Investigations into vitamin A-free basal diets. III

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Abstract
A satisfactory vit. A-free basal diet is one that gives the same growth as the best stock or colony diet when supplemented with adequate vit. A. A diet is described that satisfies this criterion, if allowance is made for slightly inferior food intake. With the diet relatively low limits of error are obtained in the biological assay of vit. A when only [male] rats are used; a 3 wks. test is no less accurate than one of 5 wks. Much of the error of such tests depends upon innate variability of animal stock, despite intensive inbreeding and raising under standardised conditions. Thus body-wt. of "normal" animals raised on full stock diet has a coefficient of variability of about 15% at 30 days of age and about 10% at 120 days. For stock animals on a stock diet, increase of wt. between 30 and 120 days and wt. at 120 days is correlated with wt. at 30 days of age. A lowering of vit. A reserves of weanling animals to a very low or zero value destroys this correlation and growth-rate becomes independent of initial wt.