combine them with other substances that make up their living tissue.
Then human beings, as well as other living creatures, get their supply
of these needed salts from the plants that they take as food, this being
the only form in which the salts can be thoroughly assimilated. These
salts are not affected by cooking unless some process is used that
removes such of them as are readily soluble in water. When this occurs,
the result is usually waste, as, for instance, where no use is made of
the water in which some vegetables are boiled. As is true of water,
mineral matter, even though it is found in large quantities in the body,
is usually disregarded when food is purchased. This is due to the fact
that this important nutritive material appears in some form in nearly
all foods and therefore does not necessitate the housewife's stopping to
question its presence.

14. Protein.--The food substance known as protein is a very important
factor in the growth and repair of the body; in fact, these processes
cannot be carried on unless protein is present in the diet. However,
while a certain quantity of protein is essential, the amount is not very
large and more than is required is likely to be harmful, or, since the
body can make no use of it, to be at least waste material. The principal
sources of protein are lean meat, eggs, milk, certain grains, nuts, and
the legumes, which include such foods as beans and peas. Because of the
ease with which they are digested, meat, fish, eggs, and milk are more
valuable sources of protein than bread, beans, and nuts. However, as the
foods that are most valuable for proteins cost more than others, a mixed
diet is necessary if only a limited amount of money with which to
purchase foods is available.

15. So much is involved in the cooking of foods containing protein that
the effect of heat on such foods should be thoroughly understood. The