The speed with which the stomach empties itself depends to a great extent on the nature of the food. Plain water leaves the stomach almost at once, salt and sugar solutions at a somewhat slower rate. On a mixed diet, emptying of the stomach in man proceeds very slowly, requiring about four hours. Cannon, by feeding with food impregnated with bismuth and using X-rays, showed that carbohydrates leave most rapidly, then mixtures of carbohydrates and proteins, then proteins, then fats and finally mixtures of fats and proteins. The diet which remains longest in the stomach is a mixture of fats and proteins—rich food, as it is popularly called. Here two factors enter to prevent rapid emptying : (I) the presence of much fat, and (2) the acid secre tion engendered by the abundant protein.
There is no doubt that fats present in fine emulsion can be de composed in the stomach. The action proceeds in a medium which is slightly acid or neutral, being entirely prevented by the presence of strong acids and alkalis. Many workers believe this gastroli pase to be of pancreatic or intestinal origin, and suppose that it gains entrance to the stomach by a reflux flow through the pylorus.
No specific enzyme for carbohydrates has been found in the stomach in man. Certainly a small amount of polysaccharide de composition takes place, but this is dependent (I) on the ptyalin which comes from the mouth, and (2) on a certain amount of hydrolysis due to the action of the free hydrochloric acid.
Pancreatic Digestion. The pancreatic juice is the secretion from the pancreas and is discharged into the duodenum. It is a clear, usually thin fluid with a specific gravity of about Ica, and with an alkaline reaction. The most important inorganic constit uent is sodium carbonate, which gives the alkaline reaction. This alkaline salt, along with that contained in the intestinal juice, plays an important part in neutralizing the acid chyme.
In the pancreatic secretion there are at least three important enzymes, each with a definite action : (a) trypsin, the proteolytic enzyme which brings about the further breakdown of the food proteins; (b) a diastase which deals with the carbohydrates, and (c) a lipase which acts on the fats.
(a) Trypsin. This ferment, in the form in which it is se creted—trypsinogen—is inert. Before it can exert its hydrolytic action it must be activated. This activation is brought about by
another enzyme found in the intestinal tract—enterokinase. The conversion is brought about as soon as the trypsinogen comes into contact with the enterokinase, the merest trace of which suffices to activate a large amount of trypsinogen.
Trypsin acts on the protein just as pepsin does, by bringing about hydrolytic changes. It differs from the latter in acting best in an alkaline or neutral medium. Its effect is much more ener getic than that of pepsin, so that the protein molecule is more completely decomposed.
The character and properties of the products formed in such digestion depend on the nature of the protein acted upon. As will be seen from the following table, proteins vary fairly widely in the proportion of amino acids which they contain.
Approximate Amino-Acid Percentage Composition of Several Proteins (b) Diastase. This ferment is found in the pancreatic juice apparently secreted in an active form, although some observers hold that it also is secreted in a zymogen form. It is practically identical in its action with the ptyalin of the saliva, converting starch into maltose. It deals with all the starchy food which has escaped conversion into the simple sugars by the ptyalin.
(c) Lipase. Most of this ferment, if not all, is apparently se creted in the form of a zymogen. There is evidence that the bile salts are the activating agent here, just as the enterokinase acts in the case of trypsin. Lipase can act in any medium acid, neutral or alkaline, and both on emulsified and non-emulsified fats. It converts the fats by a process of hydrolysis into fatty acids and glycerol. It has been found that not only can this enzyme break up fats into their components, but that it also has the power to act in the reverse direction, and in this way bring about the union of fatty acids and glycerol so as to form fats, a process which occurs in the intestinal epithelial cells after absorption.