FAT METABOLISM 723 



to form neutral fat, which gradually finds its way by the central lac- 

 teals into the lymphatics and then to the thoracic duct. No evidence 

 can be obtained that any of the fat is absorbed into the portal blood 

 although only about 60 per cent of ingested fat can be recovered from 

 the thoracic duct. 



The chemical explanation of the absorption of fat is very different from 

 that formerly held by histologists who maintained that the fine particles of 

 emulsified fat in the intestine penetrate by a mechanical process through 

 the striated border of the epithelial cell into its protoplasm. The histolog- 

 ical evidence for this view seemed very convincing, for fine fat globules can 

 readily be seen in the epithelial cells of the intestine after fatty food 

 has been taken, while they are absent during starvation. These par- 

 ticles seemed to have passed directly from the intestinal canal into the 

 epithelial cells because, when the fat was stained with characteristic fat 

 dyes before feeding it to the animal, the globules in the epithelial cells 

 were found to be similarly stained. The supporters of this mechanistic 

 view of fat absorption maintained that the appearance of the stained fat 

 globules in the epithelial cells could not be explained in any other way 

 than by supposing that the fat globules had wandered unbroken into 

 the epithelial cells. Such a conclusion is, however, unwarranted, for the 

 stains that are soluble in fat are also soluble in soap, so that when the 

 fat splits up, the stain will remain attached to the soap and be carried 

 along with it into the intestinal epithelium. 



Proof that the chemical theory is the correct one has been supplied by a large 

 number of experiments. The following may be cited: (1) When the lymph flowing 

 from the thoracic duct is examined after feeding with fatty acids instead of neutral 

 fat, it is found to contain only neutral fat, indicating that a synthesis must have 

 occurred between glycerol and fatty acid during the absorption. The glycerol for 

 this synthesis is furnished from sources which will be described later. (2) When an 

 emulsion made partly of neutral fats, and partly of some hydrocarbon, such as albo- 

 lene, is fed and the feces are examined for these substances, it has been found that 

 all the fat but none of the hydrocarbon is absorbed; the feces contain all of the albo- 

 lene but none of the fat. This experiment supplies very strong evidence against the 

 mechanistic theory, for microscopic examination of the above described emulsion shows 

 the particles of neutral fat and hydrocarbon to be of exactly the same size. (3) By 

 examining the properties of the fatty substances in the thoracic lymph collected 'during 

 the absorption of such an emulsion as that described above, nothing but neutral fat 

 has been found present. (4) Similar results are obtained when wool fat, which is an 

 ester of cholesterol and fatty acid, is fed. 



We may conclude that fatty substances which are insoluble in water or 

 can not be changed by digestion into substances (soap) that are soluble 

 in water, are not absorbed, however like fat they may be in other particulars. 



The chemical theory of fat absorption further explains why there should 

 be such large quantities of soapy substances in the intestinal contents, 

 and also why the globules of fat present in the epithelial cells of the 



