THE DIGESTION OF FATS 135 



In the case of lipids such as the esters of vitamins A and D, only a single 

 hydrolysis can occur; in hydrocarbons, as for instance carotene or lyco- 

 pene, no hydrolysis whatsoever is possible. Although other reactions, such 

 as reduction, can take place hi the lower gastrointestinal tract, these are 

 not usually concerned with absorbable material, most of which has left the 

 gastrointestinal tract prior to reaching areas where such changes can take 

 place. 



(1) Digestion in the Stomach 



Although some lipase may be secreted in gastric juice, it is a water- 

 soluble enzyme which can come in contact with only an infinitesimal amount 

 of fat unless there is some factor causing an increase in the water-fat inter- 

 face. Those fats, such as milk fat and egg-yolk fat, which are already 

 highly emulsified when ingested, provide a large surface area for attack by 

 water-soluble enzymes. Although similar fat emulsions can readily be 

 produced from ordinary fats when soap is present, the acidity of the gastric 

 contents is seldom sufficiently low to permit the formation of such soap 

 emulsions. 



The formation of fat emulsions, and the action of lipase on such emul- 

 sions are, however, favored by one physiological reaction. When any 

 considerable amount of fat is present in the food, the period of time during 

 which the food remains in the stomach is markedly increased. According 

 to Tangl and ErdeTyi 635 and von Fejer, 636 the emptying time of the stomach 

 depends upon the melting point and viscosity of the fat ingested ; the pres- 

 ence of large amounts of fat may delay the emptying time of the stomach 

 for several hours. The secretion of pepsin and of acid by the gastric mu- 

 cosa is likewise inhibited when fat is present in the stomach. The pan- 

 creatic juice and bile may pass into the stomach from the intestine as the 

 result of antiperistaltic movements. 637 ' 638 The regurgitation of intestinal 

 contents into the stomach may be sufficiently pronounced to reduce the 

 acidity of the gastric contents to a significant extent. All of these factors 

 will tend to act independently and collectively to produce some lipolysis 

 in the stomach. As the chyme is passed into the intestine, enough fatty 

 acid is already present in the free state to produce a soap emulsion immedi- 

 ately on alkalinization of the medium. 



« 36 F. Tangl and A. Erdflvi, Biochem. Z., 34, 94-110 (1911). 



636 A. von Fejer, Biochem. Z., 53, 168-178 (1913). 



637 W. Boldyreff, Arch. ges. Physiol. (Pfliiger's), 121, 13-53 (1908). 

 938 W. Boldvreff, Ergeb. Physiol, 11, 121-217 (1911). 



