THE HISTORY OF FAT. ADIPOSE TISSUE. 475 



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

 of carbon, in some combination or other ; and this surplus of carbon, if the 

 needs of the economy did not demand that it should be immediately con- 

 verted 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 24 grms. of fat. We have already seen, 

 in treating of the action of the pancreatic juice ( 218), that there is evi- 

 dence 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, 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 use- 

 less to increase the carbohydrate material of food for the purpose of fatten- 

 ing, unless a sufficient proportion of proteid material be given at the same 

 time. 



The view, however, has been proved to be untenable by several investiga- 

 tions carried out on different animals. It has been shown than an animal 

 rapidly fattened on a diet consisting of proteids with much carbohydrate will 

 store up far more fat than can possibly be accounted for by the proteids of 

 the diet. Thus a dog, the fat in whose body had been reduced to a minimum 

 by starvation, was fed for a period on measured quantities of proteids and 

 carbohydrates, and killed. The amount of fat found after death in his body, 

 making full allowance for the fat which remained after the starvation and 

 for the fat accompanying the proteids in the meat given as food, was found 

 to be far more than could be supplied by the carbon in the proteids of the 

 food, even supposing that every jot of those proteids which did not go to 

 make up the increase of the proteid " flesh " of the body taking place during 

 the fattening was used for the purpose of forming fat. Similar experiments 

 on geese and pigs have led to similar results ; and if fat be formed in this 

 way in the bodies of carnivora and omnivora, we may be sure that the same 

 holds good for the bodies of herbivora. We may therefore conclude that fat 

 can be constructed in the body on the one hand out of proteid material and 

 on the other hand by some direct conversion of carbohydrates. 



