Volume 5 — LESSON V

CHEMISTRY OF DIGESTION

DIGESTIVE ORGANS AND DIGESTIVE JUICES

First—THE MOUTH:

The three salivary glands of the mouth secrete the saliva, which is an alkaline substance containing a digestive enzym called ptyalin.

The saliva begins the digestion of starch and moistens food to facilitate swallowing.

Second—THE STOMACH:

The gastric juice secreted by the mucous lining of the stomach is an acid. It contains hydrochloric acid and pepsin, which act on proteids, changing them to proteoses ("intermediate products formed naturally in the process of digestion") and peptone.

The gastric juice also contains rennet, which acts directly on milk, and indirectly on all proteids.

Third—THE LIVER:

The liver secretes a digestive fluid called bile, which is an alkaline substance. Its chief purpose is to emulsify fats and to supply the alimentary tract with the requisite amount of moisture.

Fourth—THE PANCREAS:

The pancreatic juice, secreted by the pancreas, is an alkaline and slightly acidulous substance. It contains three enzyms, the names and action of which are as follows:

Amylopsin completes the digestion of starch.

Trypsin completes the digestion of proteids.

Steapsin converts fats into fatty acids and glycerin.

Fifth—THE SMALL INTESTINES

The intestinal juices secreted by the small intestines are alkaline substances which change sugar and maltose into glucose, and perform the last step in the process of breaking up or subdividing food so fine that it will pass through the intestinal walls into the circulation.

LESSON V

CHEMISTRY OF DIGESTION

Alternation of digestive juices

The digestive juices of the human body are five in number, namely: Saliva, gastric juice, bile, pancreatic juice, and the several intestinal juices. Beginning with the saliva these juices alternate, first an alkali, then an acid. It is the opinion of the writer that this alternating plan is carried on throughout the entire intestinal tract, as the final dissolution of food matter takes place in the intestinal canal. These five juices are secreted from the blood by special cells or glands. Each of these juices contain one or more enzyms or digestive principles. These enzyms are highly organized chemical compounds which have the property of changing other chemical compounds without being destroyed or used up themselves except in minute quantities.

Malt and yeast-cells

Malt, which was studied in the last lesson, and which is produced by the sprouting of barley, is a true digestive enzym of the barley. Yeast-cells are minute plants which secrete an enzym that causes the fermentation of bread. It was formerly thought that the fermentation of yeast could not take place except in the presence of a living cell. This has now been disproved, as a German scientist has succeeded in grinding up yeast-cells and filtering off the chemical compound or true enzym which causes the fermentation of sugar.

Fermentation due to enzyms

It is now recognized by scientists that all processes of fermentation and digestion found in plant and animal life are due to definite chemical compounds known as enzyms. The action of digestion is truly a chemical one, and could take place without the body as well as within, if we could manufacture the proper enzym and could produce the exact conditions of temperature, moisture, etc., that are found in the human digestive economy.

Predigested foods

The manufacture of predigested foods depends upon various processes of fermentation, or upon the digestion that may be carried on by inorganic chemical agents, such as acids, or by the ferments of bacteria, or other forms of life. The following are illustrations of these processes of predigestion:

1 The manufacture of glucose from starch by the action of sulfuric acid

2 The malting of starch for the production of malt-sugar or of fermented liquors

3 The making of cheese by the action of the enzym rennet which has been extracted from the stomach of a calf

A great amount of discussion, pro and con, has been raised over the subject of predigested food. The foregoing examples will show that the subject of predigested food, taken in its broadest sense, cannot be dismissed summarily with either approbation or disapproval. We must consider the particular chemical process involved in each case and the final chemical products, as well as its mechanical condition. These things must be taken into consideration when we pass an opinion upon the wholesomeness of a so-called predigested food.

With this diversion to illustrate the breadth and the importance of the action of enzyms, I will now return to the consideration of the chemical action of the human digestive organs.

SALIVA

The saliva is the digestive juice of the mouth. It is secreted by three pairs of salivary glands. The secretions from these three glands are slightly different in Starch digestion in the mouth composition, but for our purpose may be considered as one secretion. The saliva is an alkaline fluid, and the principal enzym that it contains is a starch-digesting enzym known as ptyalin, which can act only in an alkaline solution. As the gastric juice is strongly acid, the digestive action of the saliva is stopped soon after the food has entered the stomach, and the enzym is of no further use. The action of the saliva is very weak, and the amount of starch digestion which is accomplished in the mouth is comparatively insignificant.

