774 THE FORMATION OF FAT. [BOOK n. 



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 20'00 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 combina- 

 tion 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. We 

 have already seen, in treating of the action of the pancreatic 

 juice ( 249), that there is evidence of a fatty element (viz. leucin, 

 which is amido-caproic acid, and so belongs to the fatty acid 

 series) being thrown off from the complex proteid compound in 

 the very process of digestion; and though, as we have said, we 

 have no proof that this action of pancreatic juice takes place 

 largely in the normal body, its value as an example is none the 

 less important. 



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 



