CHAPTER XVII

Digestion is the breaking down of large molecules into smaller ones, a change that is sometimes dramatic

431. Everyone who is not familiar with chemistry is surprised to realize that there is a limit to the smallness of any piece of a substance. Although the limit is far below visibility it is so real that if this smallest possible particle or molecule is further divided or broken, two or more different substances are produced. The differences can be small or great. For example, a house is no longer a house if it is cut into two, but the pieces are still obviously pieces of a house. The great differences are no less amazing than it would be if a spade, being deprived of its handle, immediately modified itself into a foot-ball and a flower pot. Thus common salt, if separated into its two constituents, becomes sodium (a soft, shiny metal which tarnishes in a few minutes in air and decomposes water) and chlorine (a faintly greenish, very poisonous gas). On the other hand, common sugar, being split into two, forms two other sugars different, but not remarkably different, from their parent. Such analogies and particular examples can be quite misleading, but these may suffice for a first general idea.
    Most of the constituents of food are molecules with a much more complicated make-up than common salt, and many of them are very large, as molecules go, and much too large to be accepted into the body (except fat, some of which enters in spite of its large molecules). These large food molecules must be broken down into specific units of a suitable size and quality by an orderly process. This is digestion.

Digestion in living subjects is carried out by enzymes which, however, are not themselves alive

432.In the whole of the animal and plant kingdoms the break down or digestion of large molecules is conducted by specialized agents known as enzymes. Other enzymes serve in the building-up processes. Most of the activities inside living cells are carried out by enzymes, and often the activities outside the cells by means of enzymes also, but different ones. All enzymes are produced by living tissues. There are very many different kinds of enzymes, each capable of one particular function. Many processes other than digestion are carried out by them; the digestive enzymes are a relatively small group out of a vast array. The digestive enzymes are wonderfully designed for their particular use. For example, they are produced in the inside of living cells but they do not digest the cell that produces them. This is usually because they are produced in an inactive form, the essential parts being tied up or blocked off. (Imagine a key wrapped in plastic.) After leaving the producing cell, they meet
different conditions which remove the blocking agent. It is difficult to understand how enzymes work. It used to be thought that some special vital quality existed in them because they were produced by living tissues, but now their activities can be well explained in terms of chemistry and physics. They work equally well outside the body providing the conditions are correct (temperature, acidity, etc.).

433. The fundamental changes going on during digestion are best understood if the three main constituents of food are treated separately. We deal first with carbohydrates (i.e., sugars, starch, etc.), then with proteins (in meat, fish, cheese, and in some vegetable foods), and finally with fats and oils (as butter, margarine, etc., but plentiful in many foods and present as traces in most).

The digestion of large carbohydrate molecules produces sugars like glucose, the molecules of which are small enough to be absorbed

434. The digestion of carbohydrates is fairly easy to grasp. They form a large class and include the smaller class of sugars. There is a considerable number of sugars, ordinary sugar, i.e., cane or beet sugar being only one. Other common sugars are fructose (present in fruit) and glucose (the sugar that circulates in the blood and is fuel for bodily activities). Thus when glucose is taken no digestion is needed. It passes directly through the fine walls of the minute blood vessels in the inner surface of the small intestines. Cane sugar, on the other hand, must be digested before we can use it. It is fairly small as food molecules go, and is composed of only two other sugars, glucose and fructose. It is split into these after it has been absorbed by cells in the wall of the intestine.

435. The story with starch is different. It consists of a very long chain, more or less branched, the links of which are mostly glucose units. Several different enzymes can liberate the glucose; some can only break the chain into shorter lengths, and the glucose molecules are then chopped off the ends one at a time. Other enzymes act differently. For example, when barley grain is germinated, i.e. , made into malt, the stored starch is made available to the growing plant embryo by an enzyme that breaks off the glucose units in pairs. These two molecules of glucose joined together are called maltose. In the human, the digestion of starch begins in the mouth by means of an enzyme which is more efficient in breaking the long chain into shorter pieces than in liberating individual glucose or maltose units. The rest of the digestion is carried out by the juices from the pancreas. The glucose then passes through the intestinal walls into the blood stream and is carried to all parts of the body as a
source of energy, that is, as a fuel.

Proteins need digestion with several enzymes in order to liberate amino acids

436. Proteins are the structural units used by the body to build muscle, skin, hair, tendons, and many other tissues. Edible parts of meat and fish are thus mostly protein (apart from the water). Useful quantities are also found in peas, beans, wheat, etc. Every living organism, plant or animal, contains some protein. Enzymes are also proteins. In order to produce his own enzymes and muscles and most other parts of his body, a man must absorb the small molecules from which protein molecules are built. Most of these small molecules belong to the class called amino acids. They are liberated by the digestion of the proteins present in the food.

437. Proteins are more complex than starch. Their very large molecules are built up from twenty or so different kinds of amino acids. Protein molecules vary in kind and in size. Some are built up from only about a hundred amino acids. Others require several thousands. Sugars also are combined in some proteins. The formation of such a variety of huge and complicated molecules from only twenty different kinds of units is sometimes compared to the formation of words, sentences, and long printed pages from only twenty-six letters of the alphabet. In this analogy, a small protein would be like one sentence, and a larger one like a page of print. Many proteins contain all the amino acids required by man. Some proteins contain a super-abundance of one or two amino acids, but in general a balanced diet gives an average mixture of amino acids
regardless of which proteins are eaten.

