NORMAL FAT METABOLISM 189 



Closely related substances found in animals and probably derived from 

 cholesterol are coprosterol in feces and isocholesterol in skin and hair 



waxes. 



Fat Digestion and Absorption 



The Stomach. Digestion. Fat splitting enzymes (lipases) may ap- 

 pear in the stomach from either of two sources as part of the gastric 

 secretion or by regurgitation from the intestine. The presence in the 

 stomach of secretions from the small intestine, especially bile, has been 

 known clinically for many years, and while the tendency has been to 

 minimize the influence of these secretions on fat digestion it is realized 

 that under suitable conditions splitting of fats in the stomach may assume 

 considerable proportions. Cannon has shown that fats slow the emptying 

 of the stomach by inhibiting the production of acid, also that the pylorus 

 is kept closed by the presence of acid on the intestinal side of the sphincter. 

 In the absence of acidity the pylorus may relax or open and allow regurgi- 

 tation of intestinal contents including lipases by reverse peristalsis, and 

 under the conditions of low gastric acidity considerable lipolysis would 

 take place. Boldyreff found that after a meal rich in fat there is a reflux 

 of pancreatic secretion into the stomach. 



Quite aside from the regurgitated intestinal material the stomach has a 

 lipase of its own, a fact which was claimed many years ago by Ogata and 

 other observers. Their work received little attention until it was confirmed 

 by Volhard and his pupils. Volhard's work stimulated investigation and 

 discussion and the existence of a gastric lipase has been a much debated 

 topic since that time. One difficulty has been to rule out the possibility of 

 intestinal lipase, and when this has been successfully accomplished the low 

 values obtained for lipolysis by pure gastric juice have thrown doubt on 

 its existence in amounts worthy of consideration. Volhard found un- 

 doubted digestion of the emulsified fat of milk and egg-yolk both by gastric 

 juice obtained by siphon and by glycerin extract of the mucous membrane 

 of the fundus, and his findings have been confirmed by several workers 

 since (Davidsohn, 1912), while London and others were unable to dem- 

 onstrate lipase in gastric juice from a Pawlow stomach. Davidsohn has 

 compared the properties of gastric and of pancreatic lipase and found 



differences in their optimum reaction. For pancreatic lipase the optimum 



+ 



reaction was H = 1 X 10~ 8 , while for stomach lipase it was 2 X 10" 6 

 also that pancreatic lipase was much more sensitive to sodium fluoride. 



The probable reason for the conflicting results regarding gastric lipase 

 has recently been found by Hull and Keeton, who studied the lipase in 

 gastric juice obtained from Pawlow stomachs and in normal stomachs, 

 of which the pylorus had been ligated and the flow of secretion stimulated 

 by gastrin and by food. They found that the gastric lipase was sensitive 



