fact that rapid cooking is needed for some vegetables and slow cooking
for others, the method and the time of cooking depending on the presence
and the consistency of the cellulose that occurs in the food.

21. IMPORTANCE OF A VARIETY OF FOODS.--Every one of the five food
substances just considered must be included in a person's diet; yet,
with the exception of milk, no single food yields the right amounts of
material necessary for tissue building and repair and for heat and
energy. Even milk is in the right proportion, as far as its food
substances are concerned, only for babies and very young children. It
will thus be seen that to provide the body with the right foods, the
diet must be such as to include all the food substances. In food
selection, therefore, the characteristics of the various food substances
must be considered well. Fats yield the most heat, but are the most
slowly digested. Proteins and carbohydrates are more quickly digested
than fats, but, in equal amounts, have less than half as much food
value. Water and mineral salts do not yield heat, but are required to
build tissue and to keep the body in a healthy condition. In addition,
it is well to note that a well-balanced diet is one that contains all of
the five food substances in just the right proportion in which the
individual needs them to build up the body, repair it, and supply it
with energy. What this proportion should be, however, cannot be stated
offhand, because the quantity and kind of food substances necessarily
vary with the size, age, and activity of each person.


FOOD VALUE

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