Observations on certain factors necessary for the normal nutrition of the rat

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Abstract
Accumulated evidence is presented to show that yeast contains a factor other than vitamin B1 or B2. and that milk contains a factor other than those present in yeast, both of which are apparently required for the normal growth of rats. But even supplying both does not maintain entirely normal growth. The factor present in milk is different from other known vitamins, A, D, C or E, and is not supplied by adding lactalbumin or the essential ammo-acids to the diet. Food-intake records show that increased growth on the administration of milk may occur without appreciably increased food intake. The fact that the milk factor can be inactivated by autoclaving argues against the inorganic nature of this substance. It is also present in egg-yolk, egg-white, spinach, grass and alfalfa, but scantily in pig- and sheep-liver. Carotene or chlorophyll cannot replace the milk factor in the diet. No satisfactory method has yet been devised for extracting the substance. A method of assay is indicated. Male rats show a much more marked response to it than females. Milk and "light white casein" show7 appreciable variations in potency.