490 NUTRITION. 



that fat is formed in the body out of something which is not fat. In 421 

 we have already discussed this formation of fat out of carbohydrates. 



As one might imagine, the presence of fat or carbohydrates in the food 

 is found to decrease the amount of proteid material necessary to establish 

 nitrogenous equilibrium. For instance, with a diet of 800 grms. meat and 

 150 grms. 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 fat 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 general metabolism, actu- 

 ally reduce the fat of the body. 



We have at present no exact information concerning the nutritive differ- 

 ences between fats and carbohydrates, beyond the fact that in the final com- 

 bustion of the two, while carbohydrates require sufficient oxygen to combine 

 with their carbon only, there being already sufficient oxygen in the carbo- 

 hydrate itself to form water with the hydrogen present, fats require in addi- 

 tion oxygen to combine with some of their hydrogen. Hence in herbivora,. 

 living largely on carbohydrates, a larger portion of the oxygen consumed 

 reappears in the carbonic acid of the egesta than in carnivora, in which, 

 animals, living chiefly on proteids and fats, more of it leaves the body com- 

 bined with hydrogen to form water. This relation of the oxygen to the car- 

 bonic acid is* often expressed as the quotient of the volume of the carbonic 

 acid expired divided by the volume of the oxygen consumed, the " respira- 



CO 

 tory quotient," , which is in herbivora about 0.9 and in carnivora about 



O 2 



0.6 or 0.7. When an herbivorous animal starves, it feeds on its own fat, and 

 under these circumstances the respiratory quotient falls to the carnivorous 

 standard ; and indeed many circumstances affect this respiratory quotient. 

 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 

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

 it has been thought, however, that cane-sugar is rather more fattening than 

 starch. 



438. The effects of gelatin as food. It is a matter of common experi- 

 ence that gelatin will not supply the place of proteids as a constituent of 

 food. Animals fed on gelatin together with fat or carbohydrates die very 

 much in the same way as when they are fed on non-nitrogenous material 

 alone. Nevertheless it would appear, as might be expected, that the presence 

 of gelatin in food is not without effect. This nitrogenous equilibrium is 

 established at a lower level of real proteid food when gelatin is added. In 

 a dog, moreover, fed on a diet of gelatin and fat, the excess of nitrogen in 

 the excreta over that in the ingesta is less than when the same dog is fed on 

 a diet of fat alone ; that is to say, the gelatin has sheltered from metabolism 

 some proteid constituents of the body ; and the consumption of fat seems 

 also to be lessened by the presence of gelatin. These facts become intelli- 

 gible if we suppose that gelatin is rapidly split up into a urea and a fat 

 moiety in the same way that we have seen a certain quantity of proteid 

 material to be. It is this direct destructive metabolism of proteid matter 

 which gelatin can take up ; it seems, however, unable to imitate the other 

 function of proteid matter, and to take part in the formation of living sub- 

 stance ; or in the phraseology of a preceding paragraph ( 436), it can take 



