The Chemistry And Physiology Of Digestion

3. The Chemistry And Physiology Of Digestion

For food to be utilized by the body, it must first undergo a series of processes which we call digestion. After we perform the only really voluntary actions involved in the process of nutrition—putting the food into our mouths, chewing and swallowing—the balance of the digestive process is the function of the autonomic or involuntary nervous system.

The changes which foods undergo are largely effected by enzyme and digestive juices. The conditions under which “such action” can occur are sharply defined, and this is the logical foundation of the food combining system. Physiologists have ascertained the details of the chemistry of digestion through long and painstaking labors. It has remained for the Natural Hygienists to make practical application of this great fund of vital knowledge. Knowledge of the physiology and chemistry of digestion can lead us all to a food program that will insure better digestion and better nutrition.

3.1 Enzymes

Enzymes are proteinaceous organic catalysts in all living organisms, both plant and animal. Our digestive juices contain enzymes that accelerate chemical reaction by catalytic action, without themselves being used up in the process.

Digestive enzymes can be used over and over again but eventually are replaced by the body. There are many different kinds of enzymes, and digestive enzymes are not the same as the enzymes in raw, unprocessed food. Digestive enzymes break down the complex substances we ingest into simple components that can be utilized by the body.

There are five types of digestive enzymes in the human body.

1. Hydrolases facilitate hydrolysis, the breakdown of substances in water within the body.
2. Adding enzymes build proteins by adding amino acids, one by one.
3. Transferring enzymes transfer organic substances from one compound to another so the body may use them in various ways.
4. Isomerases, or rearranging enzymes, rearrange molecules  in  organic  substances;  they  also  rearrange amino acid enzymes.
5. Oxidases, or oxidizing enzymes, are released in the presence of oxygen and rapidly bring about a change in the color of food. They act on foods in the mouth while one is masticating or whenever the food is exposed to oxygen. These enzymes are also present in apples and other fruits. They are released when the fruit is cut and react so as to rapidly bring about, a change of color to brown.

It is not necessary to memorize the names of these enzymes, but the information about the, action of the different types of digestive enzymes will help you to understand the underlying rationale of food combining.

Each digestive enzyme is specific in its action. It acts only upon one class of food substance. Each stage in the digestion of food requires the action of a different enzyme, and the various enzymes can perform their work efficiently only if the preceding work has been properly performed.

Body chemistry is, to a large extent, determined by the the food we eat. When certain foods are eaten regularly, the digestive enzymes and secretions are of a character to handle those foods. When the diet is altered, more and more of the digestive juices secreted will be of a character to digest the foods in the new diet, and less and less of the digestive juices will be of the character to digest the foods in the old diet.

The type of digestive juice fitted for the digestion of one type of food is of no value in digesting another type of food. Therefore, it is essential that food be taken in combinations that do not interfere with enzymatic action.

3.2 The Process of Digestion

Digestive speed and efficiency vary with individuals and circumstances. However, certain general statements can be made. Fruits pass through the stomach quickly; low-protein and low-starch vegetables also pass through the stomach rapidly, with little change; vegetables containing much starch must be retained in the stomach longer, for more thorough digestion; and proteins require a still longer time »for gastric digestion. Fruits may remain in the stomach for thirty to sixty minutes, low-protein and low-starch vegetables a little longer, concentrated starches about two hours and concentrated proteins approximately four hours. Some foods may take five or six hours or more to leave the stomach. Some examples are combination starch/protein foods like legumes (including beans), grains, cooked cabbage and flesh foods.

Most digestion occurs in the stomach and small intestine. Digestion, especially starch digestion, actually begins in the mouth, with mastication and insalivation of the food. This sends the proper signals for the release of the digestive juices suited to the character of the food eaten. Digestive juices are present in the saliva and in the gastric secretions of about five million microscopic glands in the walls of the stomach.

The digestive glands supply different enzymes and juices of varied strength and character and with specific timing, depending on the different foods ingested. The digestive juices may be more or less liquid, of varying degrees of acidity or alkalinity and with complex and elaborately contrived variations.

After food is masticated, insalivated and swallowed, gastric digestion is initiated. Involuntary movements of the stomach slowly mix the food with gastric juices secreted by the glands in the walls of the stomach. Pepsin, a protein-splitting enzyme, and hydrochloric acid are separated, as
well as lipase, a fat-splitting enzyme, mucus and diluting juice, along with other factors needed in the digestive process. An alkaline secretion protects the walls of the stomach from the acids. Mucus is a natural lubricant that is secreted by the cells of the mucous membranes lining all of the hollow organs of the body. It keeps the body tissues moist and prevents them from drying and cracking.

