The nutrition of man

by R. H. Chittenden

Especially interesting and suggestive are the experiments made by Cannon[9] on the length of time the different types of foodstuffs remain in the stomach. Using cats as subjects, he found that fats remain for a long period in the stomach; they leave that organ slowly, the discharge into the intestine being at about the same rate as the absorption of fat from the small intestine or its passage into the large intestine. Carbohydrate foods, on the other hand, begin to leave the stomach soon after their ingestion. They pass out rapidly, and at the end of two hours reach a maximum amount in the small intestine almost twice the maximum for proteids, and two and a half times the maximum for fats, both of which maxima are reached only at the end of four hours. Carbohydrates remain in the stomach about half as long as proteids. Proteids, Cannon finds, frequently do not leave the stomach at all during the first half-hour after they are eaten. After two hours, they accumulate in the small intestine to a degree only slightly greater than that reached by carbohydrates an hour and a half earlier. The departure of proteids from the stomach is therefore slower at first than that of either fats or carbohydrates. When a mixture of equal parts of carbohydrates and proteids is fed, the discharge from the stomach is intermediate in rapidity. When fat is added to either carbohydrates or proteids it retards the passage of both foodstuffs through the pylorus.

It is evident from what has been stated that the gastric digestion of proteid foods is a comparatively slow process, involving several hours of time; and further, that food material in general remains in the stomach for varying periods, dependent upon its chemical composition. It would appear further, that relaxation of the pyloric sphincter, allowing passage of chyme into the intestine, must depend somewhat upon chemical stimulation, as this offers the most plausible explanation of the diversity of action seen with the different foodstuffs. As has been pointed out, gastric digestion is primarily a process for the conversion of proteid food into soluble products. It would be a mistake, however, to assume that the digestion of proteid foods is complete in the stomach. Stomach digestion is to be considered more as a preliminary step, paving the way for further changes to be carried forward by the combined action of intestinal and pancreatic juice in the small intestine. The importance of gastric digestion is frequently overrated. It is unquestionably an important process, but not absolutely essential for the maintenance of life. Dogs have lived and flourished with their stomachs removed, the intestine being joined to the œsophagus. The intestine is a much more important part of the alimentary tract; it is likewise far more sensitive to changing conditions than the stomach, and undoubtedly one function of the latter organ is to protect the intestine and preserve it from insult. The stomach may be compared to a vestibule or reservoir, capable of receiving without detriment moderately large amounts of food, together with fluid, in different forms and combinations, with the power to hold them there until by action of the gastric juice they are so transformed that their onward passage into the intestine can be permitted with perfect safety. Then, small portions of the properly prepared material may be discharged from time to time through the pylorus without danger of overloading the intestine, and in a form capable of undergoing rapid and complete digestion. Further, the stomach as a reservoir is very useful in bringing everything to a proper and constant temperature before allowing its entry into the intestine. Another fact of some importance is that, contrary to the general view, absorption from the stomach of the products of digestion is not very rapid under ordinary conditions. Even water and soluble salts pass very slowly into the circulation from the stomach. Like the partially digested food material, they are carried forward through the pyloric sphincter into the intestine, where absorption of all classes of material is most marked.

It is in the small intestine that both digestion and absorption are seen at their best. It is here that all three classes of foodstuffs are acted upon simultaneously through the agency of the pancreatic juice, intestinal juice, and bile. Here, too, are witnessed some of the most complicated and interesting reactions and changes occurring in the whole range of digestive functions. Especially noteworthy is the peculiar mechanism by which the secretion of pancreatic juice is set up and maintained. On demand, pancreatic juice is manufactured in the pancreas and poured into the intestine just beyond the pylorus through a small duct—the duct of Wirsung. Secretion is started by contact of the acid contents of the stomach with the mucous membrane of the small intestine, so that as soon as the acid chyme passes through the pyloric sphincter there commences an outflow of pancreatic juice into the intestine. While acid is plainly the inciting agent in this secretory process, its action is indirect. It does not cause secretion through reflex action on nerve fibres, but it acts upon a substance formed in the mucous membrane of the intestine, transforming it into secretin, which is absorbed by the blood and carried to the pancreas, where it excites secretory activity. As would be expected from the foregoing statements, the secretion of pancreatic juice commences very soon after food finds its way into the stomach, and naturally increases in amount with the onward passage of acid chyme into the intestine, the maximum flow being obtained in the neighborhood of the third or fourth hour, after which the secretion gradually decreases. In man, it is estimated on the basis of one or two observations that the amount secreted during 24 hours is about 700 cc., or a pint and a half. Careful experiments, however, tend to show that the quantity of secretion depends in some measure at least upon the character of the food, and also that the composition of the secretion varies with the character of the food. Thus, on a diet composed mainly of meat, the proteid-digesting enzyme is especially conspicuous, while on a bread diet, with its large content of starch, the starch-digesting enzyme is increased in amount. In other words, there is suggested the possibility of an adaptation in the composition of the secretion to the character of the food to be digested.

