[Sidenote: Proteids as tissue-builders]
The first use Nature makes of proteids in the body is in the actual
adding to or increasing of body-tissue. When an emaciated young man from
the city goes to work on a farm and gains twenty pounds, the cells of
his muscles have actually increased in size and number. This requires
proteids, which can be obtained only from the nitrogenous material in
food. The growth during early life is due to an actual increase in the
size of all the organs of the body, and is merely an accumulation of
proteid substance.
[Sidenote: Proteids form the nitrogenous part of the body]
The second use of proteids, and the one which, in matured life, is of
more importance than those already referred to, is in the formation
of the various nitrogenous products which are produced in connection
with the different processes of the body and which are destroyed by
the function of life. For example, the pepsin of the gastric juice is
a nitrogenous substance which can be formed only from proteids. All
digestive enzyms and other substances in the muscles, nerves, and in
the various organs throughout the body are of a nitrogenous nature,
and in their formation and use a certain amount of proteid material is
consumed. When the digestive enzyms are formed from proteids, they
consume more than their own weight of proteid material.
[Sidenote: Proteids replace worn-out cells]
The third form in which proteids may be consumed in the body is in
the actual replacement of worn-out cells. The skin, the hair, and the
mucous or lining membranes of the body-cavities are constantly being
cast off on the external surface, new cells being formed underneath.
When cells within the interior of the body have become injured, or have
passed their usefulness, they are removed by the phagocytes or white
blood-corpuscles, and must be replaced by other cells. In the case of
bacterial infections, as tumors, boils, or contagious dis-eases, the
bacteria feed upon the proteids of the blood. The white blood corpuscles
are destroyed in the conflict, or effort to remove the intruders, and
all these substances must be replaced by proteids from food.