A brief review of the process of digestion will help in understanding the food combining rationale. Gastric secretion is continuous (except during fevers, gastric inflammation, pain or strong emotions; fasting is indicated when any of these conditions are present). Of course, gastric secretion is unnecessary when no food is taken.

Hunger and the sight, smell, taste or thought of food stimulate gastric secretion. Usually about three pints of gastric juice is secreted every twenty-four hours and about half this amount is required to digest a hearty meal. If you eat more than two hearty meals daily, your account will be overdrawn.

As the process of digestion continues in the stomach and the food is mixed with the digestive juices, water (from the body’s reserve supply) is added to the mixture in a process called hydrolysis. During hydrolysis, digestive enzymes separate carbohydrates into simple sugars, and proteins into their constituent amino acids.

Since digestion is a mechanical as well as a chemical process, some cellulose is an important part of the diet. Although cellulose cannot be digested by humans (no enzyme secreted by humans is capable of splitting cellulose), it serves as bulk in the propulsion of food through the digestive tract. Cellulose also provides the bulk needed in the efficient elimination of food residues. Juices and refined foods contain little cellulose. However, too large a quantity of cellulose is also undesirable. Therefore, we should use fresh fruits and, when using vegetables, strive to obtain the young, tender vegetables, as these contain smaller amounts of cellulose.

Food residues, fibrous materials and particles not thoroughly masticated proceed on to the colon. Peristalsis (wave-like muscular contractions) propels the food mixture back and forth in the stomach. Periodically, the most liquid portion of the mixture is discharged into the duodenum where it meets a very acid fluid. The resultant semi-liquid mixture, known as chyme, then proceeds further—into the small intestine—where it meets a very alkaline mixture of pancreatic juice, additional digestive enzymes and bile. (Bile is secreted by the liver and stored in the gallbladder to be used when needed, particularly for emulsifying fats).

The intestinal glands secrete a juice containing enzymes similar to pancreatic enzymes. Virtually all absorption should occur by the time the food passes through the small intestine, and the residue proceeds into the large intestine (the colon).

Through all these processes, the peristaltic contractions continue, longitudinally and circularly, and propel the chyme along the alimentary canal. As you can see, foods are not digested when they have passed out of the stomach. A large part of the work of digestion takes place in the small intestine. But the role of gastric digestion is an important one in preparing the food for the next stage in the digestive process.

Coffee, tea and other such toxic infusions cause a premature emptying of the stomach and thus cause foods to leave the stomach before gastric digestion is complete.

Digestion is governed by physiological chemistry, and this must be considered in the planning of meals that are compatible with the physiological limitations of the digestive glands and their secretions. This lesson will help in understanding the principle that the digestion of different foods requires digestive juices of different characters.

The study of the processes of digestion reveals the specific action of the digestive enzymes, the careful timing of their secretion and the adjustment of the strength and character of the digestive secretions to the character of the food upon which they are to act. Carbohydrate foods receive a juice rich in carbohydrate-splitting enzymes, protein foods receive protein-splitting enzymes, and so forth.

3.3 Starch Digestion

Starch digestion begins in the mouth with the action of the enzyme ptyalin (alpha amylase) which converts (or else begins the process of converting) starches into sugar during mastication and insalivation. The salivary secretions accompany the food to the stomach and salivary digestion of starches continues in the stomach for a long time, if the food was eaten under correct conditions.

If ptyalin is the only agent in the body capable of initiating starch digestion (and this is not certain), or whether it is simply the body’s first opportunity to initiate starch digestion, we must not disregard its importance. The chewing process in the mouth should mix food with saliva, but people have the tendency to swallow the mass too quickly to permit the enzyme to complete its action. This necessitates the continuation of the salivary action in the stomach.

It is important that starches be eaten dry, not moist. So, you should eat steamed or baked potatoes dry rather than in potato soup. The eating of liquids with starches promotes the tendency to swallow moist starch without thorough mastication, insalivation and emulsification, processes that are particularly needed for the digestion of starches. Drinking liquids or eating liquidy foods softens the food artificially and may also cause you to eat more food than if you had eaten it dry. Drinking at meals dilutes the digestive juices and also prevents thorough mastication and insalivation of the food.

If the ptyalin is destroyed or its action is inhibited and the digestion of starch is interrupted, the partially digested (and probably somewhat fermented) starch proceeds to the duodenum, where further starch-splitting enzymes are secreted. Starch that escapes digestion in the stomach may later be acted upon by pancreatic and intestinal enzymes, provided too much fermentation has not already occurred. It is also very possible that the interrupted gastric digestion may never be completed.

