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.

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.

Determination of income and outgo of nitrogen.

This is all that was known about pro-teid 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.

Why proteids are converted into peptones.

Nitrogen and urea.

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.

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.

Composition of proteids.

How proteids may form body-fats.

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.

Excess of pro-teids harmful.

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.