From various estimates by different writers the average elementary composition of the human body may be presumed to be approximately as follows:

Traces of some other elements such as manganese and aluminium may perhaps be normal constituents of the body also, and even arsenic has been discussed as a possible essential element. In this book only those elements are discussed of which the amounts concerned in daily metabolism can be measured quantitatively by present methods.

Since all of the substances in the body are continually undergoing disintegration and renewal, it follows that there must be a constant metabolism or exchange of every element which enters into body structure. More or less of each element must each day be metabolized and eliminated; and, if equilibrium is to be maintained, an equal amount must be supplied.

Simple proteins furnish only five of the fifteen chemical elements which are known to be essential to human nutrition, while fats and carbohydrates are composed of but three of these five. Ten of the fifteen essential elements, or seven of the twelve which are essential in amounts sufficiently large to be measurable by present methods, must therefore be furnished by some ingredients of the intake other than simple proteins, fats, and carbohydrates. These same elements are found to remain either wholly or largely in the ash of food materials when the latter are burned in the air; and when the food is metabolized in the body they are excreted chiefly in the form of mineral matter. These elements are therefore grouped as "ash constituents," "minerals," "mineral salts," "inorganic elements," or "the inorganic foodstuffs"; and their metabolism is commonly designated as "the mineral metabolism." None of these terms is entirely appropriate. To designate the elements which remain in the ash when food is burned as ash constituents is accurate but not very instructive, since the materials of which a food ash is composed may have existed in quite different forms of combination in the food before it was burned. The terms "mineral" and "inorganic" are likely to be somewhat misleading. Some of the elements (as sodium and chlorine) do exist in the food and enter and leave the body in inorganic forms; others (as iron and sulphur) exist in the food and function in nutrition as essential constituents of organic matter and become inorganic only as the organic matter is oxidized, i.e. only in the late stages of their metabolism; still others (as phosphorus) are supplied to the body by the food in both organic and inorganic forms.

The elements concerned in "the mineral metabolism" may exist in the body and take part in its functions in at least three kinds of ways:

(1) As bone constituents, giving rigidity and relative permanence to the skeletal tissues.

(2) As essential elements of the organic compounds which are the chief solid constituents of the soft tissues (muscles, blood cells, etc.).

(3) As soluble salts (electrolytes) held in solution in the fluids of the body, giving these fluids their characteristic influence upon the elasticity and irritability of muscle and nerve, supplying the material for the acidity or alkalinity of the digestive juices and other secretions, and yet maintaining the neutrality or slight alkalescence of the internal fluids as well as their osmotic pressure and solvent power.

A man under average conditions of diet, activity, and health usually excretes daily from 20 to 30 grams of mineral salts, consisting essentially of chlorides, sulphates, and phosphates of sodium, potassium, magnesium, and calcium (as well as am-monium salts from the protein metabolism).

The purpose of this chapter and the one following is to sketch briefly the metabolism of these substances, with a more detailed quantitative study of the three elements (calcium, phosphorus, and iron) which assume an especial prominence in the practical problems of nutrition.