CAROTENOIDS AND VITAMINS A IN THE BLOOD 457 



the blood stream in general appears to be the pathway of transfer of the 

 pro\'itamins A and of the xdtamins A in the animal after absorption has 

 been completed. The postprandial levels of carotene and of vitamin A re- 

 flect to some extent the reserve of these components stored in the tissues. 



(7) ^-Carotene and Other Carotenoids in the Blood 



iS-Carotene occurs in the blood of the species of animals which do not 

 convert this pro\atamin A completely to vitamin A in the walls of the gut. 

 However, ;5-carotene occurs in the blood stream in these species only when 

 it is obtained from exogenous sources. If none of the pigment is present in 

 the diet over an extended period, the amount stored in the tissues is gradu- 

 ally depleted, so that eventually the proportion in the blood decreases or 

 entirely disappears. A number of investigators have demonstrated the 

 presence of /^-carotene in human blood^^-^^*'^^" and in that of cattle.^^-^*®-^^^-^" 



Carotenoids other than /3-carotene have been found in greater or lesser 

 amounts in the blood. The presence of carotenoids must be considered as 

 adventitious, since they reflect the composition of the diet. However, a 

 more important consideration in relation to the presence of the several 

 carotenoid components is species. In the case of man (Group A), all of the 

 dietary carotenoids are present in the blood. On the other hand, in the 

 case of cattle (Group B), carotene forms the major component, and only 

 minor amounts of the xanthophylls are present. In the animals in Group 

 C, no carotenoids appear in the blood, irrespective of their presence in the 

 diet, inasmuch as they cannot be absorbed unchanged. Thus, no carote- 

 noids occur in the blood of pigs, rats, and mice, even when they are 

 present in the diet. In the case of chickens and birds, which have been 

 referred to earlier as Group D, only carotenols are present in the blood. 



In the case of man, in addition to /3-carotene, lycopene^---^^ appears in 

 the blood. Small amomits of xanthophylls are fomid in the blood of 

 cows,^®'^^^ but not in that of newborn calves.^*^ Although Palmer and 

 Eckles^^*-^^^ considered that the latter components were free, more recent 

 work has estabhshed the fact that they occur in human blood in esterified 



25* H. Willstaedt and T. K. With, Z. physiol. Chem., 253, 40-46 (1938). 



^ J. W. Mehl, Personal communication, 1944; also cited by F. R. N. Gurd, J. L. 

 Oncley, J. T. Edsall, and E. J. Cohn, Lipoproteins, General discussion Faraday Soc, 

 No. 6, 70-74, Aberdeen Univ. Press, 1949, p. 73. 



266 B. V. Euler, H. v. Euler, and H. Hellstrom, Biochem. Z., 203, 370-384 (1928). 



257 J. Ganguly, J. W. Mehl, and H. J. Deuel, Jr., /. Nutrition, 50, 73-84 (1953). 



25S E. V. Ddniel and T. Beres, Z. physiol. Chem., 238, 160-162 (1936). 



259 L. S. Palmer and C. H. Eckles, J. Biol. Chem., 17, 211-222, 237-243 (1914). 



