810 THE FORMATION OF FAT. [BOOK n. 



fat. The carbohydrates of the food form one source. In treating 

 of digestion ( 282), we referred to the possibility of carbohydrates 

 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 organised ferments, but in the tissues through the activity 

 of the tissues themselves. We cannot 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. 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 2OOO 6'66 26'67 46'67 



Proteid 53 7'30 23'04 15-53 113 



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 giving rise to 100 grms. 

 of urea, would leave behind 139 grms. of carbon, in some com- 

 bination or other; and this surplus of carbon, 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 proteid food might furnish 42 grms. 

 of fat. 



Some observers have pushed this view of the production of fat 

 out of proteids so far as to insist that all the fat formed in the 

 body arises in this way out of proteid material, and that when 

 carbohydrate food gives rise to the formation of fat it does so by 

 shielding from oxidation the carbon moiety of the proteid food 

 taken at the same time and thus permitting it to be stored up as 

 fat. The carbohydrate itself, they argue, never becomes fat but 

 its presence allows fat to be formed out of proteid material. This 

 view has obviously a very important economical bearing, since, if it 

 be true, it is useless to increase the carbohydrate material of food 



