Fig. 80—Diagrams illustrating the relation of nutrients and the non-relation of these to alcohol. A. Inter-relation and convertibility of proteids, fats, and carbohydrates (after Hall).
B. Diagram showing disposition of alcohol if this substance is taken in quantity corresponding to that of the nutrients (F.M.W.). The alcohol thrown off as waste is unoxidized and yields no energy.
Why Alcohol is not a Food.—If the passage of alcohol through the body is followed, it is seen, in the first place, that it is a simple liquid and undergoes no digestive change; and in the second place, that it is rapidly absorbed from the stomach in both weak and concentrated solutions. This introduces it quickly into the blood, and once there, it diffuses rapidly into the lymph and then into the cells. Since the body cannot store alcohol or convert it into some nutrient that can be stored (Fig. 80), there is no way of[pg 182] regulating the amount that shall be present in the blood, or of supplying it to the cells as their needs require. They must take it in excess of their needs, regardless of the effect, at least until the organs of excretion can throw off the surplus as waste. Compared with proteid, carbohydrates, or fats, alcohol is an unmanageable substance in the body. Attempting to use it as a food is as foolish as trying to burn gasolene or kerosene in an ordinary wood stove. It may be done to a limited extent, but is an exceedingly hazardous experiment. Not being adapted to the body method of using materials, alcohol cannot be classed as a food.
Assimilation.—Digestion, absorption, circulation, and storage of foods are the processes that finally make them available to the cells in the different parts of the body. There still remains another process for these materials to undergo before they serve their final purposes. This last process, known as assimilation, is the appropriation of the food material by the cell protoplasm. In a sense the storage of fat by connective tissue cells and of glycogen by the liver cells is assimilation. The term is limited, however, to the disposition of material with reference to its final use. Whether all the materials used by the cells actually become a part of the protoplasm is not known. It is known, however, that the cells are the places where most of the oxidations of the body occur and that materials taking part in these oxidations must, at least, come in close contact with the protoplasm. Assimilation, then, is the last event in a series of processes by which oxygen, food materials, and cell protoplasm are brought into close and active relations. The steps leading up to assimilation are shown in Table II.
| TABLE II. THE PASSAGE OF MATERIALS TO THE CELLS | |||||
|---|---|---|---|---|---|
| MATERIALS | DIGESTION | ABSORPTION | ROUTE TO THE GENERAL CIRCULATION | STORAGE | CONDITION IN THE BLOOD |
| Proteids | Changed into proteoses and peptones by the action of the gastric and pancreatic juices. | In passing into the capillaries, the proteoses and peptones change into the proteids of the blood. | Through the portal vein to the liver and from there through the hepatic veins into the inferior vena cava. | Become a part of the protoplasm of all the cells. | As proteids in colloidal solution. |
| Fat | Changed into fatty acid, glycerine, and soluable soap by the bile and pancreatic juice. | In passing into the lacteals, the glycerine unites with the soluable soap and fatty acid to form the oil droplets of the blood. | Through the lacteals to the thoracic duct, by which it is emptied into the left subclavian vein. | As fat in the cells of collective tissue. | Chiefly as minute oil droplets. |
| Starch | Reduced to some of the different forms of sugar, as maltose, dextrose, etc. | Enters the capillaries as dextrose. | Through the portal vein, liver, hepatic veins, into inferior vena cava. | As glycogen chiefly by the liver, but to some extent by muscle cells. | As dextrose in solution. |
| Water | Undergoes no change. | Taken up by both the lacteals and capillaries, but to the greater extent by the capilaries. | Both routes, but mostly by way of the liver. | Is not stored in the sense that energy foods are. | As the water which serves as a carrier of all the other constituents of the blood. |
| Common salt | Undergoes no change. | Taken up by the capillaries without undergoing apparent change. | By way of portal vein, liver, and hepatic veins into inferior vena cava. | Not stored. | In solution. |
| Oxygen | Taken up by the capillaries at the lungs. | Already in the general circulation. | Is not stored. | United with the hemoglobin and to a small extent in solution in the plasma. | |
[pg 184]Tissue Enzymes.—The important part played by enzymes in the digestion of the food has suggested other uses for them in the body. It has been recently shown that many of the chemical changes in the tissues are in all probability due to the presence of enzymes. An illustration of what a tissue enzyme may do is seen in the changes which fat undergoes. In order for the body to use up its reserve fat, it must be transferred from the connective tissue cells, where it is stored, to the cells of the active tissues where it is to be used. This requires that it be reduced to the form of a solution and that it reënter the blood. In other words, it must be redigested. For bringing about these changes a substance identical in function with the steapsin of the pancreatic juice has been shown to exist in several of the tissues.
Although this subject is still under investigation, it may be stated with certainty that there are present in the tissues, enzymes that change dextrose to glycogen and vice versa, that break down and build up the proteids, and that aid in the oxidations at the cells. The necessity for such enzymes is quite apparent.
Summary.—The digested nutrients are taken up by the capillaries and the lymph vessels and transferred by two routes to the circulation. In passing from the alimentary canal into the circulation the more important of the foods undergo changes which adapt them to the needs of the body. Since materials are absorbed more rapidly than they are used, means are provided for storing them and for supplying them to the cells as their needs require. Capability of storage is an essential quality of energy-yielding foods; and substances, such as alcohol, which lack this quality are not adapted to the needs of the body. For causing the chemical changes that occur in the storage of foods, as well as the oxidations at the cells, the presence of active agents, or enzymes, is necessary.
Exercises.—1. In what respects does the absorption of food materials from the alimentary canal differ from the absorption of a simple liquid by a solid?
[pg 185]2. In what different ways is the small intestine especially adapted to the work of absorption?
3. What are the parts of a villus? What are the lacteals? Account for the name.