724 METABOLISM 



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 undetectable, 

 there is really some essential physical difference between emulsified fat and emulsified 

 hydrocarbon. In order entirely to prove the case for the chemical theory, it is neces- 

 sary to feed a neutral fat possessing some characteristic that depends on the manner of 

 union existing between fatty acid and glycerol, and then to see whether it appears in 

 an unchanged condition in the thoracic duct. If it does so, the fat must have been 

 absorbed through the intestinal epithelium in an unbroken, unsaponified condition, for 

 it is unlikely that, in the resynthesis which occurs in the intestinal epithelium, the fatty- 

 acid molecules would recombine with the glycerol 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 characterized. 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 ought to be so. In 

 order to obtain such a fat Bloor49 conducted numerous experiments with the esters of 

 stearic acid.* In a series of experiments Bloor fed isomannid-dilauratc, a synthetic 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 evi-> 

 dence 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 saponifiable 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 optic- 

 ally 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 

 and resynthesis of neutral fat occurs for the same reason that similar 

 processes occur during the absorption and assimilation of protein. It 

 will be remembered that protein is entirely disintegrated in the intestine 

 into its so-called building stones. These are absorbed separately into 

 the blood, which carries them to the tissues, in which they become re- 

 synthesized to form the body protein. And so it appears to be in the 

 case of fats. The process, in other words, permits of the rearrangement 

 of fatty-acid molecules, as a result of which the newly formed fat is more 

 adaptable for use in the organism. It comes to be more like the char- 

 acteristic fat of the animal. There may be another reason for the proc- 



*Eloor 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. 



