CARBOHYDRATES AND FATS. 919 



be converted to sugar and glycogen, hence probably also to fat. 

 Protein constitutes relatively only a small fraction of the daily 

 diet, and the modern point of view is that bodv fat is formed in 

 the first instance from food fat and food carbohydrates 



Origin of Body Fat from Food Fat.— The first proofs that 

 the food fats may be deposited as such in the fat tissues of the 

 body were obtained by feeding foreign fats to dogs and demon- 

 strating that these fats can afterward be recognized in the 

 tissues of the animals.* Linseed oil, rape-seed oil, and mutton-fat 

 were used in these experiments. Secondly, it has been made 

 probable by feeding experiments that the normal fat of the food 

 undergoes a similar fate. Thus, Hofmann used a dog weighing 

 26 kgms. and allowed it to starve until its weight was reduced to 

 16 kgms. It w^as then fed for five da3's on a little meat and large 

 quantities of fat. At the end of that time it was killed and analyzed. 

 The body contained 1353 gms. of fat, of which only 131 gms. could 

 have come from the protein used, assuming that this material 

 can serve as a fat former. Much of the fat found, therefore, was 

 probably derived from the fat of the food. 



Origin of Body Fat from Carbohydrates. — That the body 

 fat may have this origin has been made probable or certain by 

 feeding experiments. Thus, Rubner fed a dog (5.89 kgms.) for 

 two days on a diet of sugar, starch, and fat whose total carbon 

 content was equal to 176.6 gms. During this period the animal 

 excreted 87.1 gms. of carbon. There were retained in the body, 

 therefore, 89.5 gms. carbon. The fat fed, 4.7 gms., contained 

 (4.7 X 0.77) 3.6 gms. C. The total nitrogen excreted during this 

 period was 2.55 gms., which indicated a metabolism, therefore, of 

 16 gms. (2.55 X 6.25) of body protein. Making the improbable 

 assumption that all of the carbon of this protein was retained in 

 the body, this would account for 8.32 gms. C (16 X 0.52); so that 

 3.6 + 8.32 or 12 gms. C might have originated from sources other 

 than the carbohydrate of the food, leaving, therefore, 89.5 — 12 

 or 77.5 gms. of C, which could have arisen only from the carbohy- 

 drate. This quantity of carbon could have been retained only as 

 glycogen or fat. Allowing for the greatest possible storage of 

 glycogen. 78 gms. or 34.6 gms. C, there would still remain 42.9 gms. 

 of C, which could have been retained only as fat. Numerous other 

 fattening experiments of different kinds have been made in which 

 it has been shown that the fat laid on by the animal could not be 

 accounted for by the fat of the food, nor by assuming with Voit that 

 it originated from the protein. The combined testimony of these 



♦Lebedeff, "Centralblatt f. die med. Wiss.," 1881, and Munk, "Vir- 

 chow's Archiv," 95, 407, 1884. 



