FAT METABOLISM 693 



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 

 intestine are so very much smaller than those which lie on the surface of 

 the epithelium. 



It might be objected to the conclusions just stated that, although unde- 

 tectable, there is really some essential physical difference betAveen emul- 

 sified fat and emulsified hydrocarbon. In order entirely to prove the case 

 for the chemical theory, it is necessary to feed a neutral fat possessing 

 some characteristic that depends on the manner of union existing between 

 fatty acid and glycerine, and then to see whether it appears in an un- 

 changed condition in the thoracic duct. If it does so, the fat must have 

 been absorbed through the intestinal epithelium in an unbroken, unsapon- 

 ified condition, for it is unlikely that, in the resynthesis which occurs in 

 the intestinal epithelium, the fatty-acid molecules would recombine with 

 the glycerine molecules in exactly the same manner as before. 



There are, however, but very few qualities of neutral fats, apart from 

 those of the fatty acids which compose them, by which they can be char- 

 acterized. The most likely one is that of optical activity. None of the 

 ordinary fats is optically active, although from chemical considerations 

 it is quite conceivable that some should be so. In order to obtain such a 

 fat Bloor 49 conducted numerous experiments with the esters of stearic 

 acid.* In a series of experiments Bloor fed isomannid-dilaurate, a syn- 

 thetic fat of dextrorotatory power and as readily absorbed as natural fats, 

 and by examination of the neutral fat present in the chyle flowing from 

 the thoracic duct, found no evidence of the dextrorotatory fat. This result 

 confirms previous work by Frank, who found that the ethyl esters of 

 fatty acids are not absorbed unchanged. The results of both workers 

 emphasize the probability that readily saponinable fatty-acid esters do 

 not escape saponification under the favorable conditions of the normal 

 intestine. In other words, had the fats been absorbed unchanged, as 

 would be required by the mechanistic theory of fat absorption, they 

 would have appeared in the chyle in optically active conditions. 



These most important conclusions lead us to inquire as to the reason 

 for the change in fat during its absorption. It can not be for the purpose 

 of preventing the absorption of undesirable fatty substances, such as the 

 petroleum hydrocarbons or the wool fats, because such substances are 

 so rarely present in our food. It is most probable that the breakdown 



*Bloor prepared an optically active mannitan distearate, but found it to have a very high melt- 

 ing point and to be only half as digestible as the ordinary fats. Its absorption was too slow and 

 unsatisfactory to make it suitable for the above purposes. He, therefore, proceeded to prepare the 

 di-ester of isomannitan with lauric acid, and he found the resulting compounds to be as well-ab- 

 sorbvd as ordinary fat, and yet to possess very marked dextrorotatory power, which, of course, 

 they lose on saponification. This fat seemed suitable, therefore, for testing the above question. 