Saliva and mastication

The chief function of the saliva is to moisten food and to facilitate swallowing. From these statements one might first infer that the emphasis given to thorough mastication is unwarranted. In fact, the mastication of food has a much more important function than the digestion of starch by saliva. This subject will be referred to again when the physical condition of food as a factor in digestion, and the nervous control or co-ordination of the various functions of the digestive system are considered. (See "Composition of Gastric Juice," p. 147.)

GASTRIC JUICE

Chief function of the stomach

The importance of the stomach as an organ of digestion has been overestimated in modern times. From the discussions in the average text-book and physiology, one would be led to believe that the stomach is the only organ of digestion, when, as a matter of fact, the chief purpose of the stomach is that of a receptacle for the storage of food for digestion further on. I do not mean by this statement that there is no digestive action in the stomach, but I do mean to say that there are no digestive processes completed in the stomach, and that all foods which are acted on by the gastric juice can also be acted on by the digestive juices in the intestines. This has been proved by the fact that surgeons have successfully removed the entire stomach from both animals and men without seriously interfering with the nutrition of the body. They merely had to eat more often, as the depot or storage receptacle had been removed.

Inaccuracy of digestive tables

The stomach should be considered as a preliminary organ of digestion. The tables published in the physiologies giving the digestibility of various foods as so many hours, refer entirely to the length of time it takes for the food to pass out of the stomach. According to these tables boiled rice is given as one of the most digestible of foods. As a matter of fact, the chief reason why rice passes out of the stomach more quickly than other grains, is because it contains practically nothing but starch, and as starch is not digested in the stomach, the rice is passed on to the next station where it can be acted on by an alkali.

Comparison of predigested and uncooked cereals

In this connection it becomes necessary to refer to the interpretation of the experimental results obtained by investigators at the Battle Creek Sanitarium. In these experiments cereal products which had been put through various processes of predigestion were compared with uncooked whole wheat, the contents being removed from the stomach after a given period. The results of this experiment showed a greater amount of starch digestion in the case of the dextrinized or super-cooked foods. These results were published as proof that starchy foods should be put through a process of super-cooking, dextrinization or predigestion. To those who are not familiar with food chemistry, such results would appear very convincing, but to a well-informed food scientist they only illustrate how misinterpretation of scientific facts may indicate conclusions opposed to the truth.

Starchy foods are not intended by Nature to be digested in the stomach, but in the intestines, and the processes of partial digestion of these foods, by artificial means, before entering the stomach, serve only to interfere with Nature's plan, and to deprive both the stomach and the intestines of their natural functions.

COMPOSITION OF THE GASTRIC JUICE

Action of pepsin on proteids

The gastric juice contains three principal enzyms or digestive principles. These are hydrochloric acid, pepsin, and rennet. The hydrochloric acid and the pepsin are secreted by different cells, and could be considered as separate digestive juices, but as the action of one is dependent upon the other, I will consider these actions as one. Pepsin, in the presence of hydrochloric acid, acts on proteids, and changes them into proteoses Peptone and proteoses and peptone. Comparatively little food is completely peptonized in gastric digestion. Proteoses are intermediate products between food proteids and peptone, being the principal product of the action of the gastric juice. Thus it is seen that this stomach-action is only preparatory for the digestive processes of the intestines.

Action of gastric juice on fat

The gastric juice does not act on fat, but in the case of animal food, in which the membranes or connective tissues that enclose the fat-cells are formed of proteid material, the gastric juice sets the fat-globules free by dissolving these enclosing membranes.

Purpose of hydrochloric acid

The chief action of hydrochloric acid in the stomach is to aid the action of the pepsin. Pepsin alone has no digestive power. There are no other acids produced by the secretive glands of the stomach. If other acids are found in the contents of the stomach, it is because they have been taken in with the food, or produced by abnormal fermentation.

How hydrochloric acid is formed

The source of hydrochloric acid is from the sodium chlorid or common salt of the blood. The secreting cells of the stomach-glands are thought to have the power to form hydrochloric acid by uniting the chlorin of the salt with the hydrogen of the water. This is a very unusual chemical process, and has not yet been successfully produced in a laboratory.

