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8
The carbohydrates form only a very small proportion of the body
tissues, less than 1 per cent. Starches and sugars, which are very
abundant in ordinary food materials, are important food ingredients,
because they form an abundant source of energy, and are easily
digested. They may be and often are transformed into fat in the body.
REFUSE.
Food, as we buy it at the market or even as it is served on the table,
contains more or less of materials which we can not or do not eat,
and which would have little or no nutritive value if we did eat them;
such, for instance, as the bones of meat and fish, the shells of eggs, and
the skins and seeds of fruits and vegetables. In discussing the chemical
composition of foods such portions are usually counted as refuse, but
they make an important item when we consider the actual cost of the
nutrients of food. The materials grouped together as refuse contain,
in part, the same ingredients as the edible portion, though usually in
very different proportions. Thus bones are largely mineral matter,
with some fat and protein; eggshells are almost entirely mineral
matter; bran of wheat has a high content of fiber or woody material.
Generally speaking, vegetable refuse is characterized by a high content
of these latter constituents. In some cases material which is edible is
classed as refuse because the flavor is objectionable. Thus peach and
plum pits are too highly flavored to be agreeable if eaten in quantity,
and are commonly thought to be actually injurious.
FOOD AS BUILDING MATERIAL AND FUEL.
THE BODY AS A MACHINE.
Blood and muscle, bone and tendon, brain and nerve-all the organs
and tissues of the body--are built from the nutritive ingredients of
food. With every motion of the body, and with the exercise of feeling
and thought as well, material is consumed and must be resupplied
by food. In a sense, the body is a superior machine. Like other
machines, it requires material to build up its several parts, to repair
them as they are worn out, and to serve as fuel. In some ways it uses
this material like a machine; in others it does not. The steam engine
gets its power from fuel; the body does the same. In the one case
coal or wood, in the other food, is the fuel. In both cases the energy
which is latent in the fuel-the potential energy, as it is called in scientific
language-is transformed into heat and power. When the coal
is burned in the furnace a part of its potential energy is transformed
into the mechanical power which the engine uses for its work; the rest
is wasted 'n the heat which the engine does not utilize. Likewise the
potential energy of food is transformed in the body into heat and
