of the cell protoplasm, and of supplying reserve food material. That they
are available for supplying energy, and are properly regarded as _storage
material_, is shown by the rapid loss of proteid in starving animals. When
the proteids are eaten in excess of the body’s need for rebuilding the
tissues, they are supposed to be broken up in such a manner as to form
glycogen and fat, which may then be stored in ways already described.

*General Facts Relating to Storage.*—The form into which the food is
converted for storage in the body is that of _solids_—the form that takes
up the least amount of space. These solids are of such a nature that they
can be changed back into their former condition and, by dissolving,
reënter the blood.

Only energy-yielding foods are stored. Water and salts, though they may be
absorbed in excess of the needs of the body, are not converted into other
substances and stored away. Oxygen, as already stated (page 108), is not
stored. The interval of storage may be long or short, depending upon the
needs of the body. In the consumption of stored material the glycogen is
used first, then as a rule the fat, and last of all the proteids.

*Storage in the Food Canal.*—Not until three or four hours have elapsed
are all the nutrients, eaten at a single meal, digested and passed into
the body proper. The undigested food is 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