608 THE FORMATION OF FAT. [BOOK 11. 



fattening period. It is clear that fat may be formed in the body 

 out of something which is not fat. 



401. There are two possible sources of this manufactured 

 fat. The carbohydrates of the food form one source. In treat- 

 ing of digestion ( 232), we referred to the possibility of car- 

 bohydrates during digestion in the alimentary canal becoming 

 by fermentation converted into butyric acid ; and we suggested 

 that higher and more complex members of the same fatty acid 

 series might be obtained out of carbohydrates by somewhat 

 analogous changes, carried on however not in the alimentary 

 canal by means of foreign organized ferments, but in the tis- 

 sues through the activity of the tissues themselves. We can- 

 not as yet trace out the steps nor can we definitely point to 

 any particular tissues other than the fat-cells themselves as 

 the seats of any such changes ; though it is possible that the 

 fat may be manufactured in this or that tissue and subse- 

 quently transferred, for storage, to the fat-cells. But there 

 can be no doubt that carbohydrate material does in some way 

 or other give rise to fat. A carbohydrate diet is the kind of 

 diet most efficacious in producing an accumulation of fat in the 

 body : sugar or starch, in some form or other, is always a large 

 constituent of ordinary fattening foods. 



Another source of fat is to be found in the proteids. We 

 have seen that the urea of the urine practically represents the 

 whole of the nitrogen which passes through the body. Now 

 in any given quantity of urea the amount of carbon is far less 

 than that found in the quantity of proteid containing the same 

 amount of nitrogen. Thus the percentage composition of the 

 two being respectively, 



Carbon. Hydrogen. Oxygen. Nitrogen. Sulphur. 



Urea 20-00 6-66 26-67 46-67 

 Proteid 53 7-30 23-04 15-53 1-13 



100 grms. of urea contain about as much nitrogen as 300 grms. 

 of proteid; but the 300 grms. of proteid contain 139 grms. 

 (159 20) more carbon than do the 100 grms. urea. Hence 

 the 300 grms. of proteid in passing through the body and giv- 

 ing rise to 100 grms. of urea, would leave behind 139 grms. of 

 carbon, in some combination or other ; and this surplus of car- 

 bon, if the needs of the economy did not demand that it should 

 be immediately converted into carbonic acid and thrown off 

 from the body, might be deposited somewhere in the form of 

 fat. It has been calculated that in this way 100 grms. of pro- 

 teid food might furnish 42 grms. of fat. We have already 

 seen, in treating of the action of the pancreatic juice ( 210), 

 that there is evidence of a fatty element (viz. leucin, w^hich is 

 amido-caproic acid, and so belongs to the fatty acid series) being 

 thrown off from the complex proteid compound in the very pro- 



