454 FATTY AND CARBOHYDRATE FOOD. [Boon 11. 



necessary to establish nitrogenous equilibrium. For instance, with 

 a diet of 800 grins, meat and 150 grins, fat, the nitrogen in the 

 egesta became equal to that in the ingesta in a dog, in whose case 

 1800 grms.. meat had to be given to produce the same result 

 in the absence of fats or carbohydrates. 



On the other hand, it was found, that with a fixed quantity of 

 fatty or carbohydrate food, an increase of the accompanying proteid 

 led not to a storing up of the surplus carbon contained in the extra 

 quantity of proteid, but to an increase in the consumption of 

 carbon. Proteid food increases not only proteid but also non- 

 nitrogenous metabolism. This explains how an excess of proteid 

 food may, by the increase of metabolism, actually reduce the fat of 

 the body. 



There can be no doubt then that both a proteid diet and a 

 carbohydrate diet may give rise to the formation of fat within the 

 body. And the question which we have already (p. 427) partly 

 discussed comes again before us, In what way is this fat so 

 formed ? Is the sugar, arising during digestion from the carbo- 

 hydrate, converted by a series of fermentative changes into fat ? Or 

 is the sugar directly consumed by the tissues in oxidative changes, 

 by which means the fatty derivatives of the metabolized proteids 

 are sheltered from oxidation and stored up as fat ? This is a vexed 

 question which has been hotly debated. Many observers hold 

 strongly to the latter view, and hence contend that all fattening 

 food must contain a supply of proteids adequate to provide, by 

 their decomposition, the carbon of the fat which it is desired to lay 

 up. The balance of evidence, however, seems to be in favour of 

 the view that carbohydrates may be, in some way, directly converted 

 into fat and that therefore fattening foods need not necessarily 

 contain any such definite proportion of proteids. 



We have at present no exact information concerning the 

 nutritive differences between fats and carbohydrates, beyond the 

 fact that in the final combustion of the two, while carbohydrates 

 require sufficient oxygen to combine with their carbon only, there 

 being already sufficient oxygen in the carbohydrate itself to form 

 water with the hydrogen present, fats require in addition oxygen 

 to burn off some of their hydrogen. Hence in herbivora a larger 

 portion of the oxygen consumed reappears in the carbonic acid of 

 the egesta, than in carnivora, where more of it leaves the body as 

 formed water : the proportions of the oxygen in the carbonic acid 

 expired to the oxygen consumed being on an average 90 p. c. in 

 the former and 60 p. c. in the latter. When a herbivorous animal 

 starves, it feeds on its own fat, and under these circumstances the 

 oxygen proportion in the expired carbonic acid falls to the car- 

 nivorous standard. The carbohydrates are notably more digestible 

 than the fats, but on the other hand the fats contain more potential 

 energy in a given weight. As to the dietetic or rather metabolic 

 difference between starch and sugar, we know nothing very definite ; 



