Since mixtures of foods having similar deficiencies cannot greatly enhance each other's values in meeting the nutritive needs of an animal, we should expect, from the properties of the foods of the seed, tuber, fruit, root and muscle tissue group, that combinations of any articles belonging to this class, should not form a satisfactory food supply. Experience has now amply demonstrated that this is the case. The conclusion is warranted, therefore, that it is wise to select foods from different groups, based upon their nutritive qualities in order to properly balance the resulting mixture so as to make it near in its constitution, what the animal body needs for its proper maintenance. Numerous feeding experiments have shown the wisdom of this plan. Most food substances now in common use do not supplement each other in a very effective manner. Milk, and the thin leaves of plants are of especial value for enhancing the dietary properties of nearly all other foods, and they are therefore of particular interest and importance. It was for this reason that I have termed them "protective foods." In a lesser degree, eggs and the glandular organs of animals serve this purpose of improving the quality of cereals, tubers, fruits, roots and of meats of the muscle type, but this improvement is incomplete, because of their lack of sufficient calcium.

The above statement regarding the inadequacy of a diet derived from seeds, tubers, fleshy roots, fruits and muscle meats, is made again as in the first edition of this book, notwithstanding the assertions of Steenbock that certain yellow pigmented root vegetables, such as carrots and yellow turnips, and yellow maize, as contrasted with white or red varieties, are decidedly richer in fat-soluble A. There appears good reason to believe that certain varieties of vegetables are superior to certain others with respect to their content of fat-soluble A, and apparently yellow maize represents a case where the yellow pigment accompanies the superior nutritive property. The idea that the anti-ophthalmic substance is related to, or associated generally with the yellow pigments of plants has been conclusively disproved by the work of Palmer and his associates (4). But even if the statement is accepted that certain varieties of vegetables can serve as an adequate source of fat-soluble A, it would not alter in any way the truth of the generalization that diets containing yellow pigmented products would not prove satisfactory. The diets of this character would be deficient in certain mineral elements, and sufficiently so to make it impossible to secure normal development in young animals. The first of these in importance would be calcium, but sodium chlorid and phosphorus would also contribute, by their relative deficiencies, to prevent the normal nutrition of the animals.