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Comparative Assay for Vitamin B₁₂ in Certain Milk Products by Various Rat Growth Methods

Dairy Husbandry Research Branch, Agricultural Research Service, U. S. Department of Agriculture, Beltsville, Maryland

Factors affecting the values obtained by various rat growth assay methods for vitamin B₁₂ have been studied.

In the presence of maximally effective levels of vitamin B₁₂, certain milk products frequently tended to give depressed growth whether included in casein or plant protein rations. This depression was intensified by including iodinated casein in the ration. It could be alleviated in some instances by increasing the level of B₁₂ in the ration or by altering the method of substitution of the test material in the ration.

With plant protein rations containing soy protein and deficient in vitamin B₁₂, either a low response or none at all was observed when dried skim milk or crude casein was included in the ration. These results were at least partially attributable to the lactose or protein content of these milk products or to both. With corn-soy rations, a better response was obtained.

Comparative assays of one commercial lot each of dried skim milk and crude casein were made using extracted casein and corn-soy assay rations. With crude casein, essentially the same assay values were obtained in all cases when the rations were adjusted to have the same protein level. With dried skim milk assay values were affected not only by the method of substitution of the milk but also by the type of ration used, the method of depletion of the young and the amount, although not the presence, of iodinated casein in the ration.