Pancreatic juice is an alkaline fluid, rather strongly alkaline in fact, from its content of sodium carbonate, and is especially characterized by the presence of at least three distinct enzymes; viz., trypsin, a proteid-digesting ferment; lipase, a fat-splitting enzyme; and amylopsin, a starch-digesting enzyme. It has already been pointed out how dependent the secretion of pancreatic juice is upon the co-operation of the intestinal mucous membrane. A similar dependence is found when the digestive activity of the secretion is studied. As just stated, pancreatic juice contains a proteid-digesting enzyme. This statement, however, is not strictly correct, for if the secretion is collected through a cannula so that it does not come in contact with the mucous membrane of the intestine, it is found free from any digestive action on proteids. The secretion is activated, however, by contact with the duodenal membrane. Expressed in different language, pancreatic juice as it is secreted by the gland does not contain ready-formed trypsin; it does contain, however, an inactive pro-enzyme, which, under the influence of a specific substance contained in the intestinal mucous membrane, known as enterokinase, is transformed into the active enzyme trypsin. There is thus seen another suggestive example of the close physiological relationship between the small intestine and the activity of the pancreatic gland, or its secretion.

The chemical changes taking place in the small intestine are many and varied. The acid chyme, with its admixture of semi-digested food material, as it passes through the pyloric sphincter into the small intestine, is at once brought into immediate contact with bile, pancreatic juice, and intestinal juice, all of which are more or less alkaline in reaction. As a result, the acidity of the gastric juice is rapidly overcome, and the enzyme pepsin, which up to this point could exert its characteristic digestive action, is quickly destroyed by the accumulating alkaline salts. Pepsin digestion thus gives way to trypsin digestion,—most effective in an alkaline medium,—and the proteids of the food, already semi-digested by pepsin-acid, are further transformed by trypsin; aided and abetted by another enzyme, known as erepsin, secreted by the mucous membrane of the intestine. These two enzymes are much more powerful agents than pepsin. It is true that they begin work where pepsin left off, but most striking is the character of the end-products which result from their combined action, since they are small molecules and there is a surprising diversity of them. In other words, while gastric digestion breaks down the proteid foodstuffs into soluble bodies, such as proteoses and peptones closely related to the original proteids, in pancreatic digestion as it takes place in the intestine there is a profound breaking down, or disruption of the proteid molecule into a row of comparatively simple nitrogenous fragments, many of them crystalline bodies; such as leucin, tyrosin, glutaminic acid, aspartic acid, arginin, lysin, histidin, etc., known chemically as monoamino-acids and diamino-acids. We have no means of knowing to how great an extent these more profound disruptive changes of the proteid molecule take place in the intestine. Whether practically all of the ingested proteid food is broken down into these relatively simple compounds prior to absorption, or whether only a small fraction suffers this change, cannot be definitely stated.

A few years ago, the majority of physiologists held to the view that in the digestion of proteid food all that was essential was its conversion into soluble and diffusible forms which would permit of ready absorption into the blood. The belief was prevalent that, since the proteid of the food was destined to make good the proteid of the blood and through the latter the proteids of the tissues, any change beyond what was really necessary for absorption of the proteid would be uneconomical and indeed wasteful. On the other hand, due weight must be given to the fact that in trypsin digestion, proteid can be quickly broken down into simple nitrogenous compounds, and that in the enzyme erepsin, present in the mucous membrane of the intestine, we have an additional ferment very efficient in bringing about cleavage of proteoses and peptone into amino-acids. From these latter facts it might be argued that, in the digestion of proteid foodstuffs by the combined action of gastric and pancreatic juice in the alimentary tract, a large proportion of the proteid is destined to undergo complete conversion into amino-acids, and that from these fragments the body, by a process of synthesis, can construct its own peculiar type of proteid.

