The Long Hollow Tube - A Primer on Digestion

Source: The Long Hollow Tube: A Primer on the Digestive System by Sally Fallon and Mary G. Enig, PhD.

Wise men, poets and philosophers have long honored the process of digestion as the basis of good health, sound sleep and a happy attitude. "I am convinced digestion is the great secret of life," said the prolific and quotable Reverend Sydney Smith. "Now good digestion wait on appetite, and health on both!" toasts Shakespeare's Macbeth. Milton wrote, "His sleep was airy light, from pure digestion bred."

Rudolf Steiner, the Austrian philosopher and mystic, placed the highest importance on complete and thorough digestion. He described digestion as the annihilation or breaking down of the outer world, which is food, into nothingness or chaos.

A primary task of the human ego, according to Steiner, was the building up of that pregnant chaos into compounds that bear our own imprint.

But let's begin more prosaically with Gray's Anatomy, which describes the digestive tract as a "musculo-membranous tube, about thirty feet in length, extending from the mouth to the anus, and lined throughout its entire extent by mucous membrane."

Mucous membranes are soft and velvety tissues, plentifully supplied with blood; their entire surface is coated over by the secretion of mucus, which serves to protect them from foreign substances with which they are brought into contact—in the case of the digestive tract, with the food we eat and our digestive secretions.

Note also the word "musculo." The muscles that encase the digestive tract work autonomously, without our conscious involvement, in a series of peristaltic (wavelike) contractions to mix our food with digestive juices and move it along as digestion occurs.

The Incredible Journey

Digestion begins in the mouth as food is chewed and mixed with saliva. Saliva keeps the mouth moist and usually contains a starch-digesting enzyme, ptyalin, which breaks down starches into the simple sugar, glucose. Place a bit of cracker on the tongue and in a moment it will taste sweet, due to the action of this enzyme.

The introduction of food into the mouth—or even the smell of food—stimulates increased production of saliva and also signals to the stomach and small intestine that food is on the way.

Chewing is an important first step in the digestive process, especially for fruits and vegetables, as it breaks down membranes of cellulose (indigestible for humans) and liberates the nutrients they surround.

Chewing also breaks food into small pieces, creating a large amount of surface area—digestive enzymes can only work on the surface of our food. When we wolf down our food, it takes much longer to digest.

When we swallow, chewed food is forced into the pharynx, a muscular apparatus that moves the food into the esophagus, the tube from the mouth to the stomach.

The esophagus is also a muscular organ, which propels the food down to the stomach by a series of wavelike contractions. Small glands located in the mucous membrane of the esophagus secrete alkaline compounds that further lubricate the food. The esophagus is the narrowest part of the digestive tract and esophageal secretions ensure a smooth passage.

Food enters the stomach via the cardiac opening, so called because of its proximity to the heart, via a circular muscle or sphincter that opens to allow food to pass through. When empty or contracted, the interior walls of the stomach form numerous folds.

These disappear when the stomach contains food and is distended. The stomach must begin to open up as food enters; this process of relaxation begins with the sphincter muscle at the cardiac opening in response to commencement of eating.

The stomach has two main functions—the storage of food until it can pass into the intestines and the mixing of food with digestive enzymes, a process that turns the bits and chunks of food that enter the stomach into a relatively smooth and thick fluid mixture called chyme.

Mixing occurs due to the action of muscles that encase the stomach. Periodic contractions churn and knead the food into chyme and rhythmical pumping moves the food toward the pylorus, the opening at the lower end of the stomach.

The mucous membrane of the stomach is densely packed with glands that secrete hydrochloric acid and pepsin, a protein-digesting enzyme. The role of hydrochloric acid is to create a sufficiently acid environment for pepsin to be activated.

If we do not produce enough hydrochloric acid, then we cannot fully digest protein. The parietal cells that create hydrochloric acid also produce a large protein called the intrinsic factor, necessary for the assimilation of vitamin B12.

The pumping action of the stomach moves the chyme through the pyloric valve into the duodenum, the first section of the small intestine. The small intestine is about 25 feet long and one inch in diameter; it fits in coiled fashion in the abdominal cavity.

It has three parts: the first twelve inches is called the duodenum (which means "twelve" in Latin), the jejunum, which is about 10 feet long, and the ileum, the last portion of the small intestine, which is about 15 feet long. While the environment in the stomach, especially the lower part of the stomach, is highly acidic, the environment in the small intestine is alkaline.

