THE WORK OF THE DIGESTIVE JUICES
As there are five main food elements, namely, proteid, starch, sugars, fats, and salts, so also there are five main digestive fluids, the saliva, the gastric juice, the bile, the pancreatic juice, and the intestinal juice.
The saliva is an alkaline fluid that digests starch. Its work is checked by the presence of acid substances; which explains why the digestive action of saliva ceases soon after it enters the stomach. Hence the importance of giving the saliva ample opportunity to perform its function, by complete mastication, is obvious.
The gastric juice, of which about seventy ounces is formed by the stomach daily, contains in addition to hydrochloric acid, a quantity of pepsin, which with the acid dissolves all sorts of proteids or albuminous substances, like meat and eggs; and it also contains rennet, which coagulates milk. The gastric juice digests proteids by converting them into pepsin, an exceedingly soluble substance which passes readily into the blood.
The bile manufactured by the liver has the function of digesting fats. Fats are not changed chemically, as are starches and proteids. They are only broken up into particles so small that the cells of the mucous membrane can take them up and effect their removal into the blood stream.
The pancreatic juice is able to perform the work of all the three digestive fluids which we have already named. In fact, it is even more powerful than saliva in the digestion of starch, since it is able to digest raw as well as cooked starch, which the saliva cannot do. It is also able to convert proteid into peptone, as does the gastric juice; and it emulsifies fats, as does the bile.
The intestinal juice digests cane sugar, and is supposed to have a digestive influence upon all the other food elements.
The mineral salts which are taken into the body are dissolved by all the digestive fluids which we have named, some by the saliva and the juices of the intestinal tube, and others, which require acids for their solution, by the gastric juice.
Nearly all these digestive fluids are also powerful antiseptics and are able to destroy germs when the health of the body as a whole is good. The gastric juice, for instance, acts as an antiseptic, preserving the stomach contents from putrefaction during the digestive process. It is a remarkable fact that the gastric juice, although it is so essential to life, is a deadly poison, which, when introduced into the blood produces insensibility and death.
These digestive juices and organs are able completely to dispose of all the food elements which are introduced into the body, save proteid alone. The sugars and starches are either completely absorbed and oxidized, or stored up in the form of surplus fat. The oxidation or burning up of proteid, however, is never complete. There is always a certain amount of unburnable substance left behind from the processes of metabolism, which the liver and kidneys of the body have to dispose of. If only as much proteid as is needed by the body for the upbuilding of its tissue, and the repair of waste, is taken, the body can very readily handle it; but an excess of proteid is highly disadvantageous. Professor Chittenden, in his great work, “The Nutrition of Man,” has set forth in elaborate detail the process of the assimilation of proteid. It appears that there are many kinds of proteid; the proteid of eggs is different from that of meat, and that again from the proteid of beans, and so on; and human proteid is different from all. Consequently, the body is obliged to transform every kind of proteid which is brought to it. This proteid is then absorbed by the blood, and carried to the tissues, which are kept perpetually bathed in a supply of nutritive material. The taking of more proteid than is needed would not be so dangerous if it were simply passed on without being digested; nor even if it were digested and transformed, and then promptly eliminated. But what actually happens is that the new proteid taken in is passed through all the stages of assimilation, and drives out in front of it, so to speak, the proteid which has already been prepared, but has not yet been used. And the result is, of course, to throw a double strain upon the liver and the kidneys, the organs of elimination.
Professor Chittenden also points out the common blunder which is made in assuming that persons who are doing hard work need an additional amount of proteid substance. One commonly hears the phrase that laborers and athletes can eat meat in large quantities, and “work it off.” As we have seen, one can “work off” sugars and starches and fats completely; but one cannot “work off” proteid completely. Professor Chittenden is now recognized as the leading authority of the world upon this particular question; and he sets forth clearly in his book the fact that the quantity of proteid needed is not increased by muscular activity. One may work as hard as he pleases, but his body will use no more nitrogen, save only in the case where a sufficiency of other food elements is not supplied. Only as a last resort will the system undertake the labor of burning up proteid to make energy.