This latter suggestion is worthy of a moment’s further consideration. As is well known, every species of animal has its own particular type of proteid, adapted to its particular needs. The proteids of one species directly injected into the blood of another species are incapable of serving as nutriment to the body, and frequently act as poisons. Man in his wide choice of food consumes a great variety of proteids, all different in some degree from the proteids of his own tissues. Is it not possible, therefore, that it is the true function of pancreatic and intestinal digestion to break down the different proteids of the food completely into simple fragments, so that the body can reconstruct after its own particular pattern the proteids essential for its nourishment? Or, we can follow the suggestion contained in the work of Abderhalden,[10] who finds that in the long continued digestion of various proteids by pancreatic juice there results in addition to the amino-acids a very resistant residue, non-proteid in nature, which is termed polypeptid. In other words, Abderhalden believes that pepsin, trypsin, and erepsin are not capable of bringing about a complete breaking down of proteids into amino-acids, but that there always remains a nucleus of the proteid not strictly proteid in nature, though related thereto,—polypeptid,—which may serve as a starting-point for the synthesis or construction of new proteid molecules, the various amino-acids being employed to finish out the structure and give the particular character desired. This view, however, is rendered somewhat untenable by the more recent experiments of Cohnheim,[11] who claims that proteids can be completely broken down by pepsin, trypsin, and erepsin, and consequently polypeptids would hardly be available for the synthesis of proteids. Moreover, Bergell and Lewin[12] have ascertained that there is present in the liver an enzyme or ferment which has the power of digesting or breaking down certain dipeptids and polypeptids into amino-acids. Hence, it follows that if any polypeptids are absorbed from the intestine, they would naturally be carried to the liver, where further cleavage into fragments suitable for synthetical processes might occur. In any event, there is good ground for the belief that the more or less complete disruption of the proteid molecule into small fragments renders possible a synthetical construction of new proteid to meet the demands of the organism; a fact of great importance in our conception of the possibilities connected with this phase of proteid nutrition.

Fatty foods undergo little or no chemical alteration until they reach the small intestine. During their stay in the stomach they naturally become liquid from the heat of the body, and there is more or less liberation of fat from the digestive action of gastric juice on cell walls, connective tissues, etc. Most food fat is in the form of so-called neutral fat, which must undergo hydrolysis or saponification before it can be absorbed and thus made available for the body. This is accomplished by the enzyme lipase, or steapsin, of the pancreatic juice, aided indirectly by the presence of bile. Under the influence of this fat-splitting enzyme all neutral fats, whether animal or vegetable, are broken apart, through hydrolysis, into glycerin and a free fatty acid; the latter reacting in some measure with the sodium carbonate of the pancreatic juice to form a sodium salt, or soluble soap, while perhaps the larger part of the fatty acid is held in solution by the bile present. Soap, free acid, and glycerin are then absorbed from the intestine and are found again combined in the lymph as neutral fat. In this way the fats of the food are rendered available for the nourishment of the body.

The next important chemical change taking place in the small intestine is that induced by the amylopsin of the pancreatic juice, which, acting in essentially the same manner as the ptyalin of saliva, converts any unaltered starch into dextrins and sugar. The latter substance, maltose, is exposed to the action of another enzyme contained in the intestinal secretion termed maltase, which transforms it into dextrose, a monosaccharide.

In these ways the proteids, fats, and carbohydrates of the food are gradually digested, so far as conditions will admit, digestion being practically completed by the time the material reaches the ileocæcal valve at the beginning of the large intestine. Throughout the length of the small intestine absorption proceeds rapidly; water, salts, and the products of digestion passing out from the intestine into the circulating blood and lymph. At the ileocæcal valve, however, the contents of the intestine are practically as fluid as at the beginning of the small intestine, due to the fact that water is continually being secreted into the intestine. In the large intestine, the contents become less and less fluid through reabsorption of the water, and as the propulsive movements of the circular and longitudinal muscle fibres of the intestinal wall carry the material onward toward the rectum, the last portions of available nutriment are absorbed. Finally, in varying degree, certain putrefactive changes are observed in the large intestine involving a breaking down of some residual proteid matter, through the agency of micro-organisms almost invariably present, with formation of such substances as indol, skatol, phenol, fatty acids, etc. These processes, however, in health are held rigidly in check, and count for little in fitting the food for absorption. Digestion, on the other hand, extending as we have seen from the mouth cavity to the ileocæcal valve, is the handmaiden of nutrition, preparing all three classes of organic foodstuffs for their passage into the circulating blood and lymph, and thus paving the way for their utilization by the hungry tissue cells.