1 172 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



by the observation that at least 64 different fatty 

 acids have been identified in butter fat (101). 



In general, the degree of unsaturation of the fat 

 depends upon the source of the fat. Fats of aquatic 

 origin contain a wide range of unsaturated C 16 , Cis, 

 C 2 o, and C22 acids. Fats from land animals contain 

 25 to 30 per cent C^, the remainder being mostly of 

 the Cis series (102). The so-called essential fatty acids 

 (mainly linoleic) apparently cannot be synthesized 

 by animals, and must be obtained from the diet. 

 The depot fat of certain animals, such as the pig, can 

 be varied markedly in its content of linoleic acid, 

 depending on the feed (60). Similarly, the adipose 

 tissue fatty acids of man eventually reflect the dietary 

 fatty acid pattern. This is true with respect to linoleic 

 acid (103); but the medium chain fatty acids (C12 

 and below) have not been identified in the fat depots. 

 Fats of vegetable origin vary tremendously in their 

 pattern of fatty acids, as well as in their degree of 

 unsaturation. For instance, coconut oil contains only 

 a small quantity of linoleic acid, while safflower oil 

 may contain 70 per cent or more. Dietary fats should 

 not be described merely as "animal or vegetable," 

 "saturated or unsaturated," but the actual composi- 

 tion in terms of fatty acids should be identified. Thus, 

 when the effects of dietary fats on lipid metabolism 

 are being evaluated, the specific fatty acids involved, 

 their chain length, isomeric configuration, degree of 

 unsaturation, and relative proportion in the diet 

 must be considered. 



Dietary phospholipids are found as complex mix- 

 tures in organ fats and certain raw vegetable fats, 

 rather than in depot fats. Egg yolk is a rich source of 

 phospholipid. As indicated by their name the phos- 

 pholipids area group of phosphorus-containing lipids; 

 in addition, they contain a nitrogenous base. The 

 lecithins, in which the base is choline, and the cepha- 

 lins, in which the base is ethanolamine or serine, are 

 classified as mono-amino-phosphatides. The com- 

 ponent fatty acids are usually both saturated and 

 unsaturated. The inositol phospholipid contains 

 ethanolamine and tartaric acid. It is found in soy- 

 bean phospholipids and in brain tissue. Other phos- 

 pholipids include sphingomyelin, which is a diamino- 

 phosphatide containing choline and sphingosine. 

 Plasmalogens contain higher fatty aldehydes and 

 ethanolamine (69). The complexity of dietary phos- 

 pholipids is illustrated by their occurrence in egg 

 yolk: 72.8 mols per cent phosphatidyl choline; 14.8 

 per cent phosphatidyl ethanolamine; 2.1 per cent 

 lysophosphatidyl ethanolamine; 5.8 per cent sphingo- 

 myelin; 0.9 per cent plasmalogen; 0.6 per cent inositol 



phospholipid; and 0.2 per cent phosphatidyl amine 

 acids (169). One egg contains about 2 g of phos- 

 pholipid. 



The unsaponifiable fraction of food fats consists of 

 sterols, including; cholesterol (absent from vegetable 

 fats), long-chain aliphatic alcohols, glycerol ethers, 

 pigments, etc. Finally the fat-soluble vitamins, A, D, 

 E, and K, may be found in this fraction. 



FAT DIGESTION AND ABSORPTION 



Generally speaking, lipids are not readily miscible 

 in water. To be able to absorb, transport, and utilize 

 fatty acids and other lipids, man has had to evolve 

 rather elaborate mechanisms for making these water- 

 immiscible or hydrophobic materials compatible with 

 a system whose basic medium is water. The mech- 

 anisms used to deal with the water-insoluble lipids 

 as they enter the body include hydrolysis, emulsifi- 

 cation, chemical combinations with substances con- 

 taining hydrophilic groups, and complex formation 

 with substances conferring greater water miscibility 

 and dispersibility, such as bile acids and proteins. 



The mechanisms of digestion and absorption of 

 dietary fat have been subjects of controversy for many 

 decades. An early theory was proposed by Pfluger 

 (161) who described dietary fats as being emulsified 

 by bile salts in the small intestine. The triglycerides 

 were then completely hydrolyzed by pancreatic lipase 

 to fatty acids and soaps. Being water-soluble, these 

 products were readily absorbed. However, it soon 

 became apparent that intestinal pH is too low for 

 fatty acids to exist as soaps. It also became apparent 

 that the absorbed fat in lymph is mainly in triglyceride 

 form. Thus, glyceride resynthesis by the intestinal 

 mucosa was postulated. The modern concepts of fat 

 absorption arise from Frazer's work (67, 68). It is 

 now believed that hydrolysis of dietary glycerides 

 need not be complete in order for absorption to occur. 



Frazer's original "partition theory" (66) postulated 

 that fatty acids passed directly into the portal circula- 

 tion while the partial and unchanged triglycerides 

 were somehow transported across the mucosa into 

 lymph as chylomicrons. This theory has failed to 

 survive in its original form as a result of more recent 

 work (16, 145) including Frazer's own (70). Portal 

 venous transport of fat is now known to occur only 

 with fatty acids of less than ten carbons, which com- 

 prise less than 5 per cent of dietary fats. 



The digestion and absorption of long-chain fats 

 remain a subject of controversy. Frazer (70) has 



