Article #2: Digestion Of Foods by Dr. Herbert M. Shelton
Foodstuffs as we eat them constitute the raw materials of nutrition. As proteins, carbohydrates and fats, they are not usable by the body. They must first undergo a disintegrating, refining and standardizing process (more properly a series of processes) to which the term digestion has been given. Although this process of digestion is partly mechanical, as in the chewing, swallowing and "churning" of food, the physiology of digestion is very largely a study of the chemical changes foods undergo in their passage through the alimentary canal. For our present purposes, we need give but little attention to intestinal digestion but will concentrate upon mouth and stomach digestion.
Enzymic Limitations Necessitate the Combining of Compatible Foods
The changes through which foods go in the processes of digestion are affected by a group of agencies known as enzymes. Due to the fact that the conditions under which these enzymes can act are sharply defined, it becomes neccessary to give heed to the simple rules of correct food combining that have been carefully worked out on a basis of the chemistry of digestion.
Long and patient effort on the part of many physiologists in many parts of the world have brought to light a host of facts concerning enzymic limitations, but, unfortunately, these same physiologists have attempted to slur over their importance and to supply us with fictional reasons why we should continue to eat and drink in the conventionally haphazard manner. They have rejected every effort to make a practical application of the great fund of vital knowledge their painstaking labors have provided. Not so the Natural Hygienists. We seek to base our rules of life upon the principles of biology and physiology.
What Enzymes Are
Let us briefly consider enzymes in general before we go on to a study of the enzymes of the mouth and stomach. An enzyme may be appropriately defined as a physiological catalyst. In the study of chemistry it was soon found that many substances that do not normally combine when brought into contact with each other may be made to do so by a third substance when it is brought into contact with them. This third substance does not in any way enter into the combination or share in the reaction; its mere presence seems to bring about the combination and reaction. Such a substance or agent is called a catalyst and the process is called catalysis.
Plants and animals manufacture soluble catalytic substances, colloidal in nature and but little resistant to heat, which they employ in the many processes of splitting up of compounds and the making of new ones within themselves. To these substances the term enzyme has been applied. Many enzymes are known, all of them, apparently, of protein character. The only ones that need interest us here are those involved in the digestion of foodstuffs. These are involved in the reduction of complex food substances to simpler compounds that are acceptable to the bloodstream and usable by the cells of the body in the production of new cell substance.
Bacterial By Products Poisonous
As the action of enzymes in the digestion of foodstuffs closely resembles fermentation, these substances were formerly referred to as ferments. Fermentation, however, is accomplished by organized ferments—bacteria. The products of fermentation are not identical with the products of enzymic disintegration of foodstuffs and are not suitable as nutritive materials. Rather, they are poisonous. Putrefaction, also the result of bacterial action, also gives rise to poisons, some of them very virulent.
Digestive Enzymes Extremely Specialized
Each enzyme is specific in its action. This is to say, it acts only upon one class of food substance. The enzymes that act upon carbohydrates do not and cannot act upon proteins nor upon salts nor fats. They are even more specific than this would indicate. For example, in the digestion of closely related substances such as the disaccharides (complex sugars), the enzyme that acts upon maltose is not capable of acting upon lactose. Each sugar seems to require its own specific enzyme. The physiologist, Howell, tells us that there is no clear proof that any single enzyme can produce more than one kind of ferment action.
Digestion a Step-By-Step Process
This specific action of enzymes is of importance, as there are various states in the digestion of foodstuffs, each state requiring the action of a different enzyme and the various enzymes being capable of performing their work only if the preceding work has been properly performed by the enzymes that also precede. If pepsin, for example, has not converted proteins into peptones, the enzymes that convert peptones into amino acids will not be able to act upon the proteins.
The substance upon which an enzyme acts is called a substrate. Thus starch is the substrate of ptyalin. Dr. N. Phillip Norman, Instructor in gastroenterology, New York Polyclinic Medical School and Hospital, New York City, says: "In studying the action of different enzymes, one is struck by Emil Fischer's statement that there must be a special key to each lock, the ferment being the lock and its substrate the key, and if the key does not fit exactly in the lock, no reaction is possible. In view of this fact, is it not logical to believe the admixture of different types of carbohydrates and fats and proteins in the same meal to be distinctly injurious to the digestive cells? If, since it is true that similar but not identical locks are produced by the same type of cells, it is logical to believe that this admixture taxes the physiological functions of these cells to their limit?" Fischer, who was a renowned physiologist, suggested that the specificity of the various enzymes is related to the structure of substances acted upon. Each enzyme is apparently adapted to or fitted to a certain definite structure.
Chewing Is First Digestive Step
Digestion commences in the mouth. All foods are broken up into smaller particles by the process of chewing, and they are thoroughly saturated with saliva. Of the chemical part of digestion, only starch digestion begins in the mouth. The saliva of the mouth, which is normally an alkaline fluid, contains an enzyme called ptyalin, which acts upon starch, breaking this down into maltose, a complex sugar, which is further acted upon in the intestine by maltase and converted into the simple sugar dextrose.
The action of ptyalin upon starch is preparatory, as maltase cannot act upon starch. Amylase, the starch-splitting enzyme of the pancreatic secretion, is said to act upon starch much as does ptyalin, so that starch that escapes digestion in the mouth and stomach may be split into maltose and achroodextrin, providing, of course, that it has not undergone fermentation before it reaches the intestine.
