We require ten inorganic or mineral elements

Proteins consist of giant molecules which are resolved during digestion into about twenty relatively simple organic compounds known as amino-acids. We know that two of these can be synthesized by the body, and, pending further investigations, we may assume that the remaining eighteen must be supplied by the food. We must have the sugar glucose, which may be taken as such or derived from cane or milk sugar or from the various starches. These are all changed by the organs of digestion so as to introduce only glucose into the blood. We require at least ten inorganic or mineral elements: sodium, potassium, calcium, magnesium, chlorine, iodine, phosphorus, sulphur, iron, and copper. Probably this list of indispensable inorganic elements will eventually be extended to include several others, such as manganese, zinc, silicon, fluorine, and possibly nickel and cobalt, boron, etc. These last appear to be necessary for the development of plants, and may also be necessary for that of animals. At least six nutrient principles, called vitamins A, B, C, D, E, and G, are known to be required for normal nutrition. Among the twenty or more fatty acids known to chemists, at least twelve occur in the fats of our ordinary foods, but it appears from the results of experiment that only one of these, namely, linoleic acid, cannot be synthesized by the body from carbohydrate molecules. From the data available we may say with some assurance that the simplest diet which would furnish everything necessary for normal nutrition must contain appropriate proportions of at least thirty-six simple chemical substances. Actually, we eat daily many times this number in our ordinary foods of animal and vegetable origin.

Methods have been developed by means of which experiments on animals fed a single natural food (wheat, maize, etc.), which does not alone support satisfactory nutrition, supplemented with single or multiple additions of the indispensable nutrients, yield information concerning the nature of their deficiencies. Such experiments may be made nearly quantitative. Through such studies we have secured a considerable body of knowledge concerning the extent to which each of our more important natural foods-cereal grains, tubers, fruits, roots, leaves, meats, milk, eggs, etc.--furnishes the body with the indispensable nutrients. In addition, such experiments have shown us the nature and extent of the deficiencies of the different natural foods. On the basis of such data it is possible to theorize concerning which natural foods, individually lacking or deficient in one or another food principle, should, when combined, supplement each other's deficiencies. Experimental verification of predicted results confirms in a highly satisfactory manner the belief that we understand, to a great extent, the nutritive needs of the body and the dietary properties of most of our natural and manufactured foods.

Since most foods have been shown to be deficient or lacking in one or more of the nutrient principles, it is necessary for us to combine foods of unlike composition so that one will provide what the other lacks. Herein lies the cause of safety in variety in eating. The whole wheat kernel, which many people have long believed to be a complete food, is incapable, when fed as the sole source of nutriment, of supporting growth of the young or prolonged health in the adult. It is deficient in three respects: (1) it lacks sufficient calcium; (2) it lacks sufficient vitamin A; and (3) its proteins require supplementing with proteins from other sources which supply in abundance certain amino-acids which the wheat proteins contain in amounts too small to serve as building stones when food proteins are converted into body proteins. Even though wheat is supplemented with one or two of these substances in which it is deficient, the nutrition of an animal will not be so good as it will if all three are added to the diet.