It is easy now to see in the light of later experience why such skepticism existed regarding the necessity of hitherto unappreciated factors in the diet. Osborne and Mendel and McCollum had seen young rats grow for a considerable time when restricted to diets of supposedly pure food substances. These substances, however, always contained a considerable amount of lactose or milk sugar, and it was not appreciated at the time that impurities of a most important nature might still adhere to lactose of a relatively high degree of purity.

When it was found in 1913 that the addition of butter fat promoted growth on a diet, which consisted aside from this fat only of substances of accepted purity, every component of the diet being regarded as known chemically, it seemed very plausible that the only unidentified dietary essential for the rat was associated with the fat fraction. This view was at one time held by McCollum. In all such studies the question as to the degree of purity of each of the ingredients of the diet necessary to warrant the conclusion that no impurities of a significant nature were present could be decided only through critical and comparative experiments.