Some Enzymes Destroyed By Acids and Alkalines
Ptyalin is destroyed by a milk acid and also by a strong alkaline reaction. It can act only in an alkaline medium, and this must not be strongly alkaline. It is this limitation of the enzyme that renders important the manner in which we mix our starches, for if they are mixed with foods that are acid or that provide for an acid secretion in the stomach, the action of the ptyalin is brought to an end.
Some Factors That Inhibit Digestion
Stomach (gastric) juice ranges all the way from nearly neutral in reaction to strongly acid, depending upon the character of the food eaten. It contains three enzymes-pepsin, which acts upon proteins; lipase, which has slight action upon fats; and rennin, which coagulates milk. The only one of these enzymes that needs concern us here is pepsin. Pepsin is capable of initiating digestion on all kinds of proteins. This is important, as it seems to be the only enzyme with such power. Different protein splitting enzymes act upon the different stages of protein digestion. It is possible that none of them can act upon protein in stages preceding the stage for which they are specifically adapted. For example, erepsin, found in the intestinal juice and in the pancreatic juice, does not act upon complex proteins, but only upon peptides and polypeptides, reducing these to amino acids. Without the prior action of pepsin in reducing the proteins to peptides, the erepsin would not act upon the protein food. Pepsin acts only in an acid medium and is destroyed by an alkali. Low temperature, as when iced drinks are taken, retards and even suspends the action of pepsin. Alcohol precipitates this enzyme.
Just as the sight, odor or thought of food may occasion a flow of saliva, a "watering of the mouth," so these same factors may cause the flow of gastric juice, that is a "watering of the stomach." The taste of food, however, is most important in occasioning a flow of saliva. The physiologist, Carlson, failed in repeated efforts to occasion a flow of gastric juice by having his subjects chew on different substances, or by irritating the nerve endings in the mouth by substances other than those directly related to food. In other words, there is no secretory action when the substances taken into the mouth cannot be digested. There is selective action on the part of the body and, as will be seen later, there are different kinds of action for different kinds of foods.
In his experiments in studying the "conditioned reflex," Pavlov noted that it is not necessary to take the food into the mouth in order to occasion a flow of gastric juice. The mere teasing of a dog with savory food will serve. He found that even the noises or some other action associated with feeding time will occasion a flow of secretion.
It is necessary that we devote a few paragraphs to a brief study of the body's ability to adapt its secretions to the different kinds of foodstuffs that are consumed. Later we will discuss the limitations of this power. McLeod's Physiology in Modern Medicine says: "The observations of Pavlov on the responses of gastric pouches of dogs, to meat, bread and milk have been widely quoted. They are interesting because they constitute evidence that the operation of the gastric secretory mechanism is not without some power of adaptation to the materials to be digested."
Digestion Proceeds Intelligently
This adaptation is made possible by reason of the fact that the gastric secretions are the products of about five million microscopic glands embedded in the walls of the stomach, various of which secrete different parts of the gastric juice. The varying amounts and proportions of the various elements that enter into the composition of the gastric juice give a juice of varying characters and adapted to the digestion of different kinds of foodstuffs. Thus the juice may be almost neutral in reaction or it may be weakly acid or strongly acid. There may be more or less pepsin according to need. There is also the factor of timing. The character of the juice may be very different at one stage of digestion from what it is at another, as the varying requirements of a food are met.
A similar adaptation of saliva to different foods and digestive requirements is seen to occur. For example weak acids occasion a copious flow of saliva, while weak alkalies occasion no salivary secretion. Disagreeable and noxious substances also occasion salivary secretion, in this instance to flush away the offending material. It is noted by physiologists that with at least two different types of glands in the mouth able to function, a considerable range of variation is possible with reference to the character of the mixed secretion finally discharged.
An excellent example of this ability of the body to modify and adapt its secretions to the varying needs of various kinds of foods is supplied us by the dog. Feed him flesh and there is a secretion of thick, viscous saliva, chiefly from the submaxillary gland. Feed him dried and pulverized flesh, and a very copious and watery secretion will be poured out upon it, coming from the parotid gland. The mucous secretion poured out upon flesh serves to lubricate the bolus of food and thus facilitate swallowing. The thin, watery secretion, on the other hand, poured out upon the dry powder washes the powder from the mouth. Thus, it is seen that the kind of juice poured out is determined by the purpose it must serve.
Humans Ate Correctly In Nature
As we previously noted, ptyalin has no action upon sugar. When sugar is eaten there is a copious flow of saliva, but it contains no ptyalin. If soaked starches are eaten, no saliva is poured out upon these. Ptyalin is not poured out upon flesh or fat. These evidences of adaptation are but a few of the many that could be given. It seems probable that a wider range of adaptation is possible in gastric than in salivary secretion. These things are not without their significance to the person who is desirous of eating in a manner to assure most efficient digestion, although it is the custom of physiologists to gloss over or minimize them.
There are reasons for believing that man, like the lower animals, once instinctively avoided wrong combinations of foods, and there are remnants of the old instinctive practices still extant. But having kindled the torches of intellect upon the ruins of instinct, man is compelled to seek out his way in a bewildering maze of forces and circumstances by the fool's method of trial and error. At least this is so until he has gained sufficient knowledge and a grasp of proved principles to enable him to govern his conduct in the light of principles and knowledge. Instead, then, of ignoring the great mass of laboriously accumulated physiological knowledge relating to the digestion of our foodstuffs, or glossing over them as is the practice of the professional pysiologists, it behooves us, as intelligent beings, to make full and proper use of such knowledge. If the physiology of digestion can lead us to eating practices that insure better digestion, hence better nutrition, only the foolish will disregard its immense value to us, both in health and in disease.
Home > Lesson 7 - Carbohydrates - Fuel For The Human Body
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