108 METABOLISM AND NUTRITION 



no increase of combustion, for it is* evident that the contractions of the gastric 

 and intestinal walls as well as the secretion of the glands represent dissim- 

 ilative processes, taking place with the liberation of kinetic energy. 



The negative effect of fat or of carbohydrates on the total metabolism may 

 be explained in one of two ways : either the work of digesting them is too 

 small to produce a distinct rise, or the combustion in other parts of the body 

 than the digestive organs is correspondingly reduced. 



But this would apply only in case the bodily movements were suppressed 

 as much as possible. We know from the subjective feeling of improved capac- 

 ity for muscular work after eating and from an increased tonus of the muscles 

 resulting from the mere ingestion of food, that the amount' of metabolism 

 may well be increased by the foods named, if voluntary movements continue. 

 It is evident, however, that such an increase would be wholly independent of 

 the kind of food, and would be only indirectly connected with the act of 

 ingestion. 



If the food be of such a quality or quantity as to make unusual demands 

 upon the organs, the work of digestion may cause a considerable increase in the 

 metabolism. Thus in one of Kubner's experiments in which 20-30 g. bones 

 were fed, the metabolism rose ten per cent; and in an experiment of Magnus- 

 Levy where 900-1,000 g. of bones were fed the absorption of oxygen increased 

 twenty-four to thirty-three per cent during the first six hours. Likewise after 

 administration of saline purgatives, Mering and Zuntz observed a distinct rise 

 in the metabolism due to increased muscular activity of the intestine. 



F. METABOLISM AFTER INGESTION OF ALBUMOSES, FATTY ACIDS, 

 GELATIN, ALCOHOL, ETC. 



Now that we have become acquainted with the metabolism after ingestion 

 of each of the three principal kinds of organic foodstuffs, we shall consider 

 briefly the food value of some substances closely related to them, which either 

 are formed in the course of digestion, or occur more or less commonly in our 

 ordinary articles of diet. 



1. The digestion of proteid passes through a number of different stages 

 (Chapter VII). It will be of interest here to inquire whether the substances 

 representing these different stages are themselves all of equal value for the 

 nourishment of the body. 



If, with a constant quantity of fat and carbohydrate, an animal be given 

 meat on one day, and on another the so-called protoalbumose formed in proteid 

 digestion so that in both cases he receives the same quantity of nitrogen, the 

 N-excretion and the N-retention exhibit no differences whatever in the two. 

 This albumose, therefore, possesses the same food value as proteid (Blum). 

 Heteroalbumose and peptone behave quite differently. They have the power to 

 replace proteid to a certain extent, but it appears that they cannot maintain 

 the body in N-equilibrium. The reason for this doubtless is that certain carbon 

 nuclei of the proteid molecule which occur in protoalbumose are wanting in 

 heteroalbumose and peptone. Investigations on the constitution of the different 

 products of proteid digestion have shown in fact that the groups which yield 

 tyrosin and indol do not occur in heteroalbumose. Since, apparently, the aro- 

 matic groups are not built up in the body, we can readily understand why hetero- 

 albumose alone does not have the full food value of proteid. However, we are 

 not justified in concluding from this that a part of the proteid ingested becomes 



