eOMPABATIVE NUTRITIVE VALUE OF FOODS 157 



somewhat different quantities of matter will be 

 digested. 



(2) Capacity for Producing Heat, Work, and In- 

 crease. — The quantity of digestible constituents which 

 a food contains does not sufficiently indicate its nutri- 

 tive worth, owing to the unequal value of its various 

 constituents, the unequal losses which take place 

 during the processes of digestion and utilisation, and 

 the unequal labour which the process of digestion 

 requires in various cases. Thanks chiefly to the 

 laborious researches of Kellner and Zuntz, already 

 noticed, we are now able to estimate more or less 

 exactly what is the final value to the animal of a 

 digested food. 



The most accurate method of ascertaining the value 

 of any food is to experiment with it ; but as the 

 necessary investigations have as yet been made in 

 only a few instances, we must proceed at present on 

 the general plan of valuing food by summing together 

 the values of its constituents. The only common 

 function of food constituents which can be used for 

 this purpose is their capacity for producing heat in 

 the body. We have already seen (pp. 120-125) that the 

 initial value of food ; the losses which it suffers during 

 digestion ; the labour expended in this operation ; and 

 the final value of food to the animal, can all be 

 expressed in terms of heat. We must, however, bear 

 in mind that this method of determining the value of 

 food to an animal fails in one point. The amount of 

 heat which a food is capable of producing does not 

 necessarily express its power of increasing or renew- 

 ing the nitrogenous tissues of the body, this depends 

 solely on the amount of the albuminoid constituents 



