164 VITAMINS A AND CAROTENES 



individual carotenoids. Moreover, within recent years, particularly in the 

 laboratories of Deuel and Zechmeister,'* experiments have shown that the 

 stereoisomeric form of a particular carotenoid will markedly influence its 

 biological potency. For example, it was found that neo-jS-carotene U had 

 only 38 % of the potency of all trans-/3-carotene. These isomeric changes 

 can be of considerable importance, depending on the treatment to which 

 the carotenoid-containing feeds have been subjected. If corrections are 

 not made for the presence of isomers, estimates as much as 30 % in excess 

 of the true potency can result.^ 



Many of the studies on the vitamin A requirements of animals were made 

 before these differences in biopotency were fully appreciated. This would 

 account for some of the variations among the reported results leading to 

 estimates which are in error on the high side. Since under practical feeding 

 conditions, mixtures of vitamin A and carotenoids occurring in natural 

 foods are fed, these values have the advantage of more general applicability. 



In 1943 the National Research Council's Committee on Animal Nutri- 

 tion established a series of subcommittees to prepare nutritional standards 

 for domestic animals of economic importance. These subcommittees were 

 composed of members who had made a speciality of the nutrition of a par- 

 ticular species. The recommended allowances are revised when the avail- 

 ability of new data warrants. These values are to be recommended for prac- 

 tical feeding conditions, since they were established for this purpose and 

 contain, moreover, an adequate safety factor to meet exigencies that might 

 arise. 



It is not possible to summarize the requirements of all animals on a 

 uniform basis, such as diet composition or body size, because of variations 

 in experimental technique used by various investigators. Where sufficient 

 data were given in the original publication, the values have been recalcu- 

 lated to allow the use of a single basis for any one species. For the sake of 

 uniformity and to allow comparisons between species, all values are re- 

 ported in terms of international units of vitamin A. In the case of cattle, 

 horses, swine, and sheep most of the values were obtained b}' calculation 

 from their carotene equivalent, since carotene is usually administered as 

 the source of the vitamin in the studies on these species. 



2. Rats and Mice 



Numerous criteria have been used in studying the vitamin A recjuire- 

 ment of the rat. The final recommended value, of course, depended on the 

 standard employed. The results of some of these studies are sinnmarizod 

 in Table XVIII. From these data, it would seem that the daily intake allow- 



* L. Zechmeister, Vitamins and Hormones 7, 57 (1949). 



6 E. M. Bickoff and C. R. Tlionipsoii, /. Assoc. Offic. Agr. Chemists 32, 775 (1949). 



