CHAPTER IV 



THE DIGESTION, ABSORPTION, AND 



DIGESTIBILITY OF LIPIDS OTHER 



THAN FATS 



1. Introduction 



Although, from a quantitative standpoint, the neutral fats occupy by 

 far the most important position of any of the lipids, as regards digestion 

 and absorption, the utilization of the other lipids poses individual prob- 

 lems in each instance. In some cases, specific enzymes are involved in 

 the preparation of the other lipids for absorption; these usually differ 

 from the lipases required in the preliminary stages of the absorption of 

 the triglycerides. Such lipids as the hydrocarbons possess no reactive 

 group through which combination may take place. In fact, there is no 

 site in the molecule at which rupture can occur to effect a change akin to 

 hydrolysis. All lipids are insoluble in water and are fat-soluble; they 

 therefore encounter the same difficulties, and are subject to the corre- 

 sponding limitations which have been noted for triglyceride fats. 



The absorption of lipids other than fat may involve conditions entirely 

 different from those operative when neutral fat is absorbed. However, 

 most lipids are absorbed more readily when they are administered in a 

 fat solution. In the case of lecithin, it is extremely difficult to distinguish 

 its absorption from that of neutral fat when the two substances are being 

 absorbed simultaneously, since the phospholipids have been proved to be 

 synthesized in the intestinal mucosa during the absorption of fat, even in 

 the complete absence of phospholipid from the food. 



2. The Digestion and Absorption of Phospholipids 



Most of the information on the digestion and absorption of phospho- 

 lipids concerns lecithin. From a quantitative standpoint, lecithin is the 

 most widely distributed of any of the group of phospholipids in animal 

 and plant sources. 



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