250 IV. DIGESTION OF LIPIDS OTHER THAN FATS 



for the particulate absorption of fat (as postulated in the Partition Theory) , 

 then lecithin should be readily absorbed in the same manner, because of 

 the extreme ease with which it is emulsified in water. 



Artom and Swanson 18 demonstrated in a convincing manner that part 

 of the phospholipid is absorbed without hydrolysis. This conclusion 

 was based upon the demonstration that the content of phospholipid, 

 labeled with P 32 , was considerably greater in the blood and liver after the 

 feeding of phospholipid so labeled than resulted in control rats receiving 

 non-labeled phospholipid and P 32 as sodium phosphate or glycerophos- 

 phate in an equivalent amount. This suggestion is of interest in relation 

 to the early statements of Mayer. 19 While he believed that naturally 

 occurring d-lecithin (now referred to as L-lecithin) is split by lipase, he 

 stated that the unnatural form (Z-lecithin, now called D-lecithin) is ab- 

 sorbed as such, since the lipases cannot attack it. 



However, Artom and Swanson 18 state that phospholipids may be ab- 

 sorbed at various stages of hydrolysis. There appears to be definite 

 evidence that part of the phospholipid is hydrolyzed in the gastrointestinal 

 tract, so that inorganic phosphate or glycerophosphate is set free. The 

 results of Artom and Swanson 18 would seem to offer an entirely inde- 

 pendent confirmation of the Partition Theory of fat absorption as postu- 

 lated by Frazer. 20 



3. The Absorption and Digestibility of Waxes 



Although the waxes include a wide variety of components, from a quanti- 

 tative standpoint, they comprise a much smaller proportion of the diet 

 than is the case with the triglyceride fats. Waxes are usually considered 

 to be simple lipids, ordinarily solid at room temperature, in which a simple 

 fatty acid is combined with a monatomic alcohol, usually of high molecu- 

 lar weight. Two main classes of true waxes exist, namely those in which 

 the fatty acid is combined with an aliphatic alcohol such as cetyl or stearyl, 

 and those in which the alcohol component is a cyclic compound with a 

 steroid nucleus or one containing a /3-ionone residue. 



(1) Waxes Containing Aliphatic Alcohols 



Ordinarily waxes of this type are very difficult to hydrolyze in the 

 laboratory, even after prolonged saponification. Thaysen 21 proposed a 



« C. Artom and M. A. Swanson, /. Biol. Chem., 175, 871-881 (1948). 



19 P. Maver, Biochem. Z.,1, 39-52 (1906). 



20 A. C. Frazer, Physiol. Revs., 36, 103-119 (1946). 



21 T. E. H. Thaysen, Biochem. Z., 62, 89-114 (1914). 



