The relative biological efficiencies of the vitamin A and carotene of butter.

THERE is still a great deal of uncertainty regarding the relative biological activities of vitamin A and carotene. The subject has been reviewed recently by Moore [1933], who has summarised the findings of various workers and has pointed out the very wide variations in the reported minimum doses of the two substances. On the basis of his own experiments in which he directly compared f-carotene with a vitamin A concentrate from turbot-liver oil, Moore came to the conclusion that fl-carotene possesses an activity equal to that of about the same weight of vitamin A of E" 1328mu =1600. Carr and Jewell [1933] compared their richest vitamin A concentrate (E "/4328mp= 1600) with the International Carotene Standard and found that the concentrate was 1F6 times as active as the Carotene Standard. As, according to Dyer et al. [1934], pure fl-carotene has about 175 % of the vitamin A potency of the International Standard, the findings of Moore [1933] and Carr and Jewell [1933] are in good agreement. The problem of determining the relative values of vitamin A and carotene has been approached by us from a different angle. In the course of work on typical English milk, carried out at the National Institute for Research in Dairying, we have compared the vitamin A activities of Guernsey and Shorthorn butters produced under identical conditions of feeding and management. In a preliminary paper the results of biological tests on Guernsey and Shorthorn butters, together with colorimetric determinations of their vitamin A and carotene contents, have been reported [Kon and Booth, 1933]. It was found that Shorthorn butter gives a more intense blue colour with the antimony trichloride reagent than a corresponding Guernsey butter, though, as is well known, the latter is more yellow and contains more carotene. On the other hand we found no appreciable difference in the vitamin A potencies of the butters when tested on rats. It seemed that a biological estimation of the activity of butters containing different amounts of vitamin A and carotene could be combined with