The nutrition of man

by R. H. Chittenden

As a digestive secretion, saliva serves several important purposes. By moistening the food it renders mastication and deglutition possible; its natural alkalinity tends to neutralize somewhat such acidity as may be present in the food; it dissolves various solid substances, thus making a solution capable of stimulating the taste nerves; lastly, and most important, it has a marked digestive and solvent action on starchy foods. A large proportion of the non-nitrogenous food consumed by man—in most countries—is composed of some form of starch, and this the body cannot use until it has undergone conversion into soluble forms, such as dextrins and sugar. This it is the function of saliva to accomplish, and it owes its activity in this direction to the presence of a soluble ferment or enzyme known as ptyalin.

Enzymes, which play so important a part in all digestive processes, are a peculiar class of substances produced by the living cells which constitute the various secreting glands. They are of unknown composition, and are peculiar in that the chemical changes they induce are the result of what is termed catalysis, i. e., contact. That is, the enzyme or catalyzer does not enter into the reaction, it is not destroyed or used up, but by its mere presence sets in motion or accelerates a reaction between two other substances. The ordinary illustration from the inorganic world is spongy platinum, which, if placed in contact with a mixture of oxygen and hydrogen, causes the two gases to unite with formation of water, although the two gases alone at ordinary temperature will not so combine. In this reaction the platinum is not altered, neither does it apparently enter into the reaction; it is a simple catalyzer. The chemical nature of the change which most digestive enzymes produce is usually defined as hydrolytic, in which the substance undergoing transformation is made to combine with water, thus becoming hydrolyzed, this reaction generally being accompanied by a cleavage or splitting of the molecule into simpler substances. It is to be noted further that enzymes are specific in their action. An enzyme that acts upon starch, for example, cannot act on proteids or fats. Some digestive fluids have the power of producing changes in different classes of foodstuffs, but such diversity of action is always assumed to be due to the presence in the same fluid of different enzymes. Emil Fischer[6] has advanced the theory that the specificity of an enzyme is related to the geometrical structure of the substance undergoing change; i. e., that each enzyme is capable of acting upon or attaching itself only to such molecules as have a definite structure with which the enzyme is in harmony. Or, the enzyme may be considered as a key which will fit only into the lock (structure) of the molecule it acts upon.

One characteristic feature of enzymes is the incompleteness of their action. Thus, the enzyme of saliva transforms starch by a series of progressive changes into soluble starch, two or more dextrins, and the sugar maltose as the chief end-product. A mixture of starch paste and saliva under ordinary conditions, however, never results in the formation of a hundred per cent of maltose, but there always remains a variable amount of dextrin which appears to resist further change. This is apparently due to what is known as the reversible action of enzymes. Thus, the chemical reactions involved here are reversible actions, i. e., they take place in opposite directions. The catalyzer not only accelerates or incites a reaction in the direction of breaking down the substance acted upon, but it also aids in the recomposition of the products so formed into the original or kindred substance. With reversible reactions of this sort the opposite changes sooner or later strike an equilibrium, which remains constant until some alteration in the conditions brings about an inequality and the reactions proceed until a new equilibrium is established. In the body, however, where the circulating blood and lymph provide facilities for the speedy removal by absorption of the soluble products formed, the reaction may proceed until the original substance undergoing change is completely transformed into the characteristic end-product. This reversible action of enzymes is an important feature, and helps explain certain nutritional changes to be referred to later. Whether all enzymes behave in this way is not as yet determined.

Another peculiarity of digestive enzymes is their extreme sensitiveness to changes in their environment. Powerful in their ability to transform relatively large quantities of a given foodstuff into simple products better adapted for absorption and utilization by the body, they are, however, quickly checked in their action, and even destroyed, when the conditions surrounding them are slightly interfered with. They require for their best action a temperature closely akin to that of the healthy body, and any great deviation therefrom will result at once in an inhibition of their activity. Further, they demand a certain definite reaction of the fluid or mixture, if their working power is to be maintained at the maximum. Indeed, many enzymes, like the ptyalin of saliva, are quickly destroyed if the reaction is greatly changed. Enzymes are thus seen to be more or less unstable substances, endowed with great power as digestive agents, but sensitive to a high degree and working advantageously only under definite conditions. Many perversions of digestion and of nutrition are connected not only with a lack of the proper secretion of some one or more digestive enzyme, but also with the lack of proper surroundings for the manifestation of normal or maximum activity.

