CAROTENOIDS 



containing only carotenes. As Fox** rightly points out, this accumu- 

 lation must represent a secretion of carotenes manufactured either de 

 novo or from alimentary carotenoids. Further, it is difficult to see how 

 the oil fails to trap some alimentary xanthophylls. A reinvestigation of 

 this oil in the light of modern developments in carotenoid chemistry 

 is urgently reqiured. Desselberger * ^ has described a histological study 

 of the processes involved in the deposition of carotenoids in feathers. 

 The qualitative distribution of carotenoids in birds is given in 

 Table 47. 



Metabolism 



Very little advance in our understanding of carotenoid metabolism 

 in birds (apartirom that of the vitamin A-active carotenoids), especially 

 hens, has taken place since the general picture was provided by the 

 classical researches of Palmer and Kempster, * * although a considerable 

 amount of detail has been filled in. 



Xanthophylls only are laid down in the skin and shanks as esters and 

 these are mobilized in the free state into the eggs daring the laying 

 season. This transfer occurs even if during the laying period the hens 

 were maintained on a carotenoid-free diet, having had, until laying 

 began, access to a carotene-nch diet, * ' Reports differ concerning what 

 happens when laying hens are maintained on a xanthophyll-free diet ; 

 Bohren and his colleagues*' found that the xanthophylls slowly dis- 

 appeared from the yolks of the eggs, whilst Grimbleby and Black *'^ 

 found a high pigment content in the first three eggs laid after trans- 

 ference to the xanthophyll-free diet, followed thereafter by a rapid 

 decline owing to exhaustion of the body stores of xanthophylls. On the 

 other hand the laying down of xanthophylls in eggs is a rapid process * ^ 

 for hens maintained on a carotenoid-free diet will transfer xanthophylls 

 to the yolk within 48 hours of being transferred to a carotenoid-rich 

 diet. 1 ^ About five eggs must be laid before maximum colour is 

 obtained. * "* 



Titus, Fritz, and Kauffman*® found no difference in pigmentation 

 of yolks of cross breeds and white Leghorn pullets on the same diet. 



The change in relative concentrations of carotenes and xanthophylls 

 in the egg and liver compared with the food is very striking. In normal 

 green food this ratio is about 1 : 3 and in eggs and liver, although it 

 depends on the diet, it can reach 1 : 30. ^ ° This is mainly due to the 

 fact that carotene (and cryptoxanthin) are efficiently converted into 

 vitamin A ^ *. ^ o, 5 1 presumably in the ^ut wall, ** 2, 5 s ^^d not because 

 they are poorly absorbed from the lumen. It should be noted, however, 

 that the material in which the carotenoids are fed, affects the efficiency 



264 



