654 X. VITAMINS D 



upon the fact that esters which cannot be hydrolyzed have no antirachitic 

 potency''^ although it has not been unequivocally proved that such non- 

 utilization of unhydrolyzable esters is to be ascribed to their failure to be 

 al)sorbed rather than to the biologic activity of the conjugated form after 

 absorption (if, in fat, these esters are absorbed without hydrolysis). 



No differences in absorption of the several vitamins D are noted in normal 

 dogs. The absorption of vitamins Do and D3 in unchanged form after they 

 have been fed to hens is proved by the classical experiments of Bethke 

 e/ a/.'^^'^^Mn which it was demonstrated that the kind of vitamin D in the 

 egg yolk corresponds to that in the diet. A similar phenomenon was 

 reported by Bethke and co-workers'^^ for the cow, as proved by the rela- 

 tionship between the type of vitamin D in the milk and in the diet. How- 

 ever, according to McChesney,-" the chick does not absorb vitamin D2 

 effectively. Whereas the amount of vitamin D3 found in the carcasses and 

 feces accounts for all the vitamin D3 given, 35% of the vitamin D2 ad- 

 ministered cannot be traced. 



(S) The Transport of the Vitamins D 



There is no experimental e\adence concerning the pathway for the 

 transport of the vitamins D from the lumen of the intestine to the tissues. 

 However, there is no reason to suppose that this route differs from that of 

 the other fat-soluble vitamins or from that of the fats themselves. This 

 would involve the lymphatic system as the principal if not the only route 

 for the transfer of vitamin D from the epithelial cells to the blood stream. 

 The portal system would probably be of no more importance in this transfer 

 than it is in the case of vitamin A and the carotenoids, which is nil. 



Presumably, the blood effects the further transfer of the vitamin D to 

 the tissues after the vitamin has been emptied into the subclavian vein 

 from the thoracic duct. The fact that \'itamin D is present in blood 

 has been established by a number of investigators. Warkany-^^-^^^ 

 reported an average concentration of 100 I.U.% (50 to 135 T.U.%) in human 

 serum, while the mean content recorded in the case of rabbits was 50 

 I.U.%. Hess and co-workers'^^" observed that the plasma vitamin D in 

 cows was increased after irradiated ergosterol was given. The plasma of 

 cows was found to contain four times the concentration of vitamin D which 



2"E. W. McChesney, Proc. Soc. Exptl. Biol. Med., 58, 300-.303 (1945). 

 "8 J. Warkany, Am. J. Diseases Children, 52, 831-847 (1936). 

 229 J. Warkany, Biochem. Z., 293, 415-426 (1937). 



^^ A. F. Hess, M. Weinstock, and J. Gross, Proc. Soc. Exptl. Biol. Med., 30, 1357-1358 

 (1933). 



