THE FOOD-STUFFS.--The waste products we throw off indicate that the
substances which compose our bodies are being constantly broken down
and reduced to a condition such that they are useless to us. In
normal persons hunger signifies that they need material to replace
what has been used up. The substances thus required, if the wants of
the body are to be satisfied correctly, are called the food-stuffs;
and they are the same during pregnancy as at other times. The
foodstuffs are usually classified according to their chemical
properties; on this basis they are placed in five groups: (1) Water,
(2) Mineral Materials, (3) Proteins, (4) Carbohydrates, (5) Fats.

In view of the different purposes which the foodstuffs serve, it is
convenient to group them in another way. Thus, the carbohydrates and
the fats may be placed together because they are the body fuel; their
value consists in the heat and energy which they yield when acted
upon in the tissues. Water and mineral matter, on the other hand, are
never a source of energy; they assist in building new tissue or in
repairing tissue that already exists. The proteins are unique, in
that they may serve either purpose. Primarily the proteins are
tissue-builders, but in the absence of sufficient fat or carbohydrate
the body burns protein to secure heat and energy.

Each food-stuff, therefore, serves a distinct purpose, and some of
them render services which the others cannot perform. A man will die
if either water or mineral matter or protein is completely withdrawn
from his diet. Fat or carbohydrate, on the other hand, or even both
of them, may be excluded for some time without causing serious
inconvenience. It is true, nevertheless, that each food-stuff
performs some task better than any of the others can perform it, and
for that reason all of them should be included in the diet of an