FAT METABOLISM 701 



It must not be imagined, however, that no change takes place in the 

 fat during its absorption and before it becomes deposited in the tissues. 

 On the contrary, the stamp of individuality is put upon the fat, for, as 

 we have already seen, its iodine value may become altered and its melt- 

 ing point changed during the process of absorption. In other words, 

 although the absorbed fat does not become entirely adapted to conform 

 with the ordinary qualities of the depot fat, yet it tends to change in 

 this direction. 



That some of the depot fat comes from carbohydrate is well known to 

 stock raisers. When, for example, an animal is fed on large quantities 

 of carbohydrate and kept without doing muscular exercise, its tissues 

 become loaded with fat. If we desire strict scientific proof for this, we 

 do not need to go further than the old experiments of Lawes and Gil- 

 bert, who, it will be remembered, showed that the fat deposited in the 

 tissues of a growing pig is greatty in excess of the fat that could have 

 been derived from the fat or protein which was meanwhile metabolized. 

 The experiment was performed on two young pigs from the same litter 

 and of approximately equal weight ; one was killed and the exact amounts 

 of fat and nitrogen in the body determined; the other was fed for several 

 months on a diet the fat and protein contents of which were accurately 

 ascertained. When after four months this pig was killed and the fat 

 determined, it was found that much more had become deposited than 

 could be accounted for by the fat and protein of the food, even suppos- 

 ing that all the available carbon of the protein had become converted 

 into fat. The only conclusion is that the carbohydrate must have been 

 an important source of the extra fat. 



The Destination of the Depot Fat. The depot fat becomes mobilized 

 and transported by the blood to the active tissues whenever the energy 

 requirements of the body demand it. During starvation, as we nave 

 seen, the depot fat is used to supply 90 per cent of the energy on which 

 the animal maintains its existence. Before the fat is transported, it is 

 probably broken down into fatty acid and glycerine, as which it passes 

 through the cell walls to be again reconstructed into neutral fat in the 

 blood. What agency effects this constant breakdown and resynthesis 

 of fat it is difficult to say. Two ester-splitting enzymes are present in 

 blood, one acting mainly on simple esters, the other on glycerides; but 

 it has been impossible to demonstrate any evident relationship between 

 either of them and the extent of fat mobilization. 



The Fat in the Liver. The physiology of the liver fat has been very 

 diligently studied, particularly by Leathes and his pupils. 50 The out- 

 come of this work has been to show that .the liver occupies an extremely 

 important position in the metabolism of fat, being, as it were, the half- 



