798 GELATINE AS FOOD. [BOOK n. 



proteid food may, by the increase of general metabolism, actually 

 reduce the fat of the body. 



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 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 car- 

 nivora, in which animals, living chiefly on proteids and fats, more 

 of it leaves the body combined with hydrogen to form water. 

 This relation of the oxygen to the carbonic acid is often expressed 

 as the quotient of the volume of the carbonic acid expired divided 

 by the volume of the oxygen consumed, the ' respiratory quotient,' 



^ 2 , which is in herbivora about '9 and in carnivora about '6 



or '7. When a 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. 



524. The Effects of Gelatine as Food. It is a matter of com- 

 mon experience that gelatine will not supply the place of proteids 

 as a constituent of food. Animals fed on gelatine 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 gelatine in food 

 is not without effect. Thus nitrogenous equilibrium is established 

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

 dog, moreover, fed on a diet of gelatine 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 gelatine has 

 sheltered from metabolism some proteid constituents of the body ; 

 and the consumption of fat seems also to be lessened by the presence 

 of gelatine. These facts become intelligible if we suppose that 

 gelatine 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 gelatine can take up ; it seems however unable to imitate 

 the other function of proteid matter, and to take part in the 

 formation of living substance ; or in the phraseology of a preced- 

 ing paragraph ( 522), it can take the place of circulating but not 