Hydrochloric acid as an antiseptic

One of the chief functions of hydrochloric acid in the stomach is that of an antiseptic. In other words, hydrochloric acid kills bacteria. This is not true of all bacteria, for some germs can live in an acid medium, while others may live best in an alkaline solution. The alternation of the digestive juices from alkali to acid is a provision of Nature which has a dual purpose:

1 To reduce food to the finest possible solution; that is, to subdivide or to digest food elements into a form that will admit of assimilation and use

2 To destroy bacteria and enzyms of plant and animal origin that are taken into the digestive tract with food

(These two facts constitute additional reasons for the thorough mastication of food)

Object of alternating digestive juices

By such plan Nature provides for the digestion of food only by such enzyms and ferments as will produce a finished product wholly suited to the particular requirements of the body. When we attempt by artificial processes to digest our food with other enzyms than those of our own digestive organs, or take into the stomach large quantities of food without proper mastication, which causes fermentation, we may expect that the nutritive material supplied to our tissues will not be perfectly adapted to the needs of human cell-growth, and, as a natural result, consequent derangement of the body-functions will take place.

Rennet

The rennet of the gastric juice is primarily for the purpose of digestion. Other than this it has no particular function that has yet been discovered.

Why stomach does not digest itself

The problem as to why the stomach does not digest itself has puzzled scientists for many years. Investigations of the twentieth century have at last solved this fascinating question. The walls of the human stomach are composed of proteid material, and should be dissolved by the gastric juice according to all known chemical laws. The explanation formerly given was that the stomach did not digest itself because it was alive. This answer did not satisfy scientists.

Antipepsin in the blood

There has recently been discovered an enzym, known as antipepsin, which is secreted by the cells in the stomach-walls. This antipepsin destroys the action of the pepsin, thus in turn preventing its action on the stomach-wall itself. Were antipepsin secreted in sufficiently large quantities to mix with the food in the stomach-cavity, no digestion could take place. The presence of this antipepsin in the stomach-walls has been proved in the following manner: The arteries leading to a portion of the stomach-wall of a dog was severed. This portion, receiving no blood supply, did not form the usual amount of antipepsin. The secretion of pepsin went on in the remainder of the animal's stomach, but digested that portion of the stomach-wall which was receiving no blood supply; that is, secreting no antipepsin.

BILE

Function of bile

The bile is a juice secreted by the liver and is alkaline in character. It is collected by the biliary ducts to be conveyed into the duodenum. The most important constituents of bile are bile salts and sodium glycocholate. The chief purposes of bile are to emulsify fats, thus aiding them to pass through the intestinal walls, and to stimulate intestinal peristalsis.

PANCREATIC JUICE

Function of the pancreas

The pancreas is a secretive gland located entirely outside of the intestinal walls, and produces a juice which is poured into the small intestines at the point where the bile enters. Pancreatic juice is acidulous, and also strongly alkaline. As soon as the food, passing from the stomach, comes in contact with the pancreatic juice and the bile, the acid is neutralized, and the mass becomes alkaline.

The pancreatic juice contains three important enzyms:

1 Amylopsin—acts on starch

2 Trypsin—acts on proteids

3 Steapsin—a fat-splitting enzym

Pancreatic juice also has the power of coagulating milk, and is believed to contain some rennet.

Power of amylopsin

Amylopsin, the starch-digesting enzym, appears to be very similar to ptyalin in its power to digest carbohydrates. Amylopsin completes the digestion of starch that was begun by the saliva. It acts on starch with great activity. One part of amylopsin can change forty thousand times its bulk of starch to glucose. This can act only in an alkaline solution, and if any abnormal fermentation takes place in the digestive tract, producing a large quantity of acids, the digestion of starch is stopped. It is interesting to note that this enzym is entirely absent from the pancreatic juice of infants. This explains why infants cannot digest starch.

Comparison of trypsin and pepsin

The second enzym to be considered in the pancreatic juice is trypsin. This is a substance distinct from pepsin, but its action is the same. The chief distinction is that trypsin acts in an alkaline solution, while pepsin acts in an acid solution. Trypsin is much more energetic in its digestive power than the pepsin of the gastric juice. It completes the digestion of proteids that is begun in the stomach, and converts all proteids into soluble forms. A number of forms of proteid that are not acted on at all by the gastric juice are readily digested by the trypsin of the pancreatic juice.

Fat digestion and absorption

The fat-digesting enzym of the pancreatic juice is steapsin. This is the principal fat-digesting enzym of the body. This substance has power to split fats; that is, to convert them into fatty acids and glycerin of which they were originally composed. This fatty acid then combines with the alkalis of the bile and of the pancreatic juice to form soap. Soap is soluble, and passes through the walls of the small intestines in this form. Having passed through the walls of the intestines, soap is again changed into fat. The probable reason for which Nature adopts such a complex process for the absorption of fat, is because fat is insoluble. If the intestinal walls were so constructed that fat-globules could be taken directly through them, they would also be open for the entrance of germs and other foreign substances.

