This section is from the book "Health", by W. H. Coefield.
Among substances that do not contain nitrogen there are, then, two important classes, the fats and oils, and the starches, sugars, and gums. We shall see what becomes of these when they are taken in our foods.
The fats that are taken in our foods are not altered in the mouth, they are not altered in the stomach, but in the small intestine; when mixed with the pancreatic juice and bile, they form'what we call an emulsion. I can best illustrate what is meant by an emulsion by mentioning milk, which contains very various substances : one is fat, which is in the state of an emulsion, being divided into very small particles.
Fat, then, is reduced by the action of the pancreatic juice into the state of an emulsion; the fine particles of fat then pass through the walls of the villi of the small intestine into the lacteal vessels, which begin in the villi, and so get into the blood unchanged; then part of the fat combines with the oxygen in the blood, and because there is a* large quantity of hydrogen and only a small quantity of oxygen in the fat a very large amount of combination can take place, for almost all the carbon and hydrogen can combine with free oxygen in the blood, so that a large amount of oxidisation occurs, and a considerable quantity of heat is produced Fats are among the most important foods that we have for produring heat; they partly do this, and partly are deposited in the various tissues.
Another important function that fats perform is that of aiding the digestion of other substances. An animal cannot live upon a diet that contains no fat, because he cannot properly digest or absorb the other substances contained in his food.
What becomes of the sugars, starches, and gums ? They are converted in the mouth by the saliva into a form of sugar which goes by the name of grape sugar. When the food gets down into the stomach no further change occurs, but in the small intestine, if all these substances have not been changed in the mouth, the pancreatic juice changes the remainder into sugar, which is absorbed into the blood-vessels of the villi, and passes through the portal vein into the liver. It has been shown that the blood in the portal vein contains sugar, but the blood in the vein that leaves the liver does not contain sugar, and the blood generally in the body contains no sugar, or only a very small quantity: when, either from disease or other cause, the blood does contain sugar, especially if in large quantity, it is got rid of by the kidneys as sugar. What, then, becomes of the sugar that goes into the liver? It has been found by a large series of experiments that the liver is capable of preparing, out of the substances that come to it in the blood, a substance that goes by the name of liver starch, some of which it stores up, and the liver in consequence always increases in weight after the consumption of a diet containing much starch. This starch is a substance capable of very readily being turned back again into sugar, and that is why it goes also by the name of glycogen, which means sweet producer. But it is found that ordinarily there is very little or no sugar in the blood that leaves the liver; what, then, becomes of the liver starch stored up in it? The liver has at least one other property- that of manufacturing fat; but the precise way in which this is done is not understood: it is probably from the liver starch, though other substances may have something to do with the process ; so that the liver may be regarded as an important apparatus for manufacturing fat and turning it into the blood, and so we see why people who take large quantities of food containing starch frequently become stout, and how it is that animals, especially poultry, are fattened upon foods containing a large percentage of starch and very little fat. In this way these food substances, starches and sugars, are turned first into grape sugar, and then by the liver they are changed into fat, which is disposed of in the ways already described. We can perceive, then, that non-nitrogenous foods have especially the important function of combining with oxygen in the blood and producing animal heat, and for this reason they have been called calorific foods.
No mixture of non-nitrogenous substances is capable of keeping up the life of an animal, because loss of nitrogenous matter is occurring continually, and so it is necessary that foods containing nitrogen must be taken. When any tissue is used it wastes, and that waste must be replaced.
Then with regard to nitrogenous foods: the first class of these includes albumen, which we find pure in the white of an egg, also in meat; fibrin and syntonin, which we also find in flesh : it also includes casein, found in milk; legumen, found in beans and peas; and several others. These substances are known by the name of protein compounds. They are not. attacked by the saliva in the mouth, but they are attacked by the gastric juke in the stomach, and are converted into substances that go by the name of peptones, and are absorbed from the chyle into the blood through the walls of the capillaries of the villi, and also into the lacteal vessels of the villi
These substances go especially to form tissue, and so the nitrogenous substances have been called tissue-forming foods. But they do not solely contribute to that purpose, some part being oxidised in the blood, with the waste nitrogenous substances from the tissues, which are also oxidised in the blood, forming the nitrogenous waste that is'got rid of by the kidneys..
No dietary, therefore, is complete without one or more of these nitrogenous foods. Some time ago it was thought that the force we exerted was due to the oxidation of the nitrogenous substances of which our muscle is composed, but it has been shown by experiments made by different observers that this is not the case, but that the force we exert is produced by the oxidation of the non-nitrogenous substances in our foods, of the substances containing carbon, hydrogen, and oxygen, but no nitrogen, and that the greatest exertion can be supported for a time upon non-nitrogenous foods. It is clear that if force were produced by the oxidation of the nitrogenous substances, the amount of nitrogenous waste that we excrete from the kidneys during exercise ought to be greater, in proportion to the exertion. This is not found to be the case; on the contrary, it is the amount of carbonic acid that we excrete, and the amount of water that passes off, that are in proportion to the amount of work dona It is therefore certain that the main object of the non-nitrogenous foods is to produce heat by being burnt in the blood, and thus keep us at a proper temperature, and supply force for the work we do; and the chief use of the nitrogenous foods is to repair the wftste of the nitrogenous tissues, and only to aid in a secondary way the production of animal heat; in fact, the muscular tissue of the body is really the apparatus by which force is exerted, and that force is not due to the heat produced by the oxidation of the muscular tissue.