Theory of Foods 3

sustain life, these must be gotten second hand from Nature through
plants which store them up in the form of cellulose, starch and sugar,
and are then in a form suitable for human use. Although the air
contains a large percentage of nitrogen, it is not upon this we depend
for the amount necessary in our systems. It too must come from our
foods, and it is to the class of foods known as proteids—those con-
taining a relatively large percentage of nitrogen—that we look for this
essential material. Foods supplying this use are, meat, cheese, milk,
eggs, legumes, gluten of wheat, etc. In the case of animal proteid
such as meat, the nitrogen is obtained from the vegetable foods upon
which the animal feeds, hence it is gotten second hand from Nature.
Milk and eggs are Nature’s perfect foods—that is, foods containing all
the necessary nutrients in the proper proportion. These are perfect
only for the young; as, for instance, milk is too bulky for an exclu-
sive diet of the healthy adult, and eggs are perfect only for the young,
undeveloped chick. As soon as it escapes from the shell it requires
some carbohydrates to supply heat and energy. Meat is one of the
chief sources of proteid, but may be replaced by egg, cheese and le-
gumes. Cheese contains even a larger percentage of protein than
meat. One pound of cheese contains about twice as much nutritive
material as one pound of meat, but in a form possibly more difficult
of digestion to many than meat protein, on account of being so highly
concentrated. The vegetable protein (legumin) is considered more
difficult of digestion than animal protein, and is, therefore, not so
valuable a source as animal proteid. The chief use of proteid in the
body is to build up the tissue and keep it in repair as it becomes used
or worn. |

Requiring as the system does only a certain amount of each class
of foodstuffs to meet the daily demands—the amount varying with
the different ages, occupations, etc.—it is rather a troublesome problem
for the housewife to obtain the exact amount of each foodstuff for each
meal or for each day. If, however, one meal is composed of food
which is high in proteid value, the next meal might be made up of
foods rich in carbohydrates, and thus balance up the daily rations so
that the body would receive a fairly accurate amount of each and yet
not be overtaxed in disposing of waste material.

The element in fuels such as wood, coal, etc., which makes them of
fuel value, is carbon, and the same is applicable to our foods, so that
those containing a high percentage of carbon will be best suited to
this purpose. The groups of foodstuffs known as fats and carbohy-
drates are, therefore, the best source of energy, since they are rich in
carbon. They contain carbohydrates, hydrogen and oxygen. Foods
containing only these three elements are not sufficient to maintain
life for any great length of time, since their use in the body is to keep
it warm and supply energy to enable it to work, and it also requires
other materials to build up the tissues and repair them as they are
wearing out. The hydrogen and oxygen in the carbohydrates (starches