106 



MAINTENANCE RATIONS OF FARM ANIMALS. 



As was noted above, the ash supply was but partially considered in Zisterer's 

 experiments, no mention being made of the addition of ash ingredients with the 

 exception of sodium, potassium, and calcium. It seems not impossible that the 

 phosphorus compounds of the muscle protein may have had something to do 

 with its apparently greater availability. 



THOMAS'S EXPERIMENTS. 



Thomas 1 has attempted to determine the relative values of the mixed proteins 

 of different foods by a method differing somewhat from that employed in the 

 two foregoing investigations. As has been shown in previous pages, on an 

 abundant nonnitrogenous ration, especially of carbohydrates, the protein 

 katabolism of the body may be reduced to a very low limit which represents 

 more or less exactly the minimum amount of protein necessarily broken down 

 in the vital activities. If a small amount of protein be added to such a non- 

 nitrogenous ration, it will tend to be used to replace body protein, since the 

 surplus of nonnitrogenous material tends to prevent its being katabolized to 

 furnish energy. The extent, then, to which any given protein under these con- 

 ditions diminishes the loss of protein from the body may be taken as the 

 measure of its maintenance value. The principle of the method may be illus- 

 trated by the following supposititious case. 



Protein digested 



Protein katabolized 



Loss of protein from body. 



On protein- 

 free food. 



On protein 

 food. 



In this case, four parts of food protein obviously replace three parts of body 

 protein and the percentage availability of the former is therefore 75. The 

 principle of the method is similar to that of the determination of the percentage 

 availability of energy (p. 27). 



It is to be remarked concerning this method, first, that it assumes that the 

 percentage availability of the food protein is the same for all amounts below 

 the maintenance requirement; in other words, that it is a linear function. 

 This is an unproved assumption, and in view of the readiness with which 

 protein or its cleavage products in the body seem to be deamidized and utilized 

 as fuel, the assumption seems of questionable validity. 



Second, in applying the method it is necessary to know accurately the mini- 

 mum amount of protein katabolized on a nitrogen-free diet, since any error in 

 the determination of its quantity seriously affects the final result. The protein 

 katabolism, however, under these conditions, is not a constant quantity, as 

 has already been pointed out, but varies more or less, especially with the state 

 of protein nutrition of the cells. Accordingly, it must be determined as accu- 

 rately as possible for the subject at the time of the experiment, preferably 

 immediately before and immediately after. 



Third, the amount of protein fed must be less than that katabolized on the 

 nitrogen-free diet. If an excess of protein be consumed, the additional amount 

 will tend to be katabolized and used as fuel, thus rendering the comparison 

 between the two periods illusory, since it is obvious that any such oxidation 

 of protein would tend to make its availability appear too low. Thomas's 

 experiments were made upon himself and included four series, two in May to 



iArchiv fiir (Anatomie und) Physiologie, 1909, p. 219. 



