In some of these early experiments with purified food-stuffs, Osborne and Mendel (46) added about one gram per week's ration of dry feces of normally fed rats to their experimental diets. Animals which were rapidly declining on the diets of purified casein, carbohydrate, fat (lard) and a salt mixture, were markedly benefited by the addition of one per cent or thereabouts of feces of normal rats to their diet. Some were able to grow for a time and the decline of all was checked by this addition. To explain this beneficial effect of the inclusion of feces in the diet, several possibilities were suggested and discussed. One was the possibility that the bacteria thus introduced into the alimentary tract were able to elaborate substances (e. g., amino-acids) necessary to complete the defects in the purified protein contained in the diet. This seemed inadequate, because with other purified proteins serving as the sole source of amino-acids, the inclusion of feces did not render the diets complete. It is possible that the body cannot thrive unless the alimentary tract harbors a flora of certain types of microorganisms. This is a very difficult point to establish, and indeed, has not been finally settled. There was also the possibility that the small amount of protein introduced into the ration with the fecal bacteria might be sufficient to supplement the deficiencies of the protein of the diet.

In the light of later developments it appears that the rats which were confined to a diet of purified casein and otherwise of purified food substances, were benefited primarily by the inclusion of both fat-soluble A and water-soluble B, which the feces of normal rats contained. This served to so far complete the diet that the animals were given a new lease of life.