It is very significant that the quantities of milk, eggs, cheese, fish and butter contained in diet (2) over diet (1) was effective in preventing the occurrence of rickets. Ferguson did not consider the differences in the composition of these types of diets sufficient to support the view that the diet had any importance as a factor in the production of rickets. When these diets are evaluated in the light of present day knowledge, it is easy to see that neither is of very good quality for the promotion of growth or the maintenance of health, but that diet (2) is distinctly better than (1).

A similar comment may be made concerning the rachitic diet described above, which became non-rachitic on the addition of butter. The basal diet which induced rickets was poor. One factor in which it was distinctly below the optimum was its content of fat-soluble A. The addition of this substance improved the diet, not only because of its enhancement in fat-soluble A, but because any other defects in the diet would become manifest in the condition of the animals, to a lesser degree when more of the needed fat-soluble A was present. We have already seen that any single factor in a diet may be well below the optimal in quality, without its effects becoming apparent. They appear, however, when another factor is reduced in quality so as to increase the total burden which the tissues must tolerate, in the way of badly constituted nutriment.

Mellanby's hypothesis would seem to derive support from the century long use of cod liver oil as a therapeutic substance in the treatment of rickets and from such studies as those of Schabad (10), who showed that the exhibition of cod liver oil increased retention of calcium salts by the body.