LIPID METABOLISM 



11 73 



presented evidence that finely emulsified fat particles 

 of 0.5 fx or less in diameter can penetrate intact the 

 small spaces between mucosal "microvilli.'' More 

 recently, evidence has been presented that hydrolysis 

 of triglycerides in the intestinal lumen is extensive but 

 incomplete, and that approximately 65 per cent of 

 the fatty acids is absorbed in the free form while 35 

 per cent passes into the mucosa as glycerides ( 1 7, 29). 

 The mucosal cells resynthesize the free fatty acids, 

 the monoglycerides, and diglycerides into triglycer- 

 ides. Studies (48, 49, 127) have indicated that in- 

 testinal mucosal synthesis of triglycerides proceeds 

 along pathways similar to those defined by Kennedy 

 and his associates (206) for hepatic triglyceride syn- 

 thesis. In contrast to what may happen in the liver, 

 free glycerol does not appear to be a starting material, 

 although quite recently evidence for free glycerol 

 incorporation into triglyceride in the intestine in man 

 has been reported (107). Free fatty acids become 

 activated by linkage with coenzyme A. Two such 

 activated fatty acids combine with L-a-glycerophos- 

 phate to form diglyceride phosphate (phosphatidic 

 acid) which can then form diglyceride following 

 dephosphorylation by a suitable phosphatase. The 

 diglyceride reacts with a third activated fatty acid 

 to yield a triglyceride, or, like phosphatidic acid, 

 may also be converted to phospholipid. The major 

 pathway is that of triglyceride synthesis. A small 

 amount of phospholipid and cholesterol gets in- 

 corporated with the triglycerides into the "chylo- 

 microns" which range in diameter from 350 A to 

 0.5 /u. The chylomicrons are subsequently discharged 

 into the intestinal lacteals from which they drain into 

 the thoracic duct and ultimately the systemic circula- 



tion. Studies on the composition of chylomicrons in 

 man, rat, and dog (98, 160, 182) have shown that 

 they consist of 85 to 93 per cent triglyceride, 8 to 1 1 

 per cent phospholipid, 1.5 to 4.5 per cent cholesterol 

 (free and ester) and 1.9 to 2.5 per cent protein (|3- 

 globulin). Some events that occur during fat digestion 

 and absorption are summarized in figure 2. 



ADIPOSE TISSUE 



The concept that adipose tissue is a dynamic 

 "organ" capable of participating in a number of 

 metabolic processes is now generally accepted. The 

 effect of caloric abundance or inadequacy on the 

 quantity of stored fat is well recognized. A variety of 

 stimuli are known to induce an increase or a decrease 

 in body fat. However, the mechanisms involved in 

 fat deposition (lipogenesis) and fat release (lipolysis) 

 are not completely understood. Most body fat acts as 

 a highly efficient caloric reservoir. However, it must 

 be remembered that this reservoir is composed of 

 myriads of living cells the function of which includes 

 svnthesis and mobilization of fat as well as storage. 



The number of calories stored as fat is necessarily 

 a function of two variables, energy intake and energy 

 expenditure. A normal young adult man in caloric 

 balance may contain an average of 14 per cent pure 

 fat (116). Thus a young man weighing 70 kg may 

 carry approximately 10 kg of fat. This is almost two 

 and a half times the weight of his bone minerals. 

 The relative amount of fat in the body has been 

 shown to increase with age reaching, at age 55, 

 approximately 25 per cent of body weight in clini- 



LUMEN 

 TRIGLYCERIDE 



MUCOSA 



FATTY ACID 



CHOLESTEROL 



CHOLESTEROL 

 Esters and Ereel 



LYMPHATICS 



-CHYLOMICRONS* 



Triglyceride 89% 



Phospholipid 7% 



Cholesterol 2% 



Protein 2% 



fig. 2. Schema of lipid absorp- 

 tion (long -chain fats). * Propor- 

 tions of chylomicron constituents 

 vary with diet. 



