A DIET OF FAT OR OF CARBOHYDRATES. 363 



maintain their metabolism in equilibrium by eating a large amount of flesh, can 

 do so with less flesh when gelatin is added to their food. A diet of gelatin alone, 

 which produces much urea, is not sufficient for this purpose, and animals soon lose 

 their appetite for this kind of food. 



[Gelatin. Voit has shown that gelatin readily undergoes metabolism in the hody and forms 

 urea, and if a small quantity be taken, it is completely and rapidly metabolised. When ad- 

 ministered it acts just like fats and carbohydrates as an "albumin-sparing " substance. It seems 

 that gelatin is not available directly for the growth and repair of tissues.] Owing to the great 

 solubility of gelatin, its value as a food used to be greatly discussed. The addition of gelatin 

 in the form of calf's-foot jelly is recommended to invalids. [When a large amount of gelatin is 

 given as food, owing to the large and rapid excretion of urea, the latter excites diuresis.] When 

 chondrin is given along with flesh for a time, grape-sugar is found in the urine. 



[The Metabolism of Peptones. Most of the proteids absorbed into the blood 

 are previously converted into peptones by the digestive juices. It has been asserted, 

 more especially by Briicke, that some albumin is absorbed unchanged ( 192, 4), 

 and that only this is capable of forming organic albumin, while the peptones, after 

 undergoing a reconversion into albumin, undergo decomposition as such. This view 

 is opposed by many observers, who maintain that peptones perform all the func- 

 tions of proteids, so that peptones, with the other necessary constituents of an 

 adequate diet, suffice to maintain a proper standard of health.] 



239. A DIET OF FAT OR OF CARBOHYDRATES. If fat alone be given 

 as a food, the animal lives but a short time. The animal so fed excretes even less 

 urea than when it is starving ; so that the consumption of fat limits the decom- 

 position of the animal's own proteids. As fat is an easily oxidised body, it yields 

 heat chiefly, and becomes sooner oxidised than the nitrogenous proteids which are 

 oxidised with more difficulty. If the amount of fat taken be very large, all the C 

 of the fat does not reappear, e.g., in the C0 2 of the expired air ; so that the body 

 must acquire fat, whilst at the same time it decomposes proteids. The animal thus 

 becomes poorer in proteids and richer in fats at the same time. 



[The metabolism of fats is not dependent on the amount of fats taken with the 

 food. 1. It is largely influenced by work, i.e., by the activity of the tissues, and 

 in fact with muscular work C0 2 is excreted in greatly increased amount ( 127, 6). 

 2. By the temperature of the surroundings, as more C0. 2 is produced in the cold 

 ( 214, 2), and far more fatty foods are required in high latitudes. In their action 

 on the organism, proteids and fats so far oppose each other, as the former increase 

 the waste, and therefore oxidation, while the latter diminish it, probably by affect- 

 ing the metabolic activity of the cells themselves (Bauer). As a matter of fact, fat 

 animals or persons bear starvation better than spare individuals. In the latter, the 

 small store of fat is soon used up, and then the albumin is rapidly decomposed. 

 For the same reason corpulent persons are very apt to become still more so, even 

 on a very moderate diet.] 



When carbohydrates alone are given, they must first be converted by digestion 

 into sugar. The result of such feeding coincides pretty nearly with feeding with 

 fat alone. But the sugar is more easily burned or oxidised within the body than 

 the fat, and 17 parts of carbohydrate are equal to 10 parts of fat. Thus the diet 

 of carbohydrates limits the excretion of urea more readily than a purely fat diet. 

 The animals lose flesh, and appear even to use up pa'rt of their own fat. 



[The metabolism of carbohydrates also serves to diminish the proteid meta- 

 bolism, as they are rapidly burned up and thus " spare " the circulating albumin. 

 But Pettenkofer and Voit assert that they are rapidly destroyed in the body, even 

 when given in large amount, so that they differ from fats in this respect. They 

 are more easily oxidised than fats, so that they are always consumed first in a diet 

 of carbohydrates and fat. By being consumed they protect the proteids and fats 

 from consumption.] 



