DIGESTION

Any discussion in regard to the digestibility of foods must be general, because food which agrees with one may disagree with another, and a food which disagrees with one at a particular time may entirely agree with him at some other time; therefore, before one passes upon the adaptability of a food to the individual, it should be known that this food agrees or disagrees with him under varying conditions.

The digestibility of food depends largely upon the physical condition of the individual, because the amount of digestive juices poured into the alimentary canal is influenced by this condition, particularly by the condition of the nerves. If sufficient juices, in proper proportions, are not poured into the digestive tract, the foodstuffs are not made soluble for absorption into the blood. Digestion is practically synonymous with solution,—all solid foods must be reduced to a liquid state, throughdigestive juices and water, before they can pass through the walls of the stomach and intestines.

Each individual shouldlearn to likethe foods containing the nutrient elements which experience and blood tests have shown to be lacking in his case. The question of likes and of dislikes in regard to foods, is largely habit, and one can learn to like almost any food one wishes.

Where one forms the habit of discriminating too much in the food, or discarding this food or that, because at some time it has disagreed, due to the particular condition at the time, the mind approaches the table as a more or less pessimistic censor and the saliva and the gastric juices are retarded in their flow.

When one is exercising freely, so that the muscular and mucous coats of the digestive system are strong, the body will handle foods which, during sedentary habits, it would not digest. There are kinds of foods, however, which, to certain individuals, according to the chemical composition of the body, act as actual poisons, e. g., strawberries, cheese, or coffee.

It may be well to here trace, briefly, the progress of the food through the digestivetract and the action of the juices and the ferments upon it.[3]

Salivary Digestion

The food in the mouth is mixed with saliva, which dissolves the starches, converting them into sugar. The starches are the only foods whose chemical digestion is begun in the mouth. They are first broken up into dextrin and then into the more simple sugar, known as animal starch, ormaltose. Hereafter, in speaking of sugar, after it has been absorbed into the blood, the reader will bear in mind that the term refers not only to digested sugar, consumed as such, but also to digested starches (maltose), as shown on page63.

It is important that sufficient saliva be mixed with the food, through mastication, that it may enter the stomach and there continue the chemical process of digestion of starch. If starches are not thoroughly masticated, sufficient saliva will not enter the stomach to convert the starch into sugar; the food will pass into the small intestine, which must then do more than its normal work of digestion.

The saliva consists of about ninety-nine and one-half per cent water and one-half per cent solids. The solids consist of ptyalin, sodium chlorid, sodium carbonate (baking soda), mucus, and epithelium. Ptyalin, the most important of these, is an active digestive agent; the mucus lubricates the masticated food; the sodium carbonate insures the alkalinity of the food; the salt is present in all secretions; and the water dissolves the food that the juices may more readily reach and act upon each particle.

The saliva flows into the mouth, more or less, at all times, but more copiously during mastication. Its evident purpose, when food is not present, is to keep the lining of the mouth moist.

The flow of saliva is controlled, to a great degree, by nerves which have their centers in the medulla oblongata. The sight of food, pleasingly served, or even the thought of food which one likes, will increase the salivary flow. This is one instance of the control of thought materially affecting digestion, and the importance offorming the habit of cultivating a taste for all kinds of food, is apparent. The stronger the relish for the food, and the more thoroughly it is masticated, andmixed with the saliva, the more perfect will be the first step in digestion. This first step of thorough mastication is all important, not only because the chemical action upon the starch molecules is facilitated by the thorough softening and mixing with the saliva, but thorough mastication also tends to prevent overeating.

Water encourages the flow of saliva and for this reason should be drunk copiously before meals, particularly where digestion is weak. It may also be taken at rest periods during the meal. (See page44).

Stomach Digestion

As the food enters the stomach, the gastric juice pours out from the mucous lining, very much as the saliva pours into the mouth. It consists of ninety-nine and one-half per cent water and one-half per cent solids, as does the saliva. The solids of the gastric juice are composed of pepsin, rennin, hydrochloric acid, and mucus. The mucus serves to lubricate the food as in the saliva. It also prevents the digestion of the mucous lining of the stomach itself.

The hydrochloric acid and the pepsin cause the principal chemical changes in the food while in the stomach, acting alone upon the proteins. The only digestion ofstarches in the stomach is that continued by the saliva. The salivary digestion proceeds until the gastric juice is secreted in sufficient quantity to cause a marked acidity of the stomach contents, when the starches are passed into the intestines.

Gastric juice begins to flow into the stomach soon after eating, but it is not secreted in sufficient quantity to supersede salivary digestion for from twenty to forty-five minutes.