Frying fat unwholesome

Fat is not acted on by the gastric juice. This explains why the process of frying is so unwholesome. Frying causes a thin film of melted fat to spread over the surface of the starch and of the proteid atoms, with the result that these atoms cannot then be properly acted on by the saliva and the gastric juice, and therefore cannot undergo the preliminary changes necessary to normal digestion. Fat, taken in its natural form, does not interfere with other digestive processes.

INTESTINAL JUICES

In addition to the digestive juices that are poured into the small intestines from the pancreas and the liver, there is a juice which is secreted from the walls of the intestinal cells. This is called intestinal juice or succus entericus. It is a light yellow fluid with a strong alkaline reaction, due to the presence of sodium carbonate.

One action of the intestinal juice is to change sugar and maltose into glucose, which is then absorbed directly into the blood.

THE SECRETION OF DIGESTIVE JUICES

Recent discoveries concerning digestive juices

Within the past few years many remarkable discoveries have been made in regard to the secretion of the various digestive juices. Until some ten or fifteen years ago it was believed that the secretion of the digestive juices depended wholly upon the presence of food in the alimentary canal. The recent discoveries in this branch of physiology are to be accredited chiefly to Professor Palloff, a Russian scientist, and his co-workers. The facts that are now known regarding this part of Nature's work are essentially as follows:

The secretion of the various substances which make up the digestive fluids of the body depend upon two kinds of stimuli:

1 Direct nerve stimulus from the central nervous system

2 The chemical stimulus on the walls of the digestive organs

Depending upon either or both of these sources of stimulation, the digestive juices of the body are regulated in quantity, and what is much more worthy of note, in their actual chemical composition. Thus it will be readily seen how far-reaching in its effect upon scientific dietetic treatment is the knowledge of the influence of various foods, quantities, and combinations.

Comparative digestibility of foods

Professor Palloff's discoveries throw some very important light on the comparative digestibility of foods. The former method of estimating the digestibility of food was first to analyze the food, and then to analyze the intestinal residue, and subtract the undigested remnant of each particular class of food from the amount originally eaten. By such means it was possible to show that certain foods were, say 80 or 90 per cent digestible, as the case might be. By this method no allowance was made for the amount of nutrition or material that was consumed by the body in the digestion of these particular foods. According to these investigations, milk and meat were about equally digestible. It was not known that the digestion of milk requires only a small fraction of the energy that is necessary to digest meat, or proteids from vegetable sources. Thus it is obvious that when it is desirable to get a large amount of available nitrogen into the system, with as little expenditure of energy as possible, milk is a food par excellence. This is very logical inasmuch as the sole purpose of milk is food for animal life.

Comparative acidity and energy required in digestion

The amount of acidity in gastric juice that must be secreted for the digestion of meat is much in excess of that required for a given amount of vegetable food. The amount of acidity required is greatest for milk, second for meat, and least for bread. The digestive energy required is greatest for bread, second for meat, and least for milk. From this we learn that starchy foods are unsuitable for those who are afflicted with hyperchlorhydria or supersecretion of hydrochloric acid, as the excess of acid prevents their digestion by neutralizing the alkali of the intestines.

Insalivation of starchy foods and meats

The saliva secreted when nitrogenous food is eaten does not contain as much ptyalin as that secreted when starchy food is consumed; for this reason the thorough insalivation of starchy foods is much more important than that of meat, milk, and eggs. Some authorities have recently advised that people should not chew meat at all, but should swallow it as do carnivorous animals. This advice, however, is not altogether sound. In the first place, man is not a carnivorous animal, and the gastric juice of the human stomach does not act as rapidly on flesh foods as does the gastric juice of meat-eating animals, but if meat be taken into the human stomach, either in large or in small quantities, decomposition may take place before digestion has proceeded far enough to prevent the action of micro-organisms.

Mental influence upon digestive fluids

The mental influence upon the secretion of digestive fluids may originate from thought, or may be brought about reflexively by the sight, or by the smell of food. All are familiar with the experience of having one's mouth water at the sight of a particularly appetizing dish. Many of us have undergone the same experience by merely thinking of some particular food of which we are fond.

