834 GELATIN AS FOOD. [BOOK n. 



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^' 



CO 



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



2 



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. Indeed many circumstances affect 

 this respiratory quotient. When we examine quantitatively the 

 respiratory interchange after a meal of fat has been taken on a 

 fasting stomach we find that, in striking contrast to the effects 

 of a proteid meal, the increase in the production of carbonic acid 

 and consumption of oxygen is very slight indeed ; the metabolism 

 of the body is not excited by a meal of fat as it is by a proteid 

 meal. Moreover the respiratory quotient is not greatly changed 

 from what it is in the fasting state. When however a meal of 

 carbohydrates is taken both the production of carbonic acid and 

 the consumption of oxygen are increased (the effects however 

 passing off much more rapidly than those of a proteid meal) but 

 especially the former, so that the respiratory quotient is raised and 

 may even be brought to unity. Indeed in some animals at least, 

 under certain circumstances, as for instance in geese which are 

 being rapidly fattened by carbohydrate food, the respiratory quo- 

 tient may exceed unity, may rise as high as 1'34. This means 

 that more carbonic acid is produced than can be accounted for by 

 the oxygen consumed ; that is to say, that some of the carbonic 

 acid is produced by some process which is not one of "oxidation, by 

 being split off from some body, and presumably from the carbo- 

 hydrates. Now the formula for carbohydrate, for instance that for 

 sugar, may be converted into one for fat by throwing off a certain 

 amount of carbonic acid and of water; and it has been suggested 

 that the high respiratory quotient may be taken as a token of the 

 immediate conversion of carbohydrate into fat. Such a high 

 respiratory quotient is, we need hardly say, not necessary for 

 the production of fat out of carbohydrate food ; an animal can be 

 fattened on carbohydrate food without the respiratory quotient 

 exceeding or even reaching unity. 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 Gelatin as Food. It is a matter of com 

 mon experience that gelatin will not supply the place of proteids 



