THE BALANCE OF NUTRITION IQQ 



kind of material gained or lost is necessary. In the study 

 of growth, for example, it is important to know how much of 

 the increase in weight is due to a storage of protein, ash, etc., 

 i.e., to real growth, and how much to a mere storage of fat or 

 water, or both. 



For all these reasons the increase or decrease in live weight, 

 while not valueless, is by itself an entirely inadequate measure 

 of nutritive effect in investigations into the principles of nu- 

 trition. In such experiments it is essential to determine at 

 least the gain or loss of the great groups of substances of which 

 the body is composed, viz., water, ash, protein, fat and if pos- 

 sible carbohydrates, by one of the two general methods already 

 mentioned as available for this purpose, viz., the comparative 

 slaughter test or the balance experiment. 



284. The comparative slaughter test. This method seeks 

 to determine by analysis the actual weights of water, protein, 

 fat, etc., or the quantities of chemical energy, contained in the 

 body of the experimental animal at the beginning and at the 

 end of the experiment. Since, however, it is obviously im- 

 possible to analyze the same animal twice, its original stock of 

 protein, etc., is ascertained by the use of a check animal as 

 exactly like the other in age, weight, condition, conformation, 

 etc., as it is possible to select, which is slaughtered and analyzed 

 at the beginning of the experiment. Assuming initial identity of 

 percentage composition for the two animals, the results of this 

 analysis are used to compute the weights of the several ingredi- 

 ents contained in the body of the experimental animal at the 

 outset of the experiment, while the animal itself is analyzed at 

 its close. 



The method of comparative slaughter tests has the advantage of 

 being a direct determination of the amounts of each ingredient gained 

 and of requiring comparatively simple appliances. Furthermore, it 

 may be applied not only to the conventional groups of protein, fat, 

 etc., but to any substance capable of accurate analytical determina- 

 tion. Finally, in addition to the total amount of any substance, its 

 distribution between different parts of the body may be ascertained. 

 On the other hand, the method has certain drawbacks. 



In the first place, it requires relatively long experimental periods. 

 Assuming the work of weighing, sampling and analysis to be correctly 

 performed, the accuracy of the results evidently depends upon the 



