THE MAINTENANCE EATION. 



33 



dynamic effect. In experiments in collaboration with Steck he found that 

 a marked increase of the metabolism, as computed from the oxygen consumed, 

 followed the administration of urea, and likewise of sodium chlorid, to 

 men and dogs. In the case of urea he computes that the effect was equal to 

 20 to 25 per cent of that of an equivalent amount of protein. Zuntz also calls 

 attention to earlier experiments by Nering and Schmoll in which carbo- 

 hydrates added to the diet of a diabetic produced a similar increase of metab- 

 olism, although the sugar was not assimilated but excreted unchanged. Zuntz 

 ascribes the results obtained by Rubner to the fact that phlorhizin added largely 

 to the increased excretory work required by the elimination of the nitrogen and 

 of the sugar formed, pointing out also that Rubner has overestimated the 

 amount of heat produced through failure to deduct the energy of the sugar ex- 

 creted in the urine. On the other hand, in Lusk's experiments, just quoted, 

 there was an increased excretion of urea subsequent to the administration of 

 amino acids, but no increase in the carbon dioxide excreted, while Tangl 1 finds 

 that the intravenous injection of urea or sodium chlorid causes an increase in 

 the metabolism even when the kidneys have been extirpated or clamped off. 



On the whole, it can not be said that any fully satisfactory explana- 

 tion has yet been offered of the effects of feed, and in particular of 

 protein, upon the metabolism, although certain factors, especially in 

 domestic animals, are clearly evident. 



But whatever explanation we may accept — whether, following 

 Zuntz, we speak of work of digestion, or, with Rubner, avoid any 

 implication as to the cause by the use of the term specific dynamic 

 effect — the fact that the metabolizable energy of different feeding 

 substances is not equally available for maintenance is established 

 beyond question, and it is this fact which is of immediate importance 

 in considering the energy requirement for maintenance and the main- 

 tenance values of feeding stuffs. 



THE MAINTENANCE RATION. 



In accordance with the principles laid down in the foregoing para- 

 graphs, a maintenance ration as regards energy may be defined as 

 one which supplies available energy equal to the fasting katabolism. 



For example, in Rubner's experiment cited on page 27, in which 

 fat was fed, the fasting katabolism of the dog was 54 calories per 

 kilogram. Fat containing 53.4 calories of metabolizable energy di- 

 minished the loss of body tissue by 46.5 calories. Evidently, then, 

 to reduce the loss by 54 calories, that is, to reduce it to zero, would 

 have required 53.4X^6 — ^2 calories of metabolizable energy to 

 be supplied in fat. The same thing may also be expressed in a 

 slightly different wa}< : If, as there computed, only 87.08 per cent of 

 the metabolizable energy of fat is available, then to make good a total 

 loss of 54 calories will require 54-f-0.8708=62 calories of metaboliza- 



1 Biochemische Zeitschrift, vol. 34, p. 1. 

 8489°— Bull. 143—12 3 



