of the
bones and teeth; and the salts containing iron renew the hemoglobin of the
blood. Others perform important functions in the vital processes. The
mineral compound of greatest importance perhaps is sodium chloride, or
common salt.(52) This is a natural constituent of most of our foods, and
is also added to food in its preparation for the table. When it is
withheld from animals for a considerable length of time, they suffer
intensely and finally die. It is necessary in the blood and lymph to keep
their constituents in solution, and is thought to play an important role
in the chemical changes of the cells. It is constantly leaving the body as
a waste product and must be constantly supplied in small quantities in the
foods.

*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