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



(in the villi), the intestinal lymphatics, the thoracic duct, and the blood 

 stream. This is the principal pathway for fats, and practically complete 

 recoveries of ingested fat can be obtained in the lymph if one collects this 

 fluid from the thoracic duct. However, the fat appears in the lymph in 

 fine droplets of neutral fat (chyle), irrespective of A\'hether it has been in- 

 gested as neutral fat, diglycerides, monoglycerides, or fatty acids. The 

 fine fat droplets are referred to as "chylomicrons." Ordinarily it is six 

 hours before the maximum fat levels are obtained in the blood after the 

 ingestion of a fatty meal; several additional hours elapse before the func- 

 tion of transporting the fat from the intestines to the tissues is completed, 

 as indicated by the return of the blood lipids to the prefeeding levels. It is 

 believed that phospholipids are transported along with the triglycerides 

 via the lacteals, lymphatics, and the blood stream. They presumably 

 aid in maintaining the finely-emulsified state of the fat. Free fatty acids 

 are not ordinarily present in either the lymph or the blood stream, except 

 in minimal quantities. 



The second route available for fat transport from the intestines is by 

 way of the portal system. This alternate pathway originates in the 

 capillary network in each villus, from which it gradually builds up to the 

 portal system through which the blood ultimately reaches the liver. 

 Although this pathway is of little importance as a means of transporting 

 the triglycerides and other lipid components, it is of major significance in 

 the absorption of the water-soluble products arising from the digestion of 

 proteins and carbohydrates. It is likewise believed that triglycerides 

 with short-chain (hence water-soluble) fatty acids are transported by the 

 portal route. 



The blood stream is the ultimate route for the transfer of the lipids, 

 not only from the intestine to the liver and to the storage depots, but also 

 from the storage depots to the other organs, during fasting. Normally, 

 the total lipids in the blood are present, on an average, to the extent of 

 about 600 mg. %, of which the neutral fats comprise one-fourth. Fatty 

 acids, which occur not only in combination with glycerol but also in the 

 phospholipids and with cholesterol and vitamin A esters, account for 

 approximately 350 mg. %. Some of the fats occur in combination with 

 proteins in the form of the so-called lipoproteins. The level of blood lipids 

 varies not only with the ingestion of food but with such physiologic factors 

 as age, sex, race, pregnancy, lactation and time of day, with inanition, 

 with over-nutrition, and with work, as well as with many pathologic condi- 

 tions. For a more complete discussion of blood lipids, the reader is re- 

 ferred to Chapter V of Volume II, The Lipids. 



