*Importance of Water.*—Water finds its way into the body as a pure liquid, as a part of such mixtures as coffee, chocolate, and milk, and as a constituent of all our solid foods. (See table of foods, page 126.) It is also formed in the body by the oxidation of hydrogen. It passes through the body unchanged, and is constantly being removed by all the organs of excretion. Though water does not liberate energy in the body nor build up the tissues in the sense that other foods do, it is as necessary to the maintenance of life as oxygen or proteids. It occurs in all the tissues, and forms about 70 per cent of the entire weight of the body. Its presence is necessary for the interchange of materials at the cells and for keeping the tissues soft and pliable. As it enters the body, it carries digested food substances with it, and as it leaves it is loaded with wastes. Its chief physiological work, which is that of a _transporter of material_, depends upon its ability to dissolve substances and to flow readily from place to place. *Relative Quantity of Nutrients Needed.*—Proteids, carbohydrates, and fats are the nutrients that supply most of the body’s nourishment. The most hygienic diet is the one which supplies the proteids in sufficient quantity to rebuild the tissues and the carbohydrates and fats in the right amounts to supply the body with energy. Much experimenting has been done with a view to determining these proportions, but the results so far are not entirely satisfactory. According to some of the older estimates, a person of average size requires for his daily use five ounces of proteid, two and one half ounces of fat, and fifteen ounces of carbohydrate. Recent investigations of this problem seem to show that the body is as well, if not better, nourished by a much smaller amount of proteid—not more than two and one half ounces (60 grams) daily.(53) While there is probably no necessity for the healthy individual’s taking his proteid, fat, and carbohydrate in _exact_ proportions (if the proportions best suited to his body were known), the fact needs to be emphasized that proteids, although absolutely necessary, should form but a small part (not over one fifth) of the daily bill of fare. In recognition of this fact is involved a principle of health and also one of economy. The proteids, especially those in meats, are the most expensive of the nutrients, whereas the carbohydrates, which should form the greater bulk of one’s food, are the least expensive.