Digestive juices vary with different foods

Scientific investigation has shown that the secretion of saliva is only an example of what takes place in the other digestive organs. The experiments of the ingenious Russian scientist, heretofore mentioned, prove that the act of tasting and of swallowing food was the chief factor in determining the secretion of the juices from the stomach-walls. In a series of operations upon dogs, performed by skilled surgeons, certain interesting facts were observed. The esophagus was severed and made to open externally so that the food swallowed did not pass into the stomach. The secretion of gastric juice was then determined in the case of different foods which were taken into the dog's mouth and swallowed, but which did not reach the stomach. Not only did this act of pretended feeding start a flow of gastric juice, but the juice secreted in the case of different foods was especially adapted to the particular food, according to the general principle which we have already discussed.

These facts emphasize several important considerations regarding our diet:

1 We should eat slowly and get the whole taste out of food by thorough mastication, because taste largely controls the secretion of the digestive fluid

2 We should not disguise our food by high seasoning

3 Foods that do not require the same digestive principles should not be taken at the same meal

"Fermentation" and "Putrefaction" compared

Fermentation is the term generally applied to changes that take place in such food substances as carbohydrates, due to the growth of bacteria, while the term putrefaction is applied in a similar way to the changes taking place in nitrogenous or proteid materials. Both of these chemical changes are exceedingly harmful.

ABNORMAL CHEMICAL CHANGES IN THE DIGESTIVE ORGANS

Bacteria chief causes of abnormal changes

Under this heading we will consider the chemical changes which take place in the human alimentary canal, which are not beneficial or necessary to normal digestion. The cause of the most important abnormal changes in the contents of the stomach and the intestines is the presence of living micro-organisms called bacteria.

In the lesson entitled "Evolution of Man," a general survey of the history of man's development from lower forms of life is given. In this general work I do not elaborate extensively upon the method by which evolution proceeds, but those who are acquainted with the writings of Darwin, and other evolutionists, are familiar with the phrases "the survival of the fittest," and "the struggle for existence."

"Survival of the fittest" among bacteria

As we commonly think of "the survival of the fittest" in animal life, we picture the death-struggle of the captured animal, or the fight for food in times of scarcity, or, if it be in the case of plants, the crowding or the struggling for soil and sunlight. We can apply the same principle to bacteria and to other microscopic forms of life.

Bacteria, while minute masses of unconscious protoplasm, are, by the laws of growth and reproduction, struggling for existence just as truly as are the more conspicuous forms of life.

Because of the invariable presence of greater or less quantities of bacteria within the intestines of all ordinary animals, some scientists insist that their presence is in some way necessarily related to the life of the animal, and is probably beneficial.

Experiments proving accumulation of bacteria

New-born animals, however, are free from bacteria, and the bacterial germs found in the more matured animal must, therefore, have been taken into the alimentary canal with food. Ingenious scientists have taken new-born guinea pigs, and have kept them in sterile or germ-proof compartments, giving them filtered air to breathe, and absolutely sterile food. These pigs lived and thrived through the experiment as did their fellows outside the bacterial-proof dwelling. This is considered good evidence that bacteria accumulate in the digestive organs of all animals, not for a purpose connected with animal physiology, but because in order to digest and to assimilate food, conditions are established which are so nearly like those required for bacterial growth, that bacteria are produced, or take advantage of the favorable conditions, just as weeds, if given a chance, thrive in a cultivated field.

Not all bacterial growth is harmful

I have already referred to the antiseptic or germ-destroying properties of the gastric juice, and to other secretions of the digestive organs. This would suggest that the growth of bacteria is undesirable from the standpoint of man's welfare. There are many species of bacteria growing in the human intestines, hence we cannot say with certainty that all this bacterial growth is harmful, as, in order to determine this, the resulting waste-products of each particular species of bacteria would need to be considered separately. We can, however, make the general statement that bacteria are abnormal, or foreign to the human digestive canal, and that their presence is detrimental to human welfare.

Micro-organisms give off various substances as waste-products of their growth, dependent upon the species of bacteria, and the material in which they are growing. Thus the waste-products of the yeast-plant are carbon dioxid and alcohol.

Waste-products of bacterial fermentation

In the alimentary canal there exists an abundance of carbohydrate and proteid substances which form excellent food material for numerous species of bacteria. The substances produced by the growth of these various kinds of bacteria are numerous. They include the gases, carbon dioxid, hydrogen, hydrogen sulfid, marsh-gas or methane, and ammonia. Butyric, lactic, and other acids, together with alcohol, are also produced as a product of bacterial fermentation in the intestines. Perhaps the most detrimental of all are the substances produced by the bacterial putrefaction of proteids, of which indol and skatol are the two most important.

