CHAPTER I 



GENERAL CONSIDERATIONS OF 



LIPID DIGESTION, ABSORPTION. 



TRANSPORT, AND STORAGE 



1. Introduction 



The utilization of fats and other foodstuffs naturally falls into two 

 distinct divisions. The first one is largely concerned with enzymatic 

 changes of the foodstuff in the gastrointestinal tract by which the food- 

 stuff is prepared for absorption. The absorption of such products from 

 the gastrointestinal tract, together with their transport by way of the 

 lymphatics or portal system and their preliminary deposition in the tissue, 

 constitute closely related phases. These have been considered at some 

 length in Volume II of The Lipids. 



The second section into which the discussion of the biochemistry of fats 

 and other foodstuffs is obviously divided includes the so-called inter- 

 mediary metabolism of these substances. Although the term, intermediary 

 metabolism, is usually considered to encompass all changes which take 

 place from the time the substances under consideration are absorbed until 

 their end-products are excreted in the urine, feces, expired air, or through 

 the skin, the transformations which involve synthesis, oxidation, and 

 intermediate changes of these substances actually comprise the chief im- 

 portance of intermediary metabolism. These phases of intermediaiy 

 metabolism of fats will be considered in the present volume. 



The metabolism of the triglyceride fats, phospholipids, fatty acids, 

 cerebrosides, sterols, fat-soluble vitamins, and even hydrocarbons, dis- 

 plays many interrelationships. However, the metabolism of triglycerides, 

 and that of phospholipids and of fatty acids, are so intimately connected 

 that it is impossible to consider them separately. In the subsequent 

 chapters on synthesis, oxidation, and intermediary changes, the above 

 three groups of fats will be considered as a unit. Cerebrosides and hydro- 

 carbons will be dealt with separatel}^, while the metabolic interrelations of 



1 



