It was pointed out in the preceding chapter that repeated failures characterized experimental studies in which animals were confined to diets containing only the well established food principles. These failures had practically established the belief that this line of study was unprofitable, because all animals, for some unknown reason, died when confined to such a regimen. The apparently conclusive results of McCollum with such mixtures aroused hope that a new and valuable body of data could be secured which would reveal the sources of the valuable and less valuable proteins among our natural foods.

In 1909 Osborne and Mendel took up this line of investigation and proceeded to feed individual proteins with a diet which was otherwise composed of purified starch, lard and mineral salts. At the outset of their studies they accepted the evidence which McCollum had presented that this could be successfully done and planned to interpret on the basis of the capacity of young rats to grow on a constant diet, varied in successive experiments only with respect to its protein, the relative merits of such proteins as tissue builders. They did not, however, actually duplicate the experimental conditions of McCollum, but adopted a diet which they regarded as having essentially the same properties. This consisted of purified protein, such as casein of milk, starch, lard and a salt mixture. They met with complete failure in these attempts to nourish their animals with such a diet. They tried a salt mixture recommended by Rohmann (1) with which he had met with success with a diet consisting in great measure of purified food substances, and also the salt mixture described by McCollum. In all cases, however, their animals declined steadily from the time they were confined to such diets. They proceeded a step farther and established the fact that the failure of the animals was not due to lack of appetite, since the food intake was found to be sufficient to promote growth provided the composition of the food had been satisfactory. At that time no one was able to see any important difference between the quality of a purified protein, three carbohydrates, one being milk sugar, two kinds of fats, one being butter fat, and a salt mixture, as compared with a simple mixture of protein, starch, lard and salts.