438. The amino acids are tightly bound together in proteins, and their liberation is perhaps the main burden of digestion, which, in the case of proteins, takes place in three stages: one in the stomach and two in the intestines. The digestion in the stomach begins with hydrochloric acid which has an instantaneous effect on some proteins; it causes the chain of amino acids to extend into a more open structure, facilitating the attack by enzymes. The strength of the hydrochloric acid is remarkable as everyone knows who has suffered from regurgitation. Nothing can live in an acid of this strength, and many of the micro-organisms present in the food are killed; though some survive if they are protected inside resistant particles of food. The stomach itself is protected by copious layers of mucus.

439. The enzyme of the stomach (pepsin) can attack relatively few links in the chain of amino acids. Hence it breaks each protein molecule into only a few pieces, each piece still containing a number of amino acids. Nevertheless, the specific properties peculiar to each individual kind of protein are lost. The fragments produced by the action of the pepsin are further broken down in the intestine. Here, the stomach acid is neutralized, and powerful enzymes from the pancreas attack the amino acid chains at many points, liberating fragments containing only a few amino acids. As the partly digested food is moved along the intestine, it becomes mixed with a third group of enzymes which finally liberate the separate amino acids. These can then enter the blood, and be distributed for growth and maintenance, which require the linking together again, of the amino acids into proteins, but now into the characteristic order that is unique to the tissue being produced or repaired.

Fats are partly digested and partly absorbed as minute globules with the aid of the lymphatic system

440. The digestion of fats is carried out by an enzyme, or group of enzymes, known as lipase, but is assisted by bile from the liver. The bile acts as a detergent, breaking up the fat into minute globules. This is essential for adequate digestion because the lipase can act only on the surface of the fat, and the large drops of fat or oil would take a very long time to digest. Lipases are produced in a number of sites, but the most important is probably the pancreas. As mentioned above, some of the fat can be absorbed without digestion. That which is digested is resynthesized as soon as it has passed into the wall of the intestine. The result in either case is an emulsion of very fine fat droplets, and although they are very small they cannot pass through the walls of the capillaries into the blood stream. Instead they are absorbed by the lymphatic system which is adapted to receive particles that are too large to enter the blood (See Chapter VIII, Nos. 196-224).

441. In this part of the body, then, the lymph is rich in fat globules, giving it a milky appearance, so that the network of tiny transparent tubes is clearly seen. Since they look as though they contain milk, these vessels are known as lacteals, and the contents form the chyle which is mentioned in the Writings. As the chyle and the lacteals are easily seen post-mortem, they were known to the anatomists long before more advanced methods revealed many other lymphatic ducts. As related in Chapter VIII, the chyle flows into the venous blood system and then to the heart. So the fat is mixed with all the blood and is carried to sites where it may be stored for use in case of starvation; or to the tissues where it may be converted into simpler substances for energy production if required. This conversion is a more complex process than liberating glucose from starch.

There are important constituents of food that need no digestion

442. These constituents include vitamins and minerals, which form an essential part of our diet although the quantities are so small. Glucose is imbibed in much larger quantities (from grapes, raisins, and some drinks, for example) and needs no digestion. Many other substances are absorbed without digestion, but may be changed afterwards (e.g., fructose, alcohol, some medicines).

Many animals make use of bacteria to digest food that would otherwise be useless to them

443. It was mentioned above that nothing could live in the acid conditions of the stomach. As soon, however, as the acidity of the digesta is neutralized in the duodenum, the micro-organisms already there and others that have survived the acid either as spores or inside particles, grow under the new conditions and reach very high numbers. In the human they probably do not aid digestion significantly, but in the vegetarian animals that rely on cellulose for much of their energy, micro-organisms digest plant materials which otherwise would not become available. Such animals have a means of delaying the passage of food so as to give time for the micro-organism to effect digestion. In the cow, for example, the delay is in the first stomach or rumen; in the rabbit it is in the caecum and appendix; and in the horse it is in the colon. In such animals these structures are relatively large. In the human the caecum is quite short and it terminates in the appendix, which is only a narrow tube with no outlet. The micro-organisms that live in the rumen or in the caecum and colon are essential for the nutrition of mammalian herbivores which are themselves unable to digest cellulose. Without the aid of the micro-organisms they would need a richer diet than grass or
hay. (Cows do receive a richer diet to increase milk production, but if they have too little fibre the amount of cream in the milk is reduced. This is because most of the milk fat is produced from the products of the action of bacteria in the rumen.) Some of the micro-organisms are also digested.

444. As we are studying the correspondence of the Grand Man with the human body, mention of digestion in animals might seem out of place. However, we know that the whole of nature is a theatre representative of the Lord's kingdom. Moreover, "the external man…separated from the internal, is in itself no other than a wild animal, having a similar nature, desires, appetites, phantasies, and sensations, and also similar organic forms" (AC 272). It seems reasonable, therefore, to assume that the organs in animals correspond to spiritual things, as do those in man. It is, of course, from spiritual influx that "rumination" has come to mean "thinking things over." Indeed we read in AE 242:4 that man's memory corresponds to the ruminant stomachs of animals and birds.