Fat is not acted on by the gastric juice. This explains why the process of frying is so unwholesome. Frying causes a thin film of melted fat to spread over the surface of the starch and of the proteid atoms, with the result that these atoms cannot then be properly acted on by the saliva and the gastric juice, and therefore cannot undergo the preliminary changes necessary to normal digestion. Fat, taken in its natural form, does not interfere with other digestive processes.
In addition to the digestive juices that are poured into the small intestines from the pancreas and the liver, there is a juice which is secreted from the walls of the intestinal cells. This is called intestinal juice or succus entericus. It is a light yellow fluid with a strong alkaline reaction, due to the presence of sodium carbonate.
One action of the intestinal juice is to change sugar and maltose into glucose, which is then absorbed directly into the blood.
[Sidenote: Recent discoveries concerning digestive juices]
Within the past few years many remarkable discoveries have been made in regard to the secretion of the various digestive juices. Until some ten or fifteen years ago it was believed that the secretion of the digestive juices depended wholly upon the presence of food in the alimentary canal. The recent discoveries in this branch of physiology are to be accredited chiefly to Professor Palloff, a Russian scientist, and his co-workers. The facts that are now known regarding this part of Nature's work are essentially as follows:
The secretion of the various substances which make up the digestive fluids of the body depend upon two kinds of stimuli:
1 Direct nerve stimulus from the central nervous system
2 The chemical stimulus on the walls of the digestive organs
Depending upon either or both of these sources of stimulation, the digestive juices of the body are regulated in quantity, and what is much more worthy of note, in their actual chemical composition. Thus it will be readily seen how far-reaching in its effect upon scientific dietetic treatment is the knowledge of the influence of various foods, quantities, and combinations.
[Sidenote: Comparative digestibility of foods]
Professor Palloff's discoveries throw some very important light on the comparative digestibility of foods. The former method of estimating the digestibility of food was first to analyze the food, and then to analyze the intestinal residue, and subtract the undigested remnant of each particular class of food from the amount originally eaten. By such means it was possible to show that certain foods were, say 80 or 90 per cent digestible, as the case might be. By this method no allowance was made for the amount of nutrition or material that was consumed by the body in the digestion of these particular foods. According to these investigations, milk and meat were about equally digestible. It was not known that the digestion of milk requires only a small fraction of the energy that is necessary to digest meat, or proteids from vegetable sources. Thus it is obvious that when it is desirable to get a large amount of available nitrogen into the system, with as little expenditure of energy as possible, milk is a food par excellence. This is very logical inasmuch as the sole purpose of milk is food for animal life.