TRANSPORT FROM THE INTESTINE 455 



4. The Transport of Carotenoids and of Vitamins A from the 



Intestine 



Two routes are possible for the transport of nutrients from the intestine 

 to the internal organs; the first of these is via the mesenteric blood vessels 

 and the portal vein to the hver, while the second route involves the lacteals 

 in the intestinal wall, the mesenteric lymphatics, and the thoracic duct, 

 from which the contents are poured into the subcla\'ian vein to gain access 

 to the general circulation. In the case of the triglycerides, it has been 

 proved many times that the lymphatics are the chief routes of transport, if 

 not, in fact, the only path of travel. There seems to be some evidence that 

 the triglycerides composed of the short-chain acids, and these short-chain 

 acids themselves, may employ the portal vein as the route of transport. 



{1) Transport in the Lymph 



It has been definitely pro^'ed that both |S-carotene (in those species in 

 which the unchanged carotenoid is present in the blood) and \'itamin A are 

 transported exclusively by way of the lymphatic system. Drummond, 

 Bell, and Palmer,-"*^ who studied this problem in a case of chj'lothorax, 

 were able to recover administered ^atamin A quantitatively in the chyle of 

 the patient, while carotene was recovered only to the extent of 20% after 

 this provitamin A had been ingested. The failure to account more quanti- 

 tatively for the pro\'itamin A may well have been due to the fact that a 

 large portion of the provitamin A was converted to \dtamin A in the wall 

 of the gut, and was transported from the gut wall as such. 



An investigation of the composition of lymph after the feeding of ^- 

 carotene has provided cogent proof that the carotenoid must be broken 

 down to vitamin A in the intestinal wall in many species. Thus, it was 

 demonstrated by Thompson and co-workers^^ that no carotene was to be 

 found in the systemic or portal blood of pigs or of normal or vitamin A- 

 deficient rats after the feeding of carotene, although a concomitant increase 

 in ^^tamin A in the Ij^mph occurred under such conditions. An increase 

 in vitamin A has been noted in the mesenteric lymph nodes of rats" and of 

 pigs^^* after the feeding of carotene. Woytkiw and Esselbaugh^" re- 

 ported an increase in the vitamin A content of the lymph of guinea pigs 

 when either carotene or vitamin A was fed. No carotene could be detected 

 in the lymph of any of these species. A high content of vitamin A has like- 

 wise been reported in the intestinal l.ymph of sheep ^** after carotene feed- 

 ing, as well as in the intestines of bullocks under similar dietary conditions. 



2" J. C. Drummond, M. E. Bell, and E. T. Palmer, Brit. Med. J., 1935, 1, 1208-1210. 



