Vitamin A studies with rats and pigs

Abstract

MANXY aspects of the metabolism of vitamin A and of its precursors are still insufficiently known; this is especially true with regard to information available for animals other than the rat. Two of us (A. S. F. and S. K. K.) have for some time been interested in the importance of vitamin A in pig feeding. For reasons both practical and scientific we wanted to gain more knowledge of the utilization by the pig of certain carotenoids, of the relative value for the pig of carotene and of preformed vitamin A and of the minimum vitamin A requirements of this animal. Another of the authors (T. H. M.) has studied on rats the biological activity of vitamin A alcohol and of natural vitamin A esters and further comparisons using a different animal were thought likely to be of value. The present joint paper gives the results of all these inquiries. The biological tests were carried out at Shinfield and the chemical and physical manipulations were mostly done in the laboratories of The British Drug Houses, Ltd. It is generally accepted that in order to exhibit vitamin A activity a caro- tenoid pigment must contain a ,B-ionone ring or rings. Recently, however, Lund [1938] and Petersen [1939] reported that zeaxanthin was effective in preventing symptoms of vitamin A deficiency in pigs, but that 'xanthophyll' (lutein) was inactive in this respect. If confirmed, this finding would be of considerable importance, not only because it would demonstrate the unique ability of the pig to convert a dihydroxycarotene into vitamin A. but also from the practical standpoint of pig feeding. Maize, an important pig food, contains relatively more zeaxanthin than carotene and cryptoxanthin [Kuhn & The observation of Lund [1938] that the pig can make use of dihydroxy-,fcarotene but not of dihydroxyy-o-carotene (lutein, xanthophyll) is certainly puzzling, as it implies that a hydroxy-,B-ionone ring is utilized in one case but not in the other. The preparation of zeaxanthin used by Lund [1938] was admittedly contaminated with carotene, and we felt that for this and other reasons a re- investigation of the value of zeaxanthin as a source of vitamin A for pigs was amply justified.