the form of proteose and peptone. In support of this it was stated that both proteoses and peptones could be detected in the blood stream. The result of the most recent work tends to show that the material is absorbed in the form of the amino acids either simple or in complex groups, the polypeptides, and that if proteoses or peptones be absorbed they are attacked by the intra-cellular enzyme erepsin, which breaks them down into the simpler products as soon as they are within the intestinal mucous membrane. Certain proteins appear to be absorbed unchanged; for instance, blood serum disappears from the intestine without apparently any change through zymin attack. This fact is made use of in practical medicine, as, when administration of food by the mouth is impossible, patients are frequently kept alive by the giving of nutrient enemata. That the food thus given is absorbed is shown by the increase of nitrogen excretion in the urine.
In the large intestine very little absorption of nutrient matter takes place under normal conditions, mainly of course because most of the absorbable material is removed whilst the food is in the small intestine. That protein matter can be absorbed is shown by the above statement regarding nutrient enemata. The principal substance absorbed here is water; and thus the excreta become firm and formed.
V. Metabolism
In all living matter there is a constant cycle of chemical changes going on, a constant breaking down (catabolism), and a correspondingly constant building up (anabolism). Unless the former is covered by the latter wasting and finally death must supervene. These two changes together make up the metabolism, and the study of this involves a study of the fate of the food absorbed both when it is used immediately and after it has been stored in the tissues of the body. Protein matter is undoubtedly the main constituent of protoplasm, but in what form it exists there is absolutely unknown. One thing is certain, that for the maintenance of life a constant supply of protein matter is necessary. In fact it might be said that this is the essential food and keeps the body alive, fats and carbohydrates being merely subsidiary. In the mammalian organism with which we are specially concerned a supply of these latter substances is also necessary to yield the energy required. The amounts of these various food stuffs which should be present in a suitable diet are dealt with under Dietetics (q.v.). Here we are only concerned with the part played by the different materials in the various chemical changes which are the basis of vital activity.
Not many years ago physiologists were very much in the position of unskilled labourers who saw loads of heterogeneous material being “dumped” for building purposes, but who did not know for what particular purpose each individual substance was used. Thanks, however, to the brilliant work of E. Fischer we are no longer in this position. Gradually our knowledge is being broadened by actual facts obtained by direct experiment, or by inference from previous experiments. But it is still far from complete. It is only possible to outline what is at present known about the part played by the different food constituents in metabolism.
Proteins.—Since these alone contain the nitrogen necessary for the building up and repair of the tissues they are essential and will be dealt with first. In considering the digestion of proteins it was shown that in all probability all protein food was reduced in the intestine to comparatively simple crystalline bodies. O. Loewi has shown that an animal can be maintained in health without loss of weight by feeding it on a diet consisting of amino acids obtained by prolonged pancreatic digestion in place of proteins. In addition to these acids abundant carbohydrates and fats were given. It has since been shown that the presence of carbohydrate a certain amount of is absolutely essential before utilization of the amino acids can take place. Further. it has been demonstrated that only a mere fraction of the total amino acids resulting from pancreatic digestion is sufficient as the source of nitrogen supply for the animal organism. Not only so, but, in spite of the attempt to insist on the polypeptides as being the valuable nuclei for the rebuilding up of protein in the body, it has been shown that mixtures of amino acids from which the polypeptides have been removed can serve as the nitrogen supply.
What then does the body gain by breaking down food material to such simple bodies, if it is immediately to be resynthesized? This complete breakdown appears to be to facilitate rebuilding. The protein in the protoplasm of each animal is characteristic and to build up these different proteins the material must be separated into its nuclei. An experiment carried out by E. Abderhalden shows this very clearly. A protein gliadin absolutely different in constitution from the proteins of blood plasma was fed to an animal from which much of its blood had been removed, so that an active reformation had to take place. The question to be solved was whether by feeding with a protein so absolutely different in constitution the nature of the freshly forming serum protein could be radically changed. But the newly-formed serum was found to be exactly the same in constitution as the old. The tissues had selected simply those nuclei of the gliadin which were required and had rejected the others.