With these statements before us, we can readily picture for ourselves the initial results following the ingestion of starch-containing foods properly cooked; and it may be mentioned here that the cooking is an essential preliminary, for uncooked starch cannot be utilized in any degree by man. With the mind in a state of pleasurable anticipation, with freedom from care and worry, which are so liable to act as deterrents to free secretion, and with the food in a form which appeals to the eye as well as to the olfactories, its thorough mastication calls forth and prolongs vigorous salivary secretion, with which the food becomes intimately intermingled. Salivary digestion is thus at once incited, and the starch very quickly commences to undergo the characteristic change into soluble products. As mouthful follows mouthful, deglutition alternates with mastication, and the mixture passes into the stomach, where salivary digestion can continue for a limited time only, until the secretion of gastric juice eventually establishes in the stomach-contents a distinct acid reaction, when salivary digestion ceases through destruction of the starch-converting enzyme. Need we comment, in view of the natural brevity of this process, upon the desirability for purely physiological reasons of prolonging within reasonable limits the interval of time the food and saliva are commingled in the mouth cavity? It seems obvious, in view of the relatively large bulk of starch-containing foods consumed daily, that habits of thorough mastication should be fostered, with the purpose of increasing greatly the digestion of starch at the very gateway of the alimentary tract. It is true that in the small intestine there comes later another opportunity for the digestion of starch; but it is unphysiological, as it is undesirable, for various reasons, not to take full advantage of the first opportunity which Nature gives for the preparation of this important foodstuff for future utilization. Further, thorough mastication, by a fine comminution of the food particles, is a material aid in the digestion which is to take place in the stomach and intestine. Under normal conditions, therefore, and with proper observance of physiological good sense, a large proportion of the ingested starchy foods can be made ready for speedy absorption and consequent utilization through the agency of salivary digestion.

Nowhere in the body do we find a more forcible illustration of economical method in physiological processes than in the mechanism of gastric secretion. Years ago, it was thought that the flow of gastric juice was due mainly to mechanical stimulation of the gastric glands by contact of the food material with the lining membrane of the stomach. This, however, is not the case, as Pawlow has clearly shown, and it is now understood that the flow of gastric juice is started by impulses which have their origin in the mouth and nostrils; the sensations of eating, the smell, sight, and taste of food serving as psychical stimuli, which call forth a secretion from the stomach glands, just as the same stimuli may induce an outpouring of saliva. These sensations, as Pawlow has ascertained, affect secretory centres in the brain, and impulses are thus started which travel downward to the stomach through the vagus nerves, and as a result gastric juice begins to flow. This process, however, is supplemented by other forms of secretion, likewise reflex, which are incited by substances, ready formed in the food, and by substances—products of digestion—which are manufactured from the food in the stomach. Soups, meat juice, and the extractives of meat, likewise dextrin and kindred products, when present in the stomach, are especially active in provoking secretion. Substances which in themselves have less flavor, as water, milk, etc., are far less effective in this direction, while the white of eggs and bread are entirely without action in directly stimulating secretion. When the latter foods have been in the stomach for a time, however, and the proteid material has undergone partial digestion, then absorption of the products so formed calls forth energetic secretion of gastric juice. It is thus seen that there are three distinct ways—all reflex—by which gastric juice is caused to flow into the stomach as a prelude to gastric digestion. Further, it has been shown by Pawlow that there is a relationship between the volume and character of the gastric juice secreted and the amount and composition of the food ingested, thus suggesting a certain adjustment in the direction of physiological economy well worthy of note. A diet of bread, for example, leads to the secretion of a smaller volume of gastric juice than a corresponding weight of meat produces, but the juice secreted under the influence of bread is richer in pepsin and acid, i. e., it has a greater digestive action than the juice produced by meat. The suggestion is that gastric juice assumes different degrees of concentration, with different proportions of acid and pepsin, to meet the varying requirements of a changing dietary.

As has been indicated, pepsin and hydrochloric acid are the important constituents of gastric juice. It is noteworthy, however, that it is the combination of the two that is effective in digestion. Pepsin without acid is of no avail, and acid without pepsin can accomplish little in the digestion of food. Pepsin and acid are secreted by different gland cells in the stomach, and gastric insufficiency, or so-called indigestion, may arise from either a condition of apepsia or from hypoacidity. It is worthy of comment that the amount of hydrochloric acid secreted during 24 hours by the normal individual, under ordinary conditions of diet, amounts to what would constitute a fatal dose of acid if taken at one time in concentrated form. At the outset of gastric secretion, the fluid shows only a slight degree of acidity, but as secretion proceeds, the acidity rises to 0.2–0.3 per cent of hydrochloric acid. The main action of gastric juice is exerted on proteid foods, which under its influence are gradually dissolved and converted into soluble products known as proteoses and peptones. It is a process of peptonization, in which the proteid of the food is gradually broken down into so-called hydrolytic cleavage products. The enzyme, like the ptyalin of saliva, is influenced by temperature, maximum digestive action being manifested at about 38° C., the temperature of the body. Further, a certain degree of acidity is essential for procuring the highest degree of efficiency. Ordinarily, it is stated that digestive action proceeds best in the presence of 0.2 per cent hydrochloric acid, but what is more essential for vigorous digestion is a certain relationship between the acid, pepsin, and proteid undergoing digestion. As pepsin and the amount of proteid are increased, the amount of acid, and its percentage somewhat, must be correspondingly increased if digestion is to be maintained at the maximum.