The entrance of food into the duodenum stimulates the production of two hormones, secretin and cholecystokinin, which communicate important messages to the stomach, the pancreas and the gall bladder.

They tell the stomach to moderate its contractions so that the chyme does not arrive too quickly, but in measured amounts; and they signal the release of digestive secretions from the pancreas and gall bladder.

It is in the first two sections of the small intestine that most digestion and assimilation occur. Absorption takes place via the villi, small projections in the mucous membrane.

Each villus has a network of capillaries through which the broken-down components of the food are absorbed. The nutrients then pass through the epithelial cells in the inner lining of the villi, at which point they enter the capillaries.

The small intestine is attached to the rear abdominal wall by a thin sheet of membrane called the mesentery, which carries blood vessels to nourish the small intestine and carries absorbed nutrients to the liver and other parts of the body.

Once again, muscular contractions move the chyme along. Whenever a section of the small intestine becomes stretched, peristaltic movements (waves of contractions) occur at spaced intervals.

These not only move the chyme along but also mix it with digestive secretions. At the end of the small intestine is the ileocecal valve. As with the connection between the stomach and the small intestine, various hormones and feedback mechanisms regulate the passage of chyme through the ileocecal valve into the large intestine.

When the ileum becomes stretched and full, the valve opens to allow the passage of chyme and if the large intestine is too full, the valve remains closed until the bowel empties.

The small intestine actually meets the large intestine at a kind of T junction. To the left is the cecum, a kind of holding tank, and to the right the bowel. Attached to the cecum is the appendix, once considered a non-functioning or "vestigial" organ but now recognized as serving an important immunological function.

The appendix contains a high concentration of lymphoid follicles that produce antibodies to help keep the bacteria of the colon from infecting other areas of the body, such as the small intestine and the bloodstream, particularly in early life.

The large intestine or colon is five to six feet long with a diameter of about two inches and is divided by sharp turns into three major parts—the ascending colon on the right hand side of the body, the transverse colon which runs from right to left across the upper abdomen, and the descending colon which carries the mass of digested food downward to the rectum.

The purpose of the large intestine is threefold: storage of waste materials and undigested food from the small intestine—not just the breakdown products of what we take in but the residue of secretions, sloughed-off cells and dead bacteria that accumulate during the digestive process; the absorption of water and electrolytes from the food residue; and the further decomposition of solid materials by the action of millions of bacteria.

Combined contractions of circular and lengthwise muscles surrounding the colon roll over the fecal materials to ensure that all of it is exposed to the intestinal wall, so that all the fluid can be absorbed. Special cells called goblet cells lining the large intestine secrete mucus that protects the walls of the intestine, helps maintain alkalinity and provides a medium to hold the fecal matter together.

The final stage of this incredible journey is the movement of the now solid fecal matter from the transverse colon via strong contractions down the descending colon and into the rectum, a process that occurs only a few times each day—usually upon arising in the morning or immediately after breakfast. When these movements force a mass of fecal matter into the rectum, the desire to evacuate is felt.

The Second Brain

"Have you ever wondered why people get butterflies in the stomach before going on stage? Or why an impending job interview can cause an attack of intestinal cramps? And why do antidepressants targeted for the brain cause nausea or abdominal upset in millions of people who take such drugs?

The reason for these common experiences is because each of us literally has two brains—the familiar one encased in our skulls and a lesser-known but vitally important one found in the human gut. Like Siamese twins, the two brains are interconnected; when one gets upset, the other does, too." So writes science journalist Sandra Blakeslee for the New York Times.

Indeed, the human digestive tract contains over one million nerve cells, about the same number found in the spinal cord. There are actually more nerve cells in the overall digestive system than in the peripheral nervous system. Furthermore, major neurotransmitters found in the brain—including serotonin, dopamine, glutamate, norepinephrine and nitric oxide—occur plentifully in the gut as well.

Enkephalins—described as the body's natural opiates—also occur in the intestinal tract, as do benzodiazepines, psychoactive chemicals similar to mood-controlling drugs like Valium and Xanax.

