DIGESTION, ABSORPTION, AND TRANSPORT 05 1 



dehydrocholesterol) . It is known from other work that vitamin D3 can 

 be used readily by the chick, but that vitamin D2 is quite ineffective as a 

 source of vitamin D for this species. Bethke and associates"^ Ukewise 

 demonstrated that the increased vitamin present in the hen's egg after 

 the feeding of higher levels of vitamin D corresponds to the type of vitamin 

 D introduced into the diet. 



2. The Digestion, Absorption, and Transport of the Vitamins D 



(/) The Absorption of the Vitamins D 



Much less information is availal)le coiicerning the absorption of the 

 \itamins D from the intestine than about that of the other fat-soluble 

 vitamins. This is largely due to the fact that the methods hitherto em- 

 ployed for the estimation of the D vitamins were cumbersome, and required 

 a rather large amount of biologic material. However, much of this 

 difficulty has been overcome by the use of C^*-tagged vitamin D, which 

 was employed in the studies of Kodicek,-"'' as well as by the more recent 

 chemical method of Kodicek and Ashby™^ which employs reversed-phase 

 paper chromatography. 



There is no question that the vitamins D can be absorbed to an appreci- 

 able extent from the gastrointestinal tract. This is the general route of 

 administration when bioassay studies are made and, in fact, when synthetic 

 diets employing \'itamin D are given. The vitamins D can also be utihzed 

 when injected into animals. This would seem to indicate that, under 

 normal conditions, vitamin D is absorbed unchanged from the gut. 



The capacity of the gut for absorption of the vitamins D is not known. 

 Morgan and Shimotori-"^ reported the absence of any appreciable amount 

 of vitamin D from the feces of young dogs after the first day of dosing with 

 20,000 I.U. 'kg. Ijody weight, gi\'en in the form of cholesterol (an activated 

 animal sterol). Cruickshank and Kodicek -°^ were able to recover only 

 about 17% of the administered vitamin D in the feces of rats during two 

 days after 40,000 I.U. (1 mg. calciferol) had been administered in a single 

 dose. However, these workers were able to account for only 6% of the 

 total dose in the body, and it was not known what occurred to the remaining 

 77%. It has recently (1955) been demonstrated, by the use of C^^- 



'"'^ E. Kodicek, Bone Structure and Metabolism, Ciba Lecture; Unpublished data made 

 available to the author, 1955. 



205 E. Kodicek and D. R. Ashbv, Biochem. J., 57, xii (1954). 



206 A. F. Morgan and N. Shimotori, J. Biol. Chem., 147, 189-200 (19-13). 



20" E. M. Cruickshank and E. Kodicek, Biochem. J., 51, xhi (1952); also personal com- 

 munications from the authors, October, 1955. 