Ptyalin requires either an alkaline or neutral medium. Ptyalin is destroyed by even a mild acid. If fruit acids—or any acids—are taken with carbohydrates, especially with such as potatoes, beans, bananas or dates, digestion will be inhibited or prevented and fermentation may occur. Oxalic acid diluted to one part in 10,000 completely arrests the action of ptyalin. Significant amounts of oxalic acid are contained in rhubarb, spinach, Swiss chard, beet greens and purslane.

The acetic acid in a teaspoonful of vinegar can suspend salivary digestion. Tannic acid (in coffee and tea) inhibits starch digestion, as do drug acids. The combination of citric, malic and oxalic acids in tomatoes (which are released and intensified by cooking) interferes drastically with starch digestion.

People take oranges and grapefruit as part of a meal that includes cereal and/or bread, later complain that they feel great distress after such a meal and conclude that they cannot eat citrus fruit. Such a conclusion is based on their experience of a wrong combination—not of a wrong food. Fermentation frequently does occur as a result of eating acids with carbohydrates. All students of food combining know that this combination produces bad effects such as gas, sour stomach (hyperacidity) and contributes to great difficulty in digesting starches.

When foods are eaten in such incompatible mixtures, and the efficiency of digestive enzymes is inhibited, it is subjected to decomposition in the digestive tract. If the digestive enzymes cannot perform their intended functions of breaking down and hydrolyzing the food (adding water from the body’s reserve supply), bacterial decomposition may occur, resulting in fermentation and the production of alcohol and acetic acid. Sugar, particularly, will readily ferment into alcohol, especially when combined with acids or protein. Natural combinations of citric acid or malic acid or other natural fruit acids combined in the whole fruit with fructose (also called levulose or fruit sugar) do not cause fermentation unless eaten with starches.

Alcohol, acetic acids and putrefying substances are byproducts of decomposition.

Putrefaction may be defined as the decomposition (as opposed to digestion) of protein matter by micro-organisms, producing malodorous and toxic substances.

Fermentation is the decomposition of sugar and starch, and their conversion by microorganisms into carbon dioxide, alcohol and acetic acids. Dr. Shelton says that digestion reduces food to the diffusible state without depriving it of its organic qualities, while fermentation renders food diffusible by reducing it to an inorganic and useless state. Digestion puts food in a solution, but fermentation disintegrates it.

A simple way to avoid production of these poisonous substances in the digestive tract is to learn, and implement, the Hygienic rules for food combining. They are perhaps of even greater importance than food selection. Persistent adherence to food combining principles has been known to reduce, or even eliminate, many digestive, nasal, skin and other problems, even in some people who have not changed to the Hygienic diet. It is obvious that elimination of incompatible food combinations is a giant step in the right direction. Efficient digestion and good health can be possible only when we eat in such a way as to offer the least hindrance to the work of digestion.

3.4 Protein Digestion

The digestion of carbohydrates is so different from that of protein that, when they are mixed in the stomach, they interfere with the digestion of each other. Protein digestion starts in the stomach and acid enzymes are secreted when protein is eaten. Proteins require an acid medium for digestion so, upon ingestion, hydrochloric acid is secreted in order to activate pepsinogen; this immediately stops the digestion of starches.

Almost all foods contain some protein but, when we speak of protein foods in our study of food combining, we are referring to concentrated proteins like nuts and seeds, cheese, flesh foods, etc. (See Classification of Foods in Lesson No. 23 for help in identifying concentrated protein foods, concentrated carbohydrate foods, etc.)

The normal digestion, absorption and metabolism of protein requires thorough mastication of food, in order to break it down for propulsion through the digestive tract, and for action by the digestive enzymes. As previously indicated, hydrochloric acid and pepsin (and other acid gastric juices) are secreted for the initial phases of protein digestion in the stomach, and other enzymes, such as trypsin, continue the digestion in the small intestine in a slightly alkaline medium. Protein-digesting enzymes are also secreted by the pancreas.

Before the body can use proteins, they must be reduced to their constituent amino acids (the building blocks of protein). The body must break down the complex proteins in foods and synthesize its own protein out of the amino acids. Food combining rules are of major importance in the consumption of protein, since the complexity of this food element would seem to suggest that it be eaten only under ideal conditions.