Jordan Rubin describes the "brain-gut" connection very well in his book The Maker's Diet:

"Early in our embryogenesis, a collection of tissue called the 'neural crest' appears and divides during fetal development. One part turns into the central nervous system, and the other migrates to become the enteric nervous system. Both 'thinking machines' form simultaneously and independently of one another until a later stage of development.

"Then the two nervous systems link through a neural cable called the "vagus nerve," the longest of all cranial nerves. . . The vagus nerve "wanders" from the brain stem through the organs in the neck and thorax and finally terminates in the abdomen. This is your vital brain-gut connection."

Serotonin in the gut is thought to initiate peristalsis, the rhythmic movement of food through the digestive tract. Drugs like Prozac actually divert serotonin from the intestinal tract to the brain, leading to digestive problems including constipation in many patients.

The gut produces the same pain-alleviating chemicals as those in antianxiety drugs. Says Rubin, "If you overeat because you feel anxious, your body may be trying to use the extra food to produce more benzodiazepines.

We are not sure whether the gut synthesizes benzodiazepine from chemicals in our foods, from bacterial actions or from both. We do know that extreme pain appears to put the gut into overdrive in order to send benzodiazepine directly to the brain for immediate pain management."

An Ecosystem

The digestive system is far more than a collection of pipes, wiring and membranes. It is actually an ecosystem, populated by billions of organisms that produce substances necessary for digestion to occur—enzymes, vitamins and beneficial acids (especially lactic acid).

In the young, gut bacteria interact with intestinal cells, called paneth cells, to promote the development of blood vessels in the intestinal lining. In the large intestine, fermentation processes produce butyric acid and other short-chain fatty acids that nourish the intestinal wall.

But fermentation is undesirable in the small intestine. When the intestinal ecosystem is healthy, beneficial bacteria keep yeasts and other fermentation microorganisms at bay in this part of the digestive tract. An imbalance of microorganisms, called dysbiosis, results in overgrowth of fungus and other pathogens, resulting in numerous digestive disorders.

Even today, textbooks typically describe the environment of the small intestine as "sterile." Scientists thought that beneficial organisms could not survive the highly acid milieu of the stomach to pass into the small intestine. This view is no longer tenable. Good health depends on the right mix of microorganisms in both the small and large intestine.

Like all ecosystems, the delicate balance of the digestive tract can be altered by various toxins including antibiotics and other drugs, chemicals like chlorine and fluoride in our water, food additives and preservatives, stimulants like coffee, and an overabundance of difficult-to-digest foods like improperly prepared whole grains.

Digestion of Carbohydrates

Digestion of sugars and starches begins in the mouth as amylases (starch-digesting enzymes) begin the breakdown of starches into simple sugars such as maltose, fructose and glucose.

This process continues, but at a lesser rate, in the upper portion of the stomach where the enzymes provided by the saliva continue their work. Once the food moves into the lower portion of the stomach, which is highly acidic, carbohydrate digestion temporarily ceases.

In the small intestine, the breakdown of starches and sugars renews. Amylases produced by the pancreas split sugars and starches into disaccharides (such as lactose, sucrose and maltose) and enzymes from the cells lining the small intestine (called the brush border) reduce these into the monosaccharides galactose, glucose and fructose.

About 80 percent of the final product of carbohydrate digestion is glucose. These various simple sugars are selectively absorbed through the intestinal membrane.

Digestion of Protein

Digestion of proteins begins in the highly acidic medium of the lower stomach. Hydrochloric acid activates pepsin, an enzyme that breaks down proteins into shorter strings of amino acids. These products then move into the alkaline environment of the small intestine where pancreatic enzymes break down these strings into individual amino acids.

The proteolytic or breakdown enzymes are very specific for the amino-acid linkages—a specific enzyme is required for each type of amino-acid linkage. The proteins are then rapidly absorbed, usually as single amino acids but occasionally as combinations of two or three amino acids.

Digestion of Fat

Digestion of fats is more complex than that of proteins or carbohydrates. Some digestion occurs in the mouth and the upper stomach due to the action of lipases (fat-digesting enzymes) on the surface of the fat globules. But most fat digestion takes place in the small intestine.

For full digestion to occur the fat globules must be broken down; the substance that accomplishes this process (called emulsification) is bile, which is a secretion of the liver. The soap-like action of bile on fat globules increases the surface area an estimated 10,000-fold, thus allowing the lipases to liberate the fatty acids.

