The most marked deficiency of milk is in its content of iron, which is very low. The newborn mammal is provided with a store of this element in its tissues, sufficient to tide it over the period while it is on an exclusive milk diet, but if a child were confined too long to milk as its sole food it would suffer from anemia for lack of iron.

Milk is a water rich food, containing about 87 per cent of water and 13 per cent of dissolved substances consisting for the most part of proteins, milk sugar and fats (49). It is too bulky to be satisfactory as the sole food for the adult, because of the difficulty of taking a sufficient amount to cover the energy requirements. It is, therefore, to be valued essentially as a food for supplementing other foods and for specifically correcting their deficiencies. The striking fact brought out by modern nutrition investigations is that most animal and vegetable foods are singly or collectively faulty in sufficient degree to interfere seriously with the physical development of the young and to lead to early deterioration of the adult. The most important consideration to be kept in mind is, therefore, the selection of food so as to compensate the deficiencies of one article by those of another and thus make a properly balanced dietary. For this purpose milk is truly the ideal food. The property of enhancing all the deficiencies of cereal grains, legume seeds, tubers, roots and muscle meats, is possessed only by milk and the leafy vegetables, and for this reason McCollum proposed, in 1918, that in order to make these two classes of foods stand out in importance as they deserve to do, they should be designated "protective foods" (50).

The supplementary value of milk for other foods depends upon its unique calcium content, on the quality of its proteins, especially their special fitness for completing the amino-acid quota and balancing the same, in vegetable foods of the storage tissue class, and on the quality of its fat.

It is of great interest that recent investigations which were sufficiently extensive to make them conclusive, have shown that the proteins of muscle, liver and kidney, are in general more valuable for enhancing the values of cereal and legume seeds than are the proteins of milk. It is only in this respect, however, that the animal tissues surpass milk (51).

159. The Fats Of Milk A Good Source Of Fat-Soluble A

The fats of milk are the most important source in the American and European diet of the dietary essential fat-soluble A. This, as has been pointed out, is not abundant in any seeds, tubers or roots, as compared with the amounts of it contained in butter fat of good quality. This is the anti-ophthalmic substance, fat-soluble A. It is essential for growth or for normal maintenance. This substance will be considered more in detail in a later chapter in connection with the deficiency diseases.