Another important function of gastric juice is that of curdling milk, due to the presence in the secretion of a peculiar enzyme known as rennin. The latter ferment acts upon the casein of milk,—the chief proteid constituent,—transforming it into a related substance commonly called paracasein. This then reacts with the calcium salts present in milk, forming an insoluble curd or calcium compound. From this point on, the digestion of milk-casein by gastric juice is the same as that of any other solid proteid, it being gradually transformed by the pepsin-acid into soluble cleavage products. Why gastric juice should be provided with this special enzyme, capable of acting solely on the casein of milk, can only be conjectured, but we may assume that it has to do with the economical use of this important food. As the sole nutriment of the young, milk occupies a peculiar position as a foodstuff, and being a liquid, its proteid constituent might easily escape complete digestion were it to pass on too hastily through the gastro-intestinal tract. Experiment has shown that when liquid food alone is taken into the stomach it is pushed forward into the small intestine in a comparatively short time. Curdled as it is by rennin, however, casein must stay for a longer period in the stomach, like any other solid food, and its partial digestion by gastric juice thereby made certain. For the reasons above stated, it is apparent why milk should not be treated as a drink in our daily diet. Remembering that when milk reaches the stomach it is converted into a solid clot or curd, there is obvious reason for sipping it, instead of taking it by the glassful, thereby favoring the formation of small, individual clots instead of one large curd, and thus facilitating instead of retarding digestion.

Among other factors in gastric digestion, the muscular movements of the stomach walls are to be emphasized, since we have here a mechanical aid to digestion of no small moment, and likewise a means of accomplishing the onward movement of the stomach contents. The outer walls of the stomach are composed of a thick layer of circular muscular fibres, especially conspicuous at the pyloric end of the organ, where the latter is joined on to the intestine; a smaller, less conspicuous layer of longitudinal muscle fibres, and some oblique fibres. At the pylorus, the circular fibres are so arranged as to form a structure which, aided by a peculiar folding of the inner mucous membrane, serves as a sphincter, closing off the stomach from the duodenum, the beginning of the small intestine. The movements of the stomach were first made the subject of careful investigation by Dr. Beaumont in his study of the celebrated case of Alexis St. Martin, a French Canadian, who, in 1822, was accidentally wounded by the discharge of a musket, with the resultant formation of a permanent fistulous opening in the stomach. Dr. Beaumont, in the description[7] of his observations, writes that “by the alternate contractions and relaxations of these bands (of muscle) a great variety of motion is induced on this organ (the stomach), sometimes transversely, and at other times longitudinally. These alternate contractions and relaxations, when affecting the transverse diameter, produce what are called vermicular or peristaltic motions. . . . When they all act together, the effect is to lessen the cavity of the stomach, and to press upon the contained aliment, if there be any in the stomach. These motions not only produce a constant disturbance, or churning of the contents of this organ, but they compel them, at the same time, to revolve around the interior, from point to point, and from one extremity to the other.” Of more recent investigations, the most important are those made by Cannon,[8] with the X-ray apparatus. From these later studies, it is evident that Dr. Beaumont’s view of the entire stomach being involved in a general rotary movement is not correct, since in reality the movements are confined mainly to the pyloric end of the stomach, the fundus or portion nearer the œsophagus not being directly involved. This means that when food material passes into the stomach, it may remain at the fundic end for some time more or less undisturbed before admixture with the gastric juice occurs, and under such conditions, until acidity creeps in, the salivary digestion of starch can continue.

According to the observations of Cannon, the contractile movements of the stomach commence shortly after the entrance of food, the contractions starting from about the middle of the stomach and passing on toward the pylorus. These waves of contraction follow each other very closely, certainly not more than one or two minutes apart, and perhaps less, while the resulting movements bring about an intimate commingling of food and gastric juice in the pyloric portion of the stomach; followed by a gradual diffusion of the semi-fluid mixture into the fundus accompanied by a gradual displacement of the more solid food in the latter region. These movements of the stomach are more or less automatic, arising from stimuli—the acid secreted—originating in the stomach itself, although it is considered that the movements are subject to some regulation from extrinsic nerve fibres, such as the vagi and the splanchnics. As digestion proceeds and the mass in the stomach becomes more fluid, the pyloric sphincter relaxes and a certain amount of the fluid material is forced into the intestine by the pressure of the contraction wave. This is repeated at varying intervals, depending presumably in some measure upon the consistency of the mass in the stomach, until after some hours of digestion the stomach is completely emptied.