CHAPTER LII 

 THE METABOLISM OF FAT 



The Absorption of Fat. Fat is digested into fatty acids and 

 glycerine, and brought into solution. The preliminary emulsification 

 of the fat facilitates its digestion, not its absorption. The view once 

 put forward, and now abandoned, was that emulsified neutral fat is 

 absorbed in the particulate form without being split into fatty acid 

 and glycerine. The absorption of fats from the intestine depends 

 upon the solubility of free fatty acids and soaps in the bile. The 

 bile salts increase the solubility of soaps in water ; they also prevent 

 its gelatinization, and thereby greatly aid absorption. The lecithin of 

 the bile also plays an important part in the solution of fatty acids and 

 soaps. Fats of high melting-points are not absorbed so well as fats 

 with low melting-points; free fats are better absorbed than those 

 enclosed in cell membranes. 



During absorption the dissolved fatty acids and soaps pass into 

 the intestinal mucous membrane. Here, by the activity of the cells, 

 they are again synthesized with glycerine into particles of neutral 

 fat, and these pass into the lacteals, which fill with a milky white 

 lymph known as chyle. The lacteals derive their name from this 

 milky fluid, and were discovered through it. 



Leucocytes aid the passage of the synthesized neutral fat into the 

 lacteals. They are to be seen in sections of the villi stained with 

 osmic acid, crowded with particles of fat. How the particles are 

 handed on from the columnar cells to the leucocytes is unknown. 

 Not all the fat eaten finds its way into the lacteals. The fate of the 

 remainder is unknown. It probably passes into the blood, forming 

 some linkage with the protoplasm of the plasma and corpuscles. 

 The lacteals, then, act as an overflow, and protect the liver from 

 being flooded with fat. Our methods of analysis do not allow us to* 

 detect any increase of fat in the portal blood, but neither do they permit 

 us to be sure of an increase therein of sugar or amino-acids during 

 digestion. The circulation is so rapid that an immeasurably small 

 increase of any of these substances must suffice to carry them aAvay. 

 About 60 per cent, of the ingested fat is found in the chyle as 

 neutral fat, and a small quantity (4 to 5 per cent.) as soaps. 



Anabolism of Fat. From the lacteals the chyle passes to the recep- 

 taculum chyli, and thence by the thoracic duct into the venous blood. 

 During this passage the neutral fat is again broken down into soluble 

 soaps, passing as such into the blood, so that there is no danger of the 



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