Stable compounds called micelles are formed, small spherical globules consisting of long chain fatty acids, monoglycerides (a glycerol molecule attached to a single fatty acid) and bile salts. The micelles are absorbed at the surface of the intestinal mucous membrane.

Once in the intestinal mucosa the various fatty compounds are joined with small amounts of protein and formed into compounds called chylomicrons, which enter the lymph system and eventually the blood as lipoproteins—compounds with a lipid core and a protein coating that makes them soluble in water.

Bile is produced by the liver out of cholesterol and stored in the gall bladder. The gall bladder releases bile into the small intestine through the action of a hormone, cholecystokinin. When the meal contains sufficient amounts of fat, the gall bladder empties completely in about one hour. Then the gall bladder slowly fills up again, getting ready for the next meal.

Bile not only serves to break down fats but also carries a lot of waste products away from the liver and into the intestine so that they can be eliminated.

The Role of the Liver

The liver performs a multitude of wide-ranging tasks. These include the destruction of old red blood cells, the manufacture of proteins and of blood-clotting agents, the manufacture of cholesterol, the storage of carbohydrates in the form of glycogen, some storage of fats and proteins, the conversion of fats and proteins to carbohydrate, the transformation of galactose (milk sugar) into glucose, the extraction of ammonia from amino acids, the conversion of ammonia into urea, the production of bile salts, the storage of fat-soluble vitamins and the conversion of adipose fat into more combustible ketone bodies. The liver also neutralizes various drugs and poisons—everything from alcohol to barbiturates.

Unlike other organs in the body, the liver can regenerate its tissues, a trait that has led to its title of "the immortal organ" and "the seat of life." It sorts, organizes and stores the simple breakdown products of digestion, sent to it from the small intestine via the portal vein, and then uses these basic components to construct the complex substances the body needs; it also deconstructs a wide range of toxins and sends them away for elimination.

Lending a Helping Hand

The exquisite and finely tuned digestive system requires our utmost respect. From the first bite of food to the elimination of wastes, membranes, glands, muscles, hormones, secretions, enzymes, blood, nerves and microorganisms work in concert to extract nourishment from our food and deliver it to our cells.

The wrong diet can disrupt this system in two ways—by failing to provide nourishment and by delivering food that is difficult to digest.

While the medical profession turns to drugs as a solution to digestive problems, the basic remedy should be nutrient-dense foods, especially the animal foods that provide fat-soluble nutrients, combined with wise preparation methods.

Many modern foods, such as processed milk products, breads and soy foods are extremely difficult to digest; but traditional preparation methods made food easy to digest and facilitated assimilation of nutrients. They include:

• Preparation of grains by soaking and sour leavening to neutralize difficult-to-digest components and nutrient blockers.
• Long soaking and cooking, or even fermentation, of legumes.
• Fermentation of many types of tubers, such as casava.
• Lacto-fermentation of condiments and beverages to provide beneficial bacteria for the digestive tract.
• Consumption of protein foods (meat, eggs, fish and milk products) with plenty of fat.
• Use of gelatin-rich bone broths. Gelatin acts not only to bring food into contact with digestive juices, it also soothes the intestinal wall.
• Cooking of most vegetables (and even some fruits) to neutralize toxins and break down cell walls.
• Proper aging of meat to initiate the breakdown of protein. With proper aging and/or fermentation, meat is quite digestible either raw or carefully cooked at low temperature.
• Soaking and/or roasting of nuts to remove irritants and toxins.

Happy Meals

Our journey through the digestive tract teaches us that digestion is more than just a biochemical process—it is a rhythmical alchemy that is highly influenced by our emotional state.

The digestive system needs to alternate between periods of activity and rest, and that rhythm is best served by three meals per day, with nothing to eat in between. This allows the stomach to rest, the gall bladder to refill, the intestines to move at their proper pace.

Delicious smells and attractive presentation make meals a pleasurable experience, stimulating the production of feel-good chemicals in the gut; a moment of silence or prayer before the first bite allows the brain and the stomach to communicate to the body that nourishment is on its way.

Slow eating and careful chewing let your digestive tract know that it doesn't need to rush. Above all, pleasant surroundings and conversation provide support to the entire digestive process. No arguments at dinner, please—to paraphrase the great Satchell Page, they angers the stomach.