FAT METABOLISM 731 



become loaded with fat. Strict scientific proof of its production from car- 

 bohydrate is given in the old experiments of Lawes and Gilbert, who, 

 it will be remembered, showed that the fat deposited in the tissues 

 of a growing pig is greatly 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 ' have 

 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 dow r n into fatty acid and glycerol, 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 outcome of this work 

 has been to show that the liver occupies an extremely important posi- 

 tion in the metabolism of fat, being, as it were, the half way house 

 in the preparation of the fatty-acid molecule for consumption in the 

 tissues. Fat is a material containing large quantities of potential en- 

 ergy. While in the depots this potential energy is so locked away as 

 to be unavailable for tissue use. To make it available the depot fat 

 is carried to the liver, where the energy becomes unlocked but not actu- 

 ally liberated. The fat is then transported to the tissues, and the libera- 

 tion of the energy occurs. Neutral fat is like wet gunpowder: it con- 

 tains much potential energy, but not in a suitable condition for explo- 



