CHEMISTRY OF DIGESTION AND N (TUITION. 353 



directly to fat, although the chemistry of the transformation is not as yet 

 understood. Diets, such as the well-known Banting diet, intended to reduce 

 obesity are characterized, on the contrary, by a small proportion of carbo- 

 hydrates and a relative excess of proteid. 



Nutritive Value of Carbohydrates. — The nutritive importance of the 

 carbohydrates is similar in general to that of the fats ; they are oxidized aud 

 furnish energy to the body. In addition, as has been described in the pre- 

 ceding paragraph, they may be converted into fat and stored in the body as 

 a reserve supply of nourishment. As a matter of fact, the carbohydrates form 

 the bulk of ordinary diets. They are easily digested, easily oxidized in the 

 body, and from a financial standpoint they form the cheapest food-stuff. The 

 final products in the physiological oxidation of carbohydrates must be CO, and 

 H 2 0. Inasmuch as the H and O in the molecule already exist in the proper 

 proportions to form H 2 (C 6 H I2 6 , C 12 H 22 O n ), it follows that relatively less oxy- 

 gen will be needed in the combustion of carbohydrates than in the case of proteids 

 or of fats. Whatever may be the actual process of oxidation, we may consider that 

 only as much O is needed as will suffice to oxidize the C of the sugar to CO a . 



CO 



Hence the ratio of O absorbed to CO., eliminated, -=-^, a ratio that is known 



o 2 



as the respiratory quotient, will approach nearer to unity as the quantity of 

 carbohydrates in the diet is increased. From our study of the digestion of 

 carbohydrates (p. 318) we have found that most of the carbohydrates of our 

 food pass into the blood as dextrose (or levulose), and any excess above a cer- 

 tain percentage is converted temporarily to glycogen in the liver, the muscles, 

 etc., to be again changed to dextrose before being used. The sugar undergoes 

 final oxidation in the tissues to CO, and H 2 0. While it is possible that this 

 oxidation may be direct — that is, that the sugar may be burnt directly to CO a and 

 H 2 — it is usually supposed to be preceded by a splitting of the sugar mole- 

 cule, although the steps in the process are not definitely known. 



There has been discovered recently in connection with the pancreas a num- 

 ber of facts that are interesting not only in themselves, but doubly so because 

 they promise, when more fully investigated, to throw some light on the man- 

 ner of consumption of sugar by the tissues. (See also section on Internal 

 Secretions.) It has been shown by yon Mering and Minkowski ' and others 

 that if the pancreas of a dog is completely removed, the tissues lose the power 

 of consuming sugar, so that it accumulates in the blood and finally escapes in 

 the urine, causing what has been called "pancreatic diabetes." If a small 

 part of the pancreas is left in the body, even though it is not connected by its 

 duct with the duodenum, diabetes does not occur. The inference usually made 

 from these experiments is that the pancreas gives off something to the blood — an 

 internal secretion — that is necessary to the physiological consumption of sugar. 

 In what way the pancreas exerts this influence has vet to be discovered ; 

 possibly it is through the action of a specific enzyme that helps to break 

 down the sugar; possibly it is by some other means. Bui the necessity of 



1 Arrliir/iir experimentelle Pathologic uml Pharmakologie, 1893, xxxi. S. 85. 

 Vol. I.— 23 



