Five months later Osborne and Mendel (10) described experiments entirely analogous to those of McCollum and Davis, in which their animals were confined to purified protein, starch, "protein-free milk," lard and butter fat.

Although the final evidence which Osborne and Mendel offered as proof of the unsuspected value of butter fat in nutrition was of the same kind as had been presented five months earlier by McCollum and Davis, in which butter fat and egg yolk fat were shown to be similar, and superior to lard or olive oil, their paper attempted to minimize the importance of the work of McCollum and Davis. They stated that "In none of their (McCollum and Davis) published records was the recovery so rapid as in most of ours, nor was the rate or extent of growth, after reaching the previous maximum weight, any greater than on the butter fat-free diet earlier supplied." Further, "although the data furnished by McCollum and Davis strongly indicate that butter fat has a marked influence on growth, they by no means prove that butter fat contains something essential for the metabolism of growth apart from that of maintenance."

It is now clear that the reason for the better growth of Osborne and Mendel's animals when butter fat was added to the diet was due to the fact that their "protein-free milk" contained an abundance of a second dietary factor, the anti-neuritic substance which prevents beri-beri, whereas the diets of McCollum and Davis contained decidedly less of this substance which was present as an impurity in the supposedly pure milk sugar which their diet contained. It seems certain that there is no substance essential for maintenance, apart from growth, as the last quoted sentence suggests. This idea has been in the minds of several investigators, but the evidence that normal maintenance can be secured on any diet lacking in a complex necessary for growth is very slender indeed. This point will be discussed in Chapter III (The Earlier Views Of Nutrition Problems. 43. Dr. Beaumont'S Views On Digestion).

Early in 1914 Osborne and Mendel (11) confirmed the observation of McCollum and Davis that egg yolk fats and cod liver oil had the same effect on growth that butter fat exerted, and that lard did not stimulate growth as did egg and milk fats. They investigated almond oil and found it to resemble olive oil in the respect that it did not promote growth.