22. Nearly all foods are complex substances, and they differ from one
another in what is known as their _value_, which is measured by the work
the food does in the body either as a tissue builder or as a producer of
energy. However, in considering food value, the person who prepares food
must not lose sight of the fact that the individual appetite must be
appealed to by a sufficient variety of appetizing foods. There would be
neither economy nor advantage in serving food that does not please those
who are to eat it.
While all foods supply the body with energy, they differ very much in
the quantity they yield. If certain ones were chosen solely for that
purpose, it would be necessary for any ordinary person to consume a
larger quantity of them than could be eaten at any one time. For
instance, green vegetables furnish the body with a certain amount of
energy, but they cannot be eaten to the exclusion of other things,
because no person could eat in a day a sufficient amount of them to give
the body all the energy it would need for that day's work. On the other
hand, certain foods produce principally building material, and if they
were taken for the purpose of yielding only energy, they would be much
too expensive. Meats, for example, build up the body, but a person's
diet would cost too much if meat alone were depended on to provide the
body with all the energy it requires. Many foods, too, contain mineral
salts, which, as has been pointed out, are needed for building tissue
and keeping the body in a healthy condition.
23. To come to a correct appreciation of the value of different foods,
it is necessary to understand the unit employed to measure the amount of
work that foods do in the body. This unit is the CALORIE, or _calory_,
and it is used to measure foods just as the inch, the yard, the pound,
the pint, and the quart are the units used to measure materials and
liquids; however, instead of measuring the food itself, it determines
its food value, or fuel value. To illustrate what is meant, consider,
for instance, 1/2 ounce of sugar and 1/2 ounce of butter. As far as the
actual weight of these two foods is concerned, they are equal; but with
regard to the work they do in the body they differ considerably. Their
relative value in the body, however, can be determined if they are
measured by some unit that can be applied to both. It is definitely
known that both of them produce heat when they are oxidized, that is,
when they are combined with oxygen; thus, the logical way of measuring
them is to determine the quantity of heat that will be produced when
they are eaten and united with oxygen, a process that causes the
liberation of heat. The calorie is the unit by which this heat can be
measured, it being the quantity of heat required to raise the
temperature of 1 pint of water 4 degrees Fahrenheit, which is the name
of the thermometer commonly used in the home. When burned as fuel, a
square of butter weighing 1/2 ounce produces enough heat to raise 1 pint
of water 400 degrees Fahrenheit, and it will yield the same amount of
heat when it is eaten and goes through the slow process of oxidation in
the body. On the other hand, 1/2 ounce of sugar upon being oxidized will
produce only enough heat to raise the temperature of 1 pint of water
about 230 degrees Fahrenheit. Thus, as will be seen, 1/2 ounce of butter
has a value of approximately 100 calories, whereas 1/2 ounce of sugar
contains only about 57-1/2 calories.
Other foods yield heat in varying degrees, and their food value is
determined in exactly the same way as that of butter and sugar. To give
an idea of the composition of various food materials, as well as the
number of calories that 1 pound of these food materials will yield, food
charts published by the United States Department of Agriculture are here
presented. As an understanding of these charts will prove extremely
profitable in the selection of food, a careful study of them at this
time is urged. In addition, reference to them should be made from time
to time as the various kinds of foods are taken up, as the charts will
then be more easily comprehended and their contents of more value.