THE NATURAL BASIS
EATING AND DIGESTION--PART 1 A BRIEF ACCOUNT OF BODILY DIGESTION
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.
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
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).
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
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
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.
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.
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).
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
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.