysis, which 

 give rise to but little heat — in fact, heat may be lost in connec- 

 tion with digestion processes — for instance, the warming of food, 

 and especially that of the water consumed. That this loss of 

 heat is considerable is rendered evident to the senses of those 

 engaged in horse management. Nothing . is more common, 

 especially in winter, than for the horse to shiver some time after 

 watering. The temperature of the fluid has to be raised some 

 6o° F., and 4 or 5 gallons of cold water consumed in twenty-four 

 hours robs the body of a great deal of heat. 



But there are sources of loss which are not so evident, and the 

 respiration calorimeter shows that the actual value of food for 

 herbivora is less than its theoretical fuel value. The explana- 

 tion lies in the fact that so much of the latter is consumed in 

 those processes necessary to the preparation of vegetable food 

 for the use of the body — for instance, mastication, rumination, 

 fermentation, putrefaction, peristalsis, etc. Coarse fibrous 

 feeding substances, both hay and grains, are a further source of 

 loss, owing to the extensive fermentation they have to undergo 

 in order to be of any use. In fact, no comparison can safely be 

 made between the energy value of the diet of herbivora and that 

 * For definition see p. 358. 



