The curative method of vitamin A assay

THE curative method of vitamin A assay, which is still the method most widely used for the biological estimation of vitamin A, is thus summarised in the Medical Research Council's Report on "Vitamins" [1932]: "In a curative test, the animals are allowed to exhaust their reserves of vitamin A, as indicated by cessation of growth, after which doses of the test substance are administered for about 5 weeks.... Comparisons are drawn between doses which give equal average increments in weight." The method is apparently simple, but reference to the literature makes it evident that in practice the results obtained are far from satisfactory. Serious discrepancies occur not only among the results obtained by workers in different laboratories, but even among those obtained in one laboratory by the same workers using carefully standardised procedure. Thus Drummond and Morton [1929], in analysing the results obtained for the vitaminA content of a series of oils, give the case of two litter-mates of which one lost 24 g. in 4 weeks while the other, on the same dose of cod-liver oil, gained 34 g. in 5 weeks. Such extreme variations are by no means infrequent in their occurrence and could probably be paralleled by most workers from their own data.