Under ordinary conditions the bacteria themselves do not penetrate the intestinal walls, and their evil influence would be confined to mechanical disturbance of gas in the digestive organs, and to the destruction of a portion of the Solubility and distribution of bacterial waste-products nutritive material of food, were it not for the fact that these harmful and poisonous waste-products I have mentioned, are soluble, and hence pass through the intestinal walls with the digested food material, into the blood, and are thus distributed throughout the body.

It has been observed in the presence of intestinal congestion, where the food lies in the intestines for an abnormally long period, that the amount of these harmful nitrogenous decomposition products excreted by the kidneys, is considerably increased, proving that these products have circulated throughout the body.

Causes of hardening of the arteries

Arterio-sclerosis, or the hardening of the walls of the arteries, which has for many years been recognized by scientists as one of the principal causes of old age, comes from two causes:

(1) The over-consumption of starchy foods, especially of the cereal group; and (2) by the continued presence, in the blood, of small quantities of poisonous material which gradually destroys the protoplasm of the arterial walls, and causes them to be replaced by a degenerate form of tissue.

For example, alcohol and the poison of syphilis are prolific causes of the hardening of the arteries. If the diet were balanced so as to avoid excesses of starch and these toxic substances, the hardening of the arteries would not take place.

Overeating an ultimate cause of old age

The poisons produced in the intestines by bacterial decomposition, superinduced largely by overeating, are absorbed into the blood, and undoubtedly their action is similar to the other poisons herein mentioned. Thus they become a most potent factor in the cause of old age and premature death, being practically universal among all civilized tribes.

Numerous other disorders or dis-eases can be traced to this same general cause, and the subject of the poisonous products of fermentation and decomposition in the intestines will therefore be constantly referred to throughout this work.

The growth of bacteria decreased by scientific eating

From the deductions that have been made it is clearly evident that any system of feeding which will reduce the amount of bacterial growth in the intestines, would be desirable and beneficial to mankind, while foods and habits of life that increase the amount of such poisons are to be guarded against as detrimental to both health and life.

Overeating primary cause of fermentation

Overeating is perhaps the greatest of all dietetic errors in bringing about a condition which favors excessive intestinal fermentation. Overeating causes stomach prolapsus, thus reducing its mixing or peristaltic activity. This retards the process of emptying, called digestion, which is the primary cause of fermentation. Under this condition the antiseptic properties of the stomach-juices are reduced, and the bacteria from the fermenting food is vastly increased. The food, passing from the stomach in a fermenting state, produces gas in the intestines, with the resultant ills that follow, such as vertigo, dizziness, irregular heart action, and usually intestinal congestion or constipation.

THE DECOMPOSITION OF FOOD

Sugar destroys putrefying bacteria

The putrefaction of proteids in the intestines may be reduced by the liberal consumption of fresh sweet fruits. The preserving qualities of sugar depend upon the fact that putrefying bacteria cannot live where sugar is abundant. The beneficial effect of sweet fruits in reducing bacterial decomposition in the intestines, is due to the presence of relatively large quantities of sugar and of organic acids. Sour milk is known to have a prohibitive influence upon putrefaction in the alimentary canal. This is due to the Sour milk a preventive of intestinal putrefaction milk-sugar, which has been changed to lactic acid. This explains why clabbered milk, which contains a considerable portion of sugar changed into lactic acid by the action of souring bacteria, is especially beneficial in preventing intestinal putrefaction. Professor Metchnikoff, of the Pasteur Institute of Paris, became so enthusiastic upon this discovery that he proclaimed sour milk to be a remedy for old age. While Metchnikoff's enthusiasm is perhaps somewhat premature, yet the idea is worthy of much consideration.

Proper feeding chief factor in reducing bacterial growth

We do not need, however, to seek for any one specific remedy against intestinal decomposition, but should study the selections, combinations, and proportions of our food at each meal with the view of reducing to the minimum the growth in the alimentary tract.

DIGESTIVE EXPERIMENTS

It is well known that only a portion of the food taken into the alimentary canal is digested and absorbed into the circulation. It is obvious that the undigested food plays no part in the process of metabolism, therefore it is necessary to know the amount of the various food elements that are digested. For this reason we will notice briefly the method used in making digestive experiments.

