DIGESTION, ETC. OF FAT-SOLUBLE VITAMINS 323 



el a/. 473 reported that it occurs along the whole length of the small intestine. 

 The fact that an accumulation of vitamin A results in the gut wall after 

 the administration of the aldehyde indicates that the entrance of the 

 retinenei into the mucosa and its transformation into vitamin Ai are 

 rapid, as compared with the transport of vitamin A away from the gut 

 wall. It was suggested by Glover and associates 473 that the change of 

 /3-carotene into vitamin A in vivo involves the oxidation of the carotenoid 

 to retinenei, which is then rapidly reduced to vitamin A x . This mecha- 

 nism would appear to be more probable than that of hydrolytic fission. 



Morton et a,/. 475 found that, when retinene 2 was fed to vitamin A-de- 

 ficient rats, it was quickly reduced to vitamin A 2 in the gut wall; the 

 presence of vitamin A 2 could also be demonstrated in the liver. 



{2) Vitamins D 



3l. The Absorption of the Vitamins D from the Intestine. Much less is 

 known about the absorption of the vitamins D than is the case w T ith caro- 

 tene and the vitamins A. This is largely because the methods for the 

 quantitative estimation of vitamins D are cumbersome, and require much 

 larger amounts of material for analysis than is the case with the other vita- 

 mins. In fact, a bioassay is usually the only procedure by which one 

 is able to determine the correct answer. 



The utilization of vitamin D is increased when fat is present in the diet. 

 According to Knudson and Floody, 476 healing was improved in rickets 

 when a known amount of vitamin D was given in conjunction with a diet 

 containing 5% fat, as compared with the results observed when a fat- 

 free diet was employed. Diets containing 10 or 20% of fat were reported 

 to be less efficacious than the 5% fat diet, but better than the fat-free 

 regimen. It is uncertain whether this improvement in vitamin D utiliza- 

 tion is a result of improved absorption or of an effect on the vitamin D 

 after it is absorbed. Boer, 477 who reported that diets producing rickets 

 were no longer rachitogenic when 10% of the saponifiable fraction of 

 margarine was added, believed that fat exerts a sparing action on vitamin 

 D. Kon and Booth 478,479 have come to the same conclusion. As a further 

 confirmation of the effect of fats in preventing rickets, McDougall 480 



475 R. A. Morton, M. K. Salah, and A. L. Stubbs, Biochem. J., 41, xxiv (1947). 



476 A. Knudson and R. J. Floody, J. Nutrition, 20, 317-325 (1940). 



477 J. Boer, Acta Brevia Neerland. Physiol. Pharmacol. Microbiol., 9, 67-68 (1939). 



478 S. K. Kon and R. G. Booth, Biochem. J., 28, 111-120 (1934). 



479 S. K. Kon and R. G. Booth, Biochem. J., 28, 121-130 (1934). 



480 E. J. McDougall, Biochem. J., 32, 194-202 (1938). 



