Part 9

Extractives (appetizing and flavoring elements).

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, through digestive juices and water, before they can pass through the walls of the stomach and intestines.

Each individual should learn to like the 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 digestive tract and the action of the juices and the ferments upon it.

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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, or maltose. 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 page 63.

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 of forming 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, and mixed 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 page 44).

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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 of starches 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 and sugars. 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. 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 termed Indigestion.

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.