Free hydrochloric acid to the extent of only 0.003 percent is sufficient to suspend the starch-splitting action of ptyalin. Only a slight further increase in acidity not only stops the action, but destroys the enzyme. All physiologists agree that even a mild acid destroys ptyalin. It has never been shown that saliva is capable of digesting starch without the presence of ptyalin.

The function of the gastric protein-splitting enzymes, such as pepsin, are prevented by an alkali. The physiologist Stiles says, “The acid which is highly favorable to gastric digestion is quite prohibitive to salivary digestion. The power to digest proteins is manifested only with an acid reaction and is permanently lost when the mixture is made distinctly alkaline. The conditions which permit peptic digestion to take place are, therefore, precisely those which exclude the action of saliva.”

The presence of undigested starch in the stomach interferes with the digestion of protein. Physiologists have shown that undigested starch absorbs pepsin, which is necessary for the digestion of protein.

Dr. Richard C. Cabot of Harvard wrote: “When we eat carbohydrates, the stomach secretes an appropriate juice, a gastric juice of different composition from that which it secretes if it finds proteins coming down.”

3.5 Combination Foods

Single articles of food that contain starch-protein combinations (grains, legumes, and a few others) are less difficult for the body to handle than when two foods are eaten with opposite digestive needs. The body is able to adjust its juices, both as to strength and timing, to the digestive requirements of combination foods. The first response by the body is the releasing of an almost-neutral juice for digestion of the starch. After gastric digestion of starch is completed (about two hours), hydrochloric acid is secreted for digestion of the protein. The two processes do not go on simultaneously—rather, the secretions are minutely adjusted, in both character and timing, to the varying needs of the body to digest the complex food substance.

Such complex food substances are not ideal foods. They are usually cooked, and the fact that they require such complicated adjustments puts an additional strain on the body. Simple uncooked foods are easier for the body to process and offer the least hindrance to the work of digestion.

Beans: “Because of their complex character, beans, a protein-starch combination, tax the digestive powers more than simpler foods, but the gas, discomfort and other trouble that so commonly follow, eating them is not due so much to the beans themselves as to the company they keep. Baked beans are preferable to beans that are boiled and taken saturated with water. If taken thus relatively dry, well chewed and eaten in proper combinations, beans are readily digestible.” This article about beans, which was published by Dr. Shelton in 1971, is evidently a somewhat revised opinion. In his Volume II of The Hygienic System, published in 1935, he suggested that beans should not be used.

I personally was formerly unable to tolerate lentils or beans of any kind, even when sprouted. I eliminated all legumes from my diet for about six months, after which I carefully experimented with small amounts, properly combined, until I seemed to build up my ability to digest them. Today I can eat sizeable servings of cooked or sprouted legumes (which I seldom do) and I have no problem with them.

3.6 Foods with Different Digestive Requirements

When two foods are eaten that have different or even opposite, digestive needs, the precise adjustment of digestive juices to meet requirements becomes impossible. Eating proteins with carbohydrates (sugar or starch) produces the same abortive situation as combining acids with carbohydrates, since protein digestion requires the secretion of acid enzymes and juices. All acids, including those in food and the acid protein-digesting juices, destroy ptyalin, the starch-digesting enzyme.

Arther Cason, M.D., D.P.H., F.R.S.A. (London), writing in the April, 1945 issue of Physical Culture, mentions two groups of experiments made by him and his aides which showed that eating protein and carbohydrate at the same meal retards and even prevents digestion. He made control tests in which digestive rates were recorded, and a final analysis of the feces was made. He says, “Such tests always reveal that the digestion of proteins when mixed with starches is retarded in the stomach; the degree varying in different individuals, and also in the particular protein or starch ingested. An examination of the fecal matter reveals both undigested starch granules and protein shreds and fibers, whereas, when ingested separately, each goes to a conclusion.”

As indicated by Dr. Cason, there is an individual variation in the response to certain food combinations. This would seem to account for the fact that certain people exhibit overt symptoms from the use of certain food combinations, while others do not. However, the mere fact that overt symptoms are not observed is not proof, per se, that the food is being properly utilized.

Potatoes are said to be the least objectionable of any starch to be used with protein. Dr. Shelton is of the opinion that it is the rapidity with which potato starch digests that makes its combination with protein less objectionable than the combination of other starches with protein. Potato starch digests in ten minutes under ideal conditions and it would seem that the potato starch digests before the gastric juice can accumulate in quantity sufficient to interfere.

Even so, Hygienists have rarely been observed to use potatoes with nuts—either potatoes or nuts are so satisfactory as, an accompaniment to a salad that most of us would ask, “Why would we need both?”