Determination of the amount of food the body uses

The food eaten for a certain period of time is analyzed and weighed, and the intestinal excreta, corresponding to the quantity of food under study, is also weighed and chemically analyzed. The difference should show the amount of food actually digested.

There are several serious difficulties in the way of making accurate digestive experiments:

Quantity of feces and time consumed in passing food through the body

1. It is very difficult to determine the quantity of feces (intestinal excreta) that corresponds to a given quantity of food. A digestive experiment is usually conducted for a period of about one week, the man or animal being given a spoonful of lampblack at the beginning and at the close of the experiment. The lampblack being a finely powdered form of pure carbon, is insoluble in the digestive juices, hence passes through the body without change, thus blackening or marking the feces at the beginning and at the end of the test period. The subject under experiment should be given the same diet for a few days before and after the experiment, so that the error due to the inability to accurately separate the feces will be reduced to a minimum.

Measuring the digestible portion of food

2. The digestive juices, and especially the bile, pour considerable material into the alimentary canal which cannot be distinguished from the undigested elements of food. However, it is fair to assume that when large quantities of body-proteids are poured into the alimentary canal, and passed out with the feces, this amount of matter is wasted by the body, hence should be charged against the food which stimulated the secretion. For example: If grain causes a large secretion of digestive enzyms, it is no more than fair to say that grain is less digestible than milk, which wastes less body-matter in its digestion.

Certain foods may either aid or hinder digestion

3. A further difficulty with the accuracy of digestive experiments, and one to which in the past too little attention has been paid, is the influence upon the digestibility of one food by the presence of others. Some foods, such as fruits, aid the digestion of other foods, while in many cases the presence of a certain article seriously hinders the digestive process of all. This emphasizes the great necessity The mono-diet system for observing the laws of chemical harmony in combining our food at meals, and it also emphasizes the importance of limiting the diet to the fewest number of things possible, which in the opinion of the writer will lead inevitably to the mono-diet system, especially in curative or remedial feeding.

Difficulty of determining amount of undigested food

From the standpoint of the above difficulties, all digestive experiments thus far made are only approximately correct, and we are forced back to the conclusion that if we obey the laws of nutrition, Nature will give us her highest result expressed in endurance. If a single article of diet is taken by a man who is accustomed to large quantities of a highly varied bill of fare, the digestive process will not act in the usual way. On the other hand, if several articles such as nuts, grains, and milk are taken at one time, it will be impossible to determine what percentage of the proteid or of the fat from the three various sources remains undigested in the intestinal residue, hence no accurate results can be shown regarding the digestibility of each particular food.

MECHANICS OF DIGESTION

Condition of food influences chemical action

Chemistry is not the only factor in the digestive function that is to be taken into consideration. The mechanical condition of food, when it is taken into the digestive organs, very greatly influences the chemical process that takes place.

This involves the question of masticating or subdividing the food into small particles. The greater the dissolving surface, the more rapidly will solution take place. If the substance being dissolved is Necessity for thorough mastication a firm particle, the digestion or solution will take place only on the exterior surface, and the interior of the particle, however small, will remain practically unchanged. This is what occurs when food materials such as grains and nuts are taken in an uncooked state, as mastication does not dissolve them, but only divides them into small, distinct particles.

Action of enzyms during digestion

If, however, the grain be subjected to prolonged heating with water, partial solution takes place. The entire mass becomes mushy and permeated with moisture. When such a mass is brought in contact with the digestive fluids, it mixes or disintegrates with the fluid, just as molasses would mix with water. The result is that the whole mass of material is subjected to the action of the digestive fluids at once, with the result that the mass is passed from the stomach too quickly, causing congestion in the small intestines, or the whole is arrested, and fermentation and decomposition take place. In normal digestion, the enzyms are continuously secreted for a period of several hours. They begin work on the outside of the food particles, dissolving the substances gradually. Thus the enzyms are continuously used up, and the digestion proceeds slowly, but naturally, yet as fresh enzyms are continuously being secreted to act on the newly exposed surfaces, active and complete digestion is constantly taking place.

Predigested "breakfast foods"

The alleged predigestion of certain proprietary foods has neither scientific basis nor virtue. That the juices of some fruits which are already in the form of glucose, can be immediately absorbed into the tissues without any digestive process, does not prove that the mushy cooking, malting, and other forms of so-called predigestion are beneficial. The so-called "predigested breakfast foods" are not and cannot be prepared by any process for final digestion, but are in an intermediate state between starch and glucose. They are composed of a semi-soluble starch, gummy dextrin, and perhaps a little maltose which has a tendency to disturb and to interfere with the normal process of digestion.

