It was subsequently discovered by Emil Fischer * and others - that the proteins are made up of chemical units in much the same way that words are made up of letters; and just as the twenty odd letters are sufficient to form the many different words in our language, so the eighteen or more chemical units, obtained by analyzing proteins, are sufficient to form the different proteins with which we are familiar. These chemical units are known as amino-acids.

When a protein such as is found in meat is eaten, the several juices act upon it and break it up into its amino-acids. We have learnt that this breaking up of a protein in the digestive tract is a necessary prerequisite to absorption and assimilation. The system cannot absorb protein as such.** Introduce protein directly into the blood and it acts like a poison; but introduce the amino-acids out of which the proteins are built, and all goes well.

The factories in which the protein is prepared for absorption are the stomach and the small intestine. Here the proteins are changed to amino-acids, which then find their way into the blood stream, and thence to the cells.

In the cells occurs the reverse process of what takes place in the digestive tract; instead of breaking up proteins into amino-acids, the latter are joined together to form proteins. Analysis gives place to synthesis.

* Professor MeCollum suggests that " Kossel 's name be substituted for Fischer's, since Kossel did so much more pioneer work in showing the nature of the protein molecule and in the discovery of four amino-acids, whereas Fischer only discovered one. I have felt that more credit is due to Kossel than is usually given him."

** A little may be absorbed unchanged, but the amount is negligible.

But why this breaking down of proteins into their units if these act merely as nuclei for rebuilding protein? A perfectly fair question. But remember that we are not rebuilding the same protein. The same letters will give you different words, depending upon the arrangement of the letters. The same bricks will give you different houses, depending upon the arrangement of the bricks. The same amino-acids will give you different proteins, depending upon the arrangement of these amino-acids. That is why the Germans call the amino-acids the Bausteine or building-stones.

Not all proteins contain the same amino-acids. Some proteins contain more of one amino-acid than another; others are deficient in one or more of the acids.

In order to build up that peculiar protein which we find in the tissues, the tissue protein, the cell selects those amino-acids it needs and discards the rest.

You may ask, why if what the cell needs is not the protein but the amino-acid, the diet should not rather consist of amino-acids, fats and carbohydrates, rather than proteins, fats and carbohydrates? Unfortunately, nature insists upon supplying us with the more complicated proteins; just why we do not know.* But nature has considerately supplied us with a factory, known as the digestive tract, where the proteins can be changed into amino-acids.

* This is not quite true. The physical chemist will tell you that proteins are colloids, whereas amino-acids are crystalloids, and there are reasons why plant and animal material should be in the colloidal state.

But before we dismiss the general metabolism of proteins - the changes that substances undergo in the system - we must refer to another topic, because of its relationship to the subject of just how much protein the body needs. We have already seen in earlier chapters how the subject of protein needs has given rise to much discussion. Most of the experiments conducted in this direction have tended to point to an amount of protein considerably less than that ordinarily used by man. But the more recent studies have shown us that not all of the protein ingested is utilized by the cells. As we have just seen, only a select number of the amino-acids formed from the protein are taken up by the cells to form tissue protein; the rest are discarded. Is it not therefore necessary to eat more protein than would at first hand appear, if only to insure an adequate supply of those amino-acids needed for tissue building? Arguments such as these have led to the revision of the diets of our soldiers in favor of a relatively high protein content.