THE PATHWAYS FOR LIPID ABSORPTION 115 



amino acids present in casein and glycine were given. Lysine, glycine, 

 and the D-amino acids were not involved in this effect; the ketogenic amino 

 acids (leucine, tyrosine, and phenylalanine) were found to provoke a chole- 

 poietic response, which was augmented by the addition of methionine, 

 threonine, and valine. Vitamin B 12 produces a transient increase in cholate 

 output, but cortisone acetate and testosterone propionate do not have this 

 effect. 



4. The Pathways for Lipid Absorption 



(1) The Sites of Absorption in the Gastrointestinal Tract 



In order that a substance may be absorbed from the gastrointestinal 

 tract, it is necessary that the material be present in water-soluble form, 

 that it be allowed to remain in contact with the surface of the alimentary 

 canal for an adequate length of time, and finally, that the anatomical struc- 

 ture of the gastrointestinal tract be suitable for the process of absorption. 



a. Mouth. Under normal conditions, no absorption whatsoever occurs 

 in the mouth. In the first place, the fats are not in solution at this stage. 

 No fat-splitting enzymes are present in saliva, and there is no mechanism 

 whereby emulsification of the fat may occur. However, the mucosa of the 

 mouth and esophagus is capable of absorbing small amounts of water- 

 soluble material if the substance is retained in the oral cavity. Meltzer 584 

 reported that, following tying of the esophagus, rats were quickly poisoned 

 if phenol or nicotine was placed in their mouths. It has also been reported 

 by Mendell 685 and by Mendel et al. m that, in man, codeine, morphine, 

 atropine, strophanthin, medinal, nitroglycerol, and pyramidon can be 

 absorbed from the mouth if the drug is kept there for a sufficient period. 

 There is also evidence that strychnine can be absorbed fairly readily from 

 the pharynx and esophagus of the dog. 587 



b. Stomach. No appreciable absorption of lipids normally occurs in the 

 stomach. Although fats may be emulsified to a considerable extent in 

 this organ, the hydrolysis of the fat molecules is kept at a minimum. The 

 presence of lipase has been postulated in gastric juice; however, its action 

 is weak at the pH existing in the gastric cavity. Highly emulsified fats 



584 S. J. Meltzer, 1899; cited by F. Verzar and E. J. McDougall, Absorption from the 

 Intestine, Longmans, Green, London and New York, 1936, p. 4, and by F. Mendell, 

 Munch, med. Wochschr., 69, 1593-1595 (1922); 70, 1526 (1923). 



585 F. Mendell, Munch, med. Wochschr., 69, 1593-1595 (1922); 70, 1526 (1923). 



586 B. Mendel, A. Wittgenstein, and E. Wolffenstein, Klin. Wochschr., 3, 470-472 

 (1924). 



587 S. J. Meltzer, Am. J. Med. Sci., 118, 560-570 (1899). 



