held in reserve, awaiting
digestion, and is only gradually absorbed as this process takes place. It
may properly, on this account, be regarded as _stored material_. That such
storage is of advantage is shown by the observed fact that substances
which digest quickly (sugar, dextrin, "predigested foods," etc.) do not
supply the needs of the body so well as do substances which, like starch
and proteids, digest slowly. Even substances digesting quite slowly
(greasy foods and pastry), since they can be stored longer in the food
canal, may be of real advantage where, from hard work or exposure, the
body requires a large supply of energy for some time. These "stay by" the
laborer, giving him strength after the more easily digested foods have
been used up. Storage by the food canal is limited chiefly to the stomach.

*Regulation of the Food Supply to the Cells.*--The storage of food
materials is made to serve a second purpose in the plan of the body which
is even more important than that of supplying nourishment to the cells
during the intervals when no food is being taken. It is largely the means
whereby the rate of supply of materials to the cells is regulated. The
cells obtain their materials from the lymph, and the lymph is supplied
from the blood. Should food substances, such as sugar, increase in the
blood beyond a low per cent, they are converted into a form, like
glycogen, in which they are held in reserve, or, for the time being,
placed beyond the reach of the cells. When, however, the supply is
reduced, the stored-up materials reenter the blood and again become
available to the cells. By this means their rate of supply to the cells is
practically constant.

We are now in a position to understand why carbohydrates, fats, and
proteids are so well adapted to the needs of the body, while other
substances, like alcohol, which may also liberate energy, prove injurious.
It is because foods are of such a chemical nature that they are adapted in
all respects to the body plan of taking up and using materials, while the
other substances are lacking in some particular.

[Fig. 80]

Fig. 80--*Diagrams illustrating the relation of nutrients* and the
non-relation of these to alcohol. _A._ Inter-relation and convertibility
of proteids, fats, and carbohydrates (after Hall).

_B._ Diagram showing disposition of alcohol if this substance is taken in
quantity corresponding to that of