The result of gastric digestion of proteins is their conversion, first, into albumin, then into proteosis and, lastly, into peptone, which is protein in a more simple, soluble, and diffusible form. In the form of peptone, the proteins are in condition to be absorbed.

If the food has been properly cooked and masticated the gastric digestion will be completed in one and one-half to three hours. If not properly cooked and masticated, the stomach digestion may continue one to two hours longer. It should, however, be completed in three hours.

The most readily digested animal foods remain less time in the stomach. Meat, as a rule, is easily digested, because the action of the digestive juices of the animal has converted the starches andsugars. The white meat of chicken, being soft, is digested in a shorter time than the red or the dark meat.

Fluids leave the stomach more rapidly than solids. Seven ounces of water leave the stomach in one and one-half hours, seven ounces of boiled milk in about two hours.

The flow of gastric juice, as the flow of saliva, is governed by the nerves;—the sight, taste, and smell of food, and the attitude of mind toward it, to a certain extent, regulates its flow.

After the food has extensively accumulated, during the progress of a meal, the stomach begins a series of wave-like movements called peristaltic waves.[4]These waves work downward through the length of the stomach towards its lower opening, known as the pyloric orifice. As the food is moved down the stomach by these motions, it is thoroughly mixed with the gastric juice.

During the early stages of digestion, the sphincter muscles of the pylorus keep the lower end of the stomach closed, but, as digestion progresses, the pylorus gradually relaxes to let the digested,soluble portion of the food pass into the intestine. If the food still remains in a solid form, by reason of being improperly cooked or poorly masticated, as it touches the pylorus, these sphincter muscles, almost as if they were endowed with reasoning faculties, close, forcing the undigested mass back to be further acted upon by the gastric juice,—the solid mass is not allowed to pass until dissolved.

If the individual continues to abuse the stomach and to cause it to work overtime, it becomes exhausted and demands rest; it refuses to discharge the gastric juice in proper proportion; the peristaltic movements are weak; and food is not promptly or forcefully moved along the stomach and mixed with the gastric juice. This demand for a rest is termedIndigestion.

To sum up,—digested sugar is dextrose; digested starch is first dextrin, then maltose (animal, sugar); digested protein is peptone; and, digested fat is saponified fat.

Intestinal Digestion

The food passes from the stomach, through the pylorus into the small intestine. The first twelve inches of the small intestine is known as the duodenum. In the duodenum it is acted upon bythe pancreatic juice from the pancreas, the bile from the liver, and the intestinal juices. These juices act upon proteins, fats, and carbohydrates. The bile acts upon the fats, while the pancreatic and intestinal juices act chiefly upon the carbohydrates.

As the food enters the intestine, it is changed, by the sodium carbonate, from the acid condition produced in the stomach, to alkaline reaction.

The bile exercises an important influence upon digestion, any disturbance in the flow of this greenish-brown secretion being very quickly shown both in stomach and intestinal digestion. It emulsonizes and saponifies the fats, it aids in their absorption, and it lubricates the intestinal mass, facilitating its passage through the entire length of the intestines. Thus, it is a very potent agent in regulating the bowel movements. A diminution in the flow of bile quickly expresses itself in constipation.

Fats are almost entirely digested in the small intestine. The presence of fat stimulates the flow of pancreatic juice, which, in turn, stimulates the flow of bile from the liver. For this reason, if the liver is sluggish, fatty foods are desirable. Olive oil is prescribed for gall stones to stimulate the action of the bile ducts.

Before the fat molecules can be absorbed, they must first be broken up into glycerin and fatty acids and further changed to a fine emulsion, which gives the contents of the small intestine a milky appearance. After they are broken up into these fatty acids and thus brought to the finest state of emulsion, they are readily saponified, being then soluble in water and in a state to be absorbed by the walls of the intestines. The fats are absorbed almost entirely in the small intestine,—mostly in the duodenum.

As a rule, the starches, or dextrin, will not be fully digested by the saliva and those which have failed of salivary digestion are acted upon by amylase (one of the solids of the intestinal juice) and changed to maltose, while the trypsin from the pancreas, together with the intestinal juice, acts upon any protein which has failed to be fully digested in the stomach, changing it into peptone. In the form or peptone it is absorbed through the “sucking” villi of the intestinal walls.

The food is forced along the intestinal tract by peristaltic or muscular relaxation and contraction waves, as in the stomach. As it is so forced, the nutrient elements, after being put into condition for absorption,are taken up through the villi of the intestinal walls by the portal veins and the lacteals of the sub-mucous lining. (See page78).

