ose found in the
body, they cannot be utilized in their original form, but must be
split into simpler substances from which the cells of the various
tissues throughout the body may select those particularly adapted for
their purpose. These transformed substances are known as amino acids,
the production of which is a result of digestion in the digestive
tract. There are about seventeen of these acids entering into the
construction of the common proteins. One scientist has likened these
units to letters of the alphabet, which, being combined, spell many
proteins. When a protein contains all of the essential units, it may
be said to be "complete," the best example of which may be seen in
milk, eggs, and meat. When a protein lacks some of the essential
elements, or letters of the protein alphabet, it is said to be
incomplete. Gelatin is the best example of this type of protein, but
the cereals and beans must likewise be supplemented by other
substances; milk being the one most generally used for this purpose.
For the purpose of building young tissues, and maintaining those
already mature, it is logical to use foods containing the foodstuffs
in their best form; that is, those that not only contain the complete
protein, but also the requisite mineral salts and vitamines. Foods
lacking in some of these respects become adequate when supplemented by
these foods which can supply the missing constituents; hence, the use
of such incomplete protein foods need not necessarily be abandoned,
for, as in the case of cereals, the foods are both economical and
palatable, and, when used in addition to milk, furnish valuable
adjuncts to the dietary.

~Classification of Proteins.~--A brief description of some of the more
important proteins with which we are chiefly concerned will serve to
simplify the formulation of a diet. Those assuming the most important
position in nutrition and food are ~globulins~, ~albumens~,
~nucleoproteins~, ~phosphoproteins~, ~hemoglobins~, and ~derived
proteins~ such as proteoses and peptones. The albumens and globulins
associated together occur in the tissues of both animals and plants.
The albumens are richer in sulphur than the globulins and are found
more abundantly in the animal fluids, such as the blood, while the
globulins predominate in the more solid tissues of animals and in
plants. The close association of these two proteins is particularly
noticeable in the blood and cells. They have different characte