2 I. LIPID DIGESTION, ABSORPTION, TRANSPORT, AND STORAGE 



the sterols will constitute a separate chapter, as will also the discussion of 

 the metabolism of the fat-soluble vitamins. 



2. The Digestion and Absorption of Triglycerides, Phospholipids, 

 and Fatty Acids in the Gastrointestinal Tract 



There are two main theories to explain the mechanism by which fats are 

 absorbed. According to one of these, the so-called Lipolytic Theory, 

 which has been supported by Pfliiger, Bloor, and most recently by Verzar, 

 triglyceride fats are completely hydrolyzed in the intestine, by pancreatic 

 lipase, to fatty acids and to glycerol. The bile acids are believed to aid 

 in the absorption of the fatty acids by forming a soluble coordination com- 

 pound, which is called a choleic acid. As soon as this is transported into 

 the intestinal mucosa, the complex is broken down, and the bile acids are 

 excreted into the lumen of the intestine to aid in the absorption of addi- 

 tional fatty acid molecules. The fatty acids thus set free in the intestinal 

 mucosa combine directly with glycerol to yield triglycerides. It has been 

 suggested that this synthesis involves the intermediate formation of the 

 phospholipids, particularly that of lecithin. However, current evidence 

 indicates that the rate of lecithin turnover in the intestinal mucosa is not 

 sufficiently great to account for more than a fraction of the total fat passing 

 through this intermediate stage. It is obvious that, with a total hydroly- 

 sis of the triglyceride molecules in the gut, followed by their resynthesis in 

 the intestinal mucosa, a complete realignment in the orientation of the 

 fatty acids in the triglyceride molecules may be expected, with the result 

 that the newly synthesized fat may have properties which vary consider- 

 ably from those of the ingested fat. 



The second theory which has been proposed to explain the mechanism of 

 fat absorption is known as the Particulate Theory. This hypothesis has 

 been largely developed by Frazer and his colleagues. According to this 

 concept, only a minimum hydrolysis of fat occurs in the gastrointestinal 

 tract. The small amount of fatty acid and monoglycerides so formed, 

 together with bile salts, stabilize the emulsion of the remaining triglyceride 

 fat most effectively. It is believed that these fat droplets, which measure 

 less than 0.5 fi, are capable of passing directly into the cell. The small 

 proportion of free fatty acid, monoglycerides, and diglycerides, are built 

 up into triglycerides in the intestinal mucosa. Frazer is now of the opinion 

 that the fat can be worked over in the intestinal mucosa to bring about a 

 reorientation of the fatty acids in the glycerides. Moreover, Frazer 



