576 IX. CAROTENOIDS AND VITAMINS A 



believed to play a role in color differentiation. It is also restricted to the 

 retina in the case of the chicken. It would therefore seem probable that 

 the carotenoids and vitamins A have a molecular structure which renders 

 them particularly adaptable in photoreception. For a further discussion 

 of the distribution and photoreceptor function of carotenoids and of vita- 

 mins A, the reader is referred to the classical review of Wald,^^'* and to the 

 discussion in The Lipids, Vol. I, pages 680 to 683. 



9. The Requirements of Animals for Carotenoids and for Vitamins A 



(1) Introduction 



In assessing the requirement of an animal for vitamin A, one is faced with 

 the problem as to what criteria should be employed for such an evaluation. 

 In the past, it has usually been considered that the minimum requirement 

 necessary to provide satisfactory growth and to prevent lesions and dys- 

 functions due to vitamin A deficiency represents an approximation of the 

 level in question. However, the above procedure for determining the 

 vitamin A requirement is arbitrary. 



Various physiologic and pathologic conditions cause a variation in the 

 vitamin A requirement. Moreover, factors which affect the absorption of 

 vitamin A will influence the amount needed in the diet. Any situation 

 which alters the conversion of ;S-carotene to vitamin A will influence the 

 quantity of vitamin A required insofar as the source of the vitamin is pro- 

 vitamin A. The vitamin requirements of a number of species have been 

 reviewed recently, including that of the rat by Brown and Sturtevant,^^^ 

 that of the mouse by Morris,^" that of chickens, pigeons, turkeys, and 

 ducks by Bird,^^* that of the cotton rat and the hamster by Schweigert,^^^ 

 and that of the guinea pig by Mannering.^^" The most up-to-date review 

 of vitamin A requirements is that of Rubin and De Ritter,^^^ published in 

 1954. Other recent articles include the summary of the requirements of 

 animals by Mattson,'^^^ and of human beings reported by Mason. ^^^ 



The criteria which have been employed for determining the vitamin A 

 requirement include the amount necessary for good or optimum growth, ^^^ 



"6 R. A. Brown and M. Sturtevant, Vitamins and Hormones, 7, 171-199 (1949). 

 ''' H. P. Morris, Vitamins and Hormones, 5, 175-195 (1947). 

 ^'8 H. R. Bird, Vitamins and Hormones, 6, 163-173 (1947). 

 ^'^ B. S. Schweigert, Vitamins and Hormones, 6, 55-67 (1948). 

 9« G. J. Mannering, Vitamins and Hormones, 7, 201-221 (1949). 

 "» S. H. Rubin and E. De Ritter, Vitaynins and Hormones, 12, 101-135 (1954). 

 9^2 K. E. Mason, Requirements of Human Beings, in W. H. Sebrell, Jr., and R. S. 

 Harris, The Vitamins, Vol. I, Academic Press, New York, 1954, pp. 171-175. 