In addition to this breakdown of protein in the intestine, another factor of importance comes into play. After absorption from the lumen of the gut the amino acids are not wholly conveyed as such by the portal blood to the liver. That the portal blood contains a greater amount of ammonia than the systemic blood has long been known, and Jacoby and Lang have shown that many tissues, and among them the intestinal tissues, are able to split off from the amino acids their amino group NH2. Thus it would seem probable that any excess of the amino acids formed does not reach the liver as such but denitrified as members of the fatty acid series. The ammonia split off is also conveyed to the liver and is excreted for the most part as urea, within the first few hours after a protein meal. Thus, in all probability very early after absorption and before the products of digestion enter into combination or any synthesis occurs, all excess of the absorbed nitrogen is disposed of. The rest of the products circulate in the blood, yielding to the cells the materials of which they are in need. On the other hand some investigators still, hold that resynthesis into a neutral protein like serum albumin takes place in the intestinal wall immediately after absorption of the digest products. That the leucocytes play an important part in carrying the products of protein digestion to the tissues is indicated by the enormous increase in their number which occurs during the digestion and absorption of protein foods. How they act, whether simply as carriers of the products of protein digestion combined or uncombined, and how they give the material to the tissues is unknown.
Carbohydrates are generally assumed simply to serve the purpose of yielding energy in their combustion to CO2 and H2O, and to act as protein sparers, i.e. they save the ingestion of large amounts of costly protein material as a source of energy. There may, however, be other activities in which the ingested sugars play a part, for instance, in the utilization of the nitrogen of proteins. It has already been indicated that the nitrogen in the products of pancreatic digestion can be used only when a sufficient amount of carbohydrates is given at the same time. Only carbohydrates seem to be able to do this, for it has been found that when isodynamic amounts of fat are given the utilization does not take place.
When taken into the body in excess of the immediate requirements the sugar is not utilized all at once, but any excess is stored in the form of glycogen both in the liver and the muscles. This glycogen is an insoluble polysaccharide, and is only utilized according to the requirements of the body, especially during muscular exertion. Carbohydrates, when taken in in excess, are also stored in the tissues in the form of fat. This was demonstrated by the feeding experiments of Lawes and Gilbert at Rothamstead. They took two young pigs of a litter, killed and analysed one, then fed the other for a definite time upon food of known composition, determining the amount of protein absorbed by analysing the urine and the faeces. They then killed the pig and by analysis ascertained the amount of fat put on. They found that this was far in excess of the amount of the protein of the food which had been absorbed and was also in excess of what could have been formed from the small amount of fat in the food. The fat must therefore have been formed from the carbohydrates of the food. The consumption of larger amounts of sugar than can be used or stored as glycogen results in its passing straight through the body and being excreted in the urine. This condition is known as alimentary glycosuria. The power of using and storing sugar varies greatly in different individuals and in the same individual at different times.
Fats.—The fats simply serve as stores of energy. After ingestion, if in small amount, they are, like carbohydrates, oxidized to the same final products CO2, and H2O. If in larger amount they are stored as fat, to serve as a reserve in case of need, in the body tissues. Like the carbohydrates they serve as the sources of part of the energy dissipated as heat, but they are not so efficient as sparers of protein material, evidently in part at least because they are less easily digested and absorbed.
Factors which influence Normal Metabolism.
1. Fasting.—During fasting the body draws upon its own reserve of stored material for the requirements in the production of energy, and the rate of breakdown varies with the energy requirements. An individual who is kept warm in bed therefore stands fasting longer than one who is compelled to take exercise in a cold place. The breakdown of tissue during the early days of a fast is much greater than later, for as the fast progresses the body becomes more economical in its utilization of tissue. During a fast the tissues do not all waste at an equal rate; those which are not essential are utilized at a much greater rate than those which are essential to the maintenance of the organism. For instance, it has been shown that during a fast the skeletal muscles may lose over 40% of their weight, whereas an essential organ like the heart loses only some 3%.
The essential tissues obtain their nourishment from the less essential probably by ferment action, a process which has been