Comparative digestibility of cooked and uncooked starch

I do not advocate the use of uncooked grain, but I wish to correct a popular error in regard to the digestibility of uncooked cereal starch. Nearly all works on physiology and diet make the statement without reserve that raw starch is indigestible. This theory has been established by putting samples of cooked and uncooked starch into two test tubes, and treating them with some digestive enzym. The cooked starch, being soluble, is all exposed to the digestive enzyms at one time, and started on its way through the numerous changes in the complex chemical process of changing starch into glucose, while in the sample of uncooked starch, the digestive enzym attacks the particles from the outside, and slowly digests or eats off the exterior of the starch grains. After a given length of time the chemist adds iodin to the two test tubes. With starch, iodin gives a blue color. In the test tube containing the cooked starch, all of which has undergone a certain amount of digestion, no blue color is discerned, for no pure starch is left, while in the other tube, in which some of the particles remain unchanged, owing to the fact that Nature does all her work slowly, a blue reaction is of course obtained, and the chemist proclaims that uncooked starch is indigestible.

At one of the United States Experiment Stations in the state of Kansas, a comparison of two diets, consisting chiefly of several varieties of grains, was recently made. The diets were alike in every respect with the exception that in one Government experiments with cooked and uncooked grains instance all the grains were boiled for two hours, while in the other case they were taken in an uncooked state. In the case of the uncooked grains, no starch whatever passed through the body in an undigested form. In the case of the cooked grains, the same results were found; that is, no starch was found in the intestinal residue. Other substances, however, remaining undigested in the cooked diet, were much in excess of that in the uncooked, yet no starch was present. In the case of cooked grains, the digestive processes may start with more rapidity than in uncooked grains, yet they are not thoroughly completed, and various decomposition products occur, as well as undigested proteid, which is not likely to occur with foods taken in their natural state.

Uncooked starch harmless

Moreover, if uncooked starch be taken in excess of the digestive capacity, and passed through the body wholly unchanged, no harm results. The starch grain, in its unchanged state, is a fine, white glistening granule, and its presence in the digestive tract would have no harmful effect upon the body functions. Without solution, no material can have any effect upon the physiological processes, except by irritating the mucous surfaces of the digestive organs; in the latter respect, starch granules are harmless.

Condition in which food should enter digestive organs

With the exception of articles that are in solution, the condition in which all foods should enter the digestive organs is in finely divided, yet distinct particles, and not in pasty or gummy masses. In this latter form "bolting" is encouraged, and mastication greatly discouraged.

THE MUSCULAR MOVEMENT OF DIGESTIVE ORGANS

Peristaltic movement in alimentary canal

Another point to be considered in digestion, and which may well be classed under the mechanics of digestion, is the muscular action or peristalsis of the alimentary tract. The best example is the swallowing action observed in the throat of a horse, or of a cow, when drinking. At each swallow, what appears to be a lump goes down the throat. This is a wave-like relaxation of the muscular walls of the esophagus, followed closely by a muscular contraction. This is the action that takes place in the intestinal tract, and that which Nature employs to move the contents along toward the final point of excretion.

X-ray examination of peristalsis

A very fascinating and scientific demonstration may be performed in the following manner: A cat may be given food mixed with some such substance as bismuth subnitrate, which is opaque to X-rays. Upon placing the animal under an X-ray during digestion, this peculiar peristaltic motion can be observed, one "swallow" passing rapidly after another down the alimentary tract.

This method of investigation has also shown that peristaltic action stops immediately in the case of fright, or anger, but is shown to proceed with regularity during sleep, contrary to the antiquated idea that digestion ceases when sleep begins.

Milk for relieving constipation

Peristaltic action in the lower parts of the alimentary canal is stimulated by taking food into the stomach. This explains the laxative action of such foods as fruits, or, sometimes, milk, taken at frequent intervals. When all other methods fail, constipation can oftentimes be relieved by taking a glass of milk every thirty minutes until four glasses have been consumed.

Moisture and peristaltic action

The longer food remains in the intestines, the more completely is the water absorbed from the residue. The object to be obtained in relieving constipation is to increase the moisture and the peristaltic action. Whatever will accomplish these things will relieve and perhaps cure intestinal congestion.

The subject of intestinal congestion and purgative medicines will be discussed at length in Lessons IX and XI, Vol. II, p. 375 and p. 436, respectively.