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.
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