292 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



The total nitrogen excreted = 2.55 grams. The carbon contained in the pro- 

 teid thus broken down plus that in the 4.7 grams of fat = 13 grams. If we 

 make the assumption, that all of the C from these two sources was retained 

 within the body, there would still be a balance of 76.5 grams C (89.5 13.0) 

 which must have been stored in the body either as glycogen or as fat. The 

 greatest possible storage of glycogen was estimated at 78 grams = 34.6 grams 

 C, so that 76.5 34.6 = 41.9 grams C as the minimal amount which must 

 have been retained as fat and must have arisen from the carbohydrates of the 

 food. Similar experiments have been made upon herbivorous animals, and 

 as the result of investigations of this character we are compelled to admit that 

 the carbohydrates form one source, and possibly the main source, from which 

 the body-fats are derived. This belief accords with the well-known fact that 

 in fattening stock the best diet is one containing a large amount of carbo- 

 hydrate together with a certain quantity of proteid. On the view that fats 

 were formed only from proteids, the efficacy of the carbohydrates in such g. diet 

 was supposed to lie in the fact that they protected a part of the proteid from 

 oxidation, and thus permitted the formation of fat from proteid ; but it is now 

 believed that the carbohydrates of a fattening diet are, in part, converted 

 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 and 

 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 2 and 

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

 proportions to form H 2 O (C 6 H 12 O 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 2 . 



CO 



Hence the ratio of O absorbed to CO 2 eliminated, -^, a ratio which 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. 257) 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 2 and H 2 O. While it is possible that this 

 oxidation may be direct that is, that the sugar may be burnt directly to CO 2 and 