It is now believed that a larger proportion of food is digested and absorbed than was heretofore realized, and that the excretions from the intestines are, in many cases, made up almost entirely of refuse, and of the catabolic waste of the system. In an ordinary, mixed diet, it is stated that about ninety-two per cent of the proteins, ninety-five per cent of the fats, and ninety-seven per cent of the carbohydrates are retained by the body.

In digestion, it is of the utmost importance that the muscular, mucous, and the sub-mucous coats, and the secreting glands of the stomach and intestines be kept thoroughly strong and active, that the digestive juices may be freely poured out, the nutriment be freely absorbed, and the food be moved along the digestive tract. The strength of any organ is gained through the nutriment in the blood; therefore, daily exercise, which calls the blood freely to these organs, is imperative.

Absorption of Food

The greater part of the food is absorbed through the intestines, yet some proteins, which have been fully digested by the gastric juice, and certain fats, particularly the fats in milk, which are in a natural state of emulsion, may be absorbed through the walls of the stomach. However, the absorption through the stomach is small compared to that through the intestines.

The small intestine is particularly fitted for absorption. Every inch or so along its course the mucous lining is thrown up into folds, as if to catch the food as it passes toward the large intestine, and to hold it there until the villi have the opportunity to absorb it. These transverse folds of the intestinal walls are called valvulæ conniventes. The villi are fingerlike projections of the mucous lining of the intestines, which stand out upon the lining somewhat as the nap on plush. They have been called “sucking” villi, because during the movements of the intestines they seem to suck in the liquid food. As soon as the foodstuffs,—proteins, carbohydrates, and fats, are put in a dissoluble state ready for absorption, they are very promptly absorbed by the villi. If, forany reason, they remain unabsorbed, they are liable to ferment by the action of the trypsin, or to be attacked by the bacteria always present in the intestines.

The peptones, sugars, and saponified fats are rapidly absorbed, while the undigested portion, together with the unabsorbed water, the bile, mucus and bacterial products, are passed through the ileo-cecal valve into the large intestine.

That the large intestine is also adapted to the absorption of fats is shown by clinical experiments with patients who cannot retain food in the stomach, the food in such cases being given through rectal injections.

In the large intestine, the mass passes up the ascending colon, across the transverse colon, and down the descending colon, losing, by absorption, foodstuffs not absorbed in the stomach and small intestine.

While water and salt are absorbed both in the stomach and in the small intestine, the evident purpose in leaving the larger part of the water to be absorbed in the large intestine is that it may assist the intestinal contents in passing along. The water also stimulates the peristaltic movement.

As the food is absorbed through the walls of the alimentary canal, it is picked up by the rootlets of the mesenteric veins[5]and by the lymph channels,—the latter, through the abdominal cavity, are called lacteals. Nearly all of the fats are absorbed through the lacteals. The whitish color given to the contents of the lacteals, by the saponified fats, gives rise to the term lacteal, meaning “whitish.”

Nearly all of the proteins and sugars pass through the mesenteric veins and the portal veins into the liver. Here the sugars are at once attacked by the liver cells and built up into glycogen as described on page81and the proteins are passed through the liver into the arterial blood stream. A small portion of the proteins, however, do not go to the liver, but are passed directly into the lymphatics and thus into the blood stream, where they are again carried to the liver.

To sum up,—the larger part of the absorption of sugars, starches, proteins, and fats is through the small intestine, though some are absorbed in the stomach and a very little through the large intestine; while some water and salts are absorbedin the stomach and small intestine, these are largely absorbed in the large intestine.

FOOTNOTES:[3]A knowledge of the mucous lining of the stomach and intestines, and of the tributary glands, such as the liver and pancreas, is important to a thorough understanding of digestion, and the reader is referred to “The Vital Organs: Their Use and Abuse” of this series. This takes up the study of the secretion of digestive juices, the conditions favoring normal secretions, etc.[4]See “The Vital Organs; Their Use and Abuse” by Susanna Cocroft.[5]For illustration see the frontispiece of “The Circulation, Lungs, Heart,” of this series.

[3]A knowledge of the mucous lining of the stomach and intestines, and of the tributary glands, such as the liver and pancreas, is important to a thorough understanding of digestion, and the reader is referred to “The Vital Organs: Their Use and Abuse” of this series. This takes up the study of the secretion of digestive juices, the conditions favoring normal secretions, etc.

[4]See “The Vital Organs; Their Use and Abuse” by Susanna Cocroft.

[5]For illustration see the frontispiece of “The Circulation, Lungs, Heart,” of this series.

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