[Sidenote: Determination of income and outgo of nitrogen]
The gain or loss of body-proteids is indicated by the gain or loss
of nitrogen. The income of nitrogen can be ascertained by analyzing
the food. The outgo of nitrogen is computed by analyzing the products
excreted from the body. If the body at the beginning and at the end of
an experimental period is carefully watched, and the income and the
outgo of nitrogen determined, we can compute the amount of gain in the
body that is nitrogenous tissue. The other gain or loss of body-weight
must be fat. These calculations cannot be made exact, owing to the
amount of food and water that may be in the digestive organs at the time
the various weighings are made.
[Sidenote: Why proteids are converted into peptones]
We have learned that in the digestive tract foods are converted into a
soluble form of proteid known as peptone. The purpose of this conversion
and the fine subdivisions of food produced by the various digestive
juices are to reduce it to a form which will readily pass through the
walls of the alimentary canal.
[Sidenote: Nitrogen and urea]
This is all that was known about proteid metabolism until within very
recent years. The older scientists followed proteid digestion until the
soluble peptone stage was reached, at which point all track was lost of
the chemical changes and processes until the nitrogen was again excreted
by the kidneys in the form of urea.
No scientist attempted to explain how the radically different proteids,
such as egg-albumin, milk-casein, and wheat-glutin could appear in the
body as blood-globulin, brain-lecithin, or as a myosis of the muscles.
The history of all these investigations cannot be fully explained here,
but the discussion must be confined to that which actually takes place
in the metabolism of proteids.
[Sidenote: Composition of proteids]
Proteids, as the student will remember, contain carbon, hydrogen,
oxygen, and nitrogen, and sometimes small quantities of sulfur,
phosphorus, or iron. These forms of proteids are now known to be
chemically changed, by the digestive enzyms of the intestines, into
simpler compounds containing these same elements.
[Sidenote: How proteids may form body-fats]
These simple nitrogenous substances pass into the liver. Just as the
liver regulates the supply of blood sugar, so it regulates the supply of
nitrogenous compounds in the blood. A certain amount of proteid-forming
material is passed through the liver, and goes on to perform the various
functions for which proteid is utilized in the body. All nitrogenous
material in excess of the amount required by the body is secreted by
the liver, and the nitrogen, together with a portion of the carbon,
hydrogen, and oxygen, is split off, forming urea, which is excreted by
the kidneys. The remainder of the proteid substance, having been robbed
of its nitrogen, is now essentially the same as carbohydrates, and goes
to form glucose or blood-sugar, which may in turn form body-fats.
[Sidenote: Excess of proteids harmful]
In the light of this explanation, we can understand several things
already mentioned. It has been stated that proteid is the most essential
food material of the body because it alone contains the nitrogenous
compounds from which the body-tissues, and the chemical enzyms which
control all living processes, can be constructed. But we now see that
as important as is a supply of proteid materials, any excess above the
body-needs is immediately turned into glucose and urea. The glucose,
though useful to the body, could be taken in a simpler and less
expensive form, while the urea is a waste-product, harmful to life, and
must be immediately excreted by the kidneys.
The nitrogen that is actually used in the body serves a different
purpose from that which is split off from the excessive proteid taken
as food. The food proteid is simply split by the chemical addition of
water, much the same as starch and other carbohydrates are changed into
glucose. The proteid that is really used by the body is oxidized, and is
excreted by the kidneys chiefly in the form of creatinin and uric acid.