The digestive system or alimentary canal is a long muscular tube in which food passes through our body that starts with the mouth and ends at the anus.
Along the way, a set of organs, glands, and their complex physiological functions are able to process and transform large, insoluble food molecules into small, water-soluble molecules that our body can utilize.
For example, potatoes, even when cooked, are chemically too complex to be absorbed and utilized by the body.
They need to be reduced into smaller and simpler molecules.
Through digestion, they are getting broken down into their elemental constituents: glucose, fructose, amino acids, fatty acids, mineral salts and vitamins.
This catabolic process is facilitated by specific kinds of proteins called digestive enzymes.
Digestive enzymes are chemical substances secreted by the salivary glands and the cells that line our stomach, pancreas, and small intestine to help with the digestion of food.
Their job is to split the large, complex molecules that comprise proteins, carbohydrates, and fats (macronutrients) into smaller ones, allowing the nutrients from these foods to be easily absorbed into the bloodstream and carried throughout the body.
Phases of Digestion
The process of digestion can be distinguished into 3 distinct phases :
1. Cephalic Phase
The process of digestion begins with the sight of food or even, its image in our mind.
The thought, smell, or look of food immediately send signals to the brain, which then activates the glossopharyngeal nerve, the main parasympathetic nerve that controls the three salivary glands, namely, parotid, submandibular and sublingual glands.
The salivary glands, when activated, begin manufacturing and secreting large quantities of mucus-laden saliva, rich in water, mineral salts, salivary amylase and lipase, anti-microbial substances and the enzyme lysozyme, which both combat invading microbes (non-specific defense).
Once food is ingested, it is chewed (masticated) by the teeth and moved around the mouth by the tongue and the muscles of the cheeks.
It is mixed with saliva and formed into a soft mass or bolus, ready for swallowing.
The amylase family of enzymes begins breaking down carbohydrates such as starches- complex arrangements of sugars linked together (also known as polysaccharides) into simpler fragments.
The lipases begin the process of breaking down triglycerides, a major dietary fat, into their fatty acid components.
Saliva’s high water and mucus content help lubricate the food bolus to aid in chewing and to help the food move more easily along the mouth and esophagus.
The duration of chewing is dependent on the consistency of the food.
Also, the taste buds of the tongue are stimulated only by chemical substances in solution, and thus dry foods stimulate the sense of taste only after thorough mixing with saliva.
After the bolus formation and chewing is complete, swallowing occurs, initially voluntarily, and then by involuntary action.
The presence of food in the mouth stimulates the vagus nerve, the parasympathetic nerve that controls the upper part of the digestive system, from the esophagus to the mid-colon .
Branches of the vagus nerve activate two different types of cells, the parietal and chief cells in the body of the stomach.
Even before food makes its way from the mouth into the stomach, the parietal cells are producing and releasing hydrochloric acid (HCL), and the chief cells secrete the enzyme precursor pepsinogen.
With the mouth closed, the voluntary muscles of the tongue and cheeks push the bolus backward into the pharynx.
The presence of bolus in the pharynx stimulates a wave of peristalsis that propels it through the oesophagus to the stomach.
The walls of the oesophagus are lubricated by mucus, which assists the passage of the food bolus during peristaltic contractions of the muscular wall.
Unless the bolus is swallowed, the walls of the oesaphagus stay relaxed, without initiating peristalsis.
2. Gastric/Stomach Phase
Ahead of a peristaltic wave, the lower oesophageal sphincter guarding the entrance to the stomach relaxes to allow the descending food bolus to pass into the stomach.
When the food bolus itself makes its way into the stomach, secretion of both hydrochloric acid (HCL) and pepsinogen – which have already started due to reflex stimulation of the vagus nerve – increase significantly and begin the gastric phase of digestion.
In addition, the cells lining the stomach secrete a thick mucus coating that protects the stomach from the acid and pepsin, which in itself is only active in a very acidic environment that HCL provides of about 1.5 to 2 pH.
In such an environment, pepsin begins the process of breaking down complex food proteins into smaller fragments.
The presence of food stimulates vigorous contractions of the stomach muscles generating a churning action that breaks down the bolus and mixes it with gastric juice.
Normally, constriction of the lower oesophageal sphincter prevents the reflux of gastric acid from the stomach into the oesophagus.
The stomach is a J-shaped digestive organ that functions as a temporary holding pouch for food, allowing time for the digestive enzymes pepsins to act.
It consists of four layers of tissue and is divided into three sections: the fundus, the body, and the pylorus .
When the stomach is empty, its mucous membrane lining is thrown into longitudinal folds or rugae, and when it is full the rugae are “ironed out”, giving its surface a smooth, velvety appearance and increasing its food volume storage capacity.
It is different from the other regions of the alimentary canal, as it has three layers of smooth muscle instead of two, namely, outer longitudinal, middle circular, and inner oblique muscles.
This arrangement allows for the churning action, characteristic of gastric activity as well as peristalsis.
The stomach plays a very important role in the initial stages of digestion.
It squeezes and churns the food bolus, and secretes a thin, watery, acidic digestive fluid known as “gastric juice.”
Gastric juice is comprised of water, mineral salts, mucus, hydrochloric acid, intrinsic factor and inactive enzyme precursors – pepsinogens secreted by the chief cells .
Each of its components carries a specific function:
It further liquefies the food swallowed.
Hydrochloric acid (HCL)
Acidifies the food and stops the action of salivary amylase, kills ingested microbes and provides the acid environment necessary for the action of pepsins.
They are activated to pepsins by hydrochloric acid and by pepsins already present in the stomach.
These enzymes begin the digestion of proteins, breaking them down into smaller molecules.
Pepsins have evolved to act most effectively at a very low pH, between 1.5 and 3.5.
A protein necessary for the absorption of vitamin B12 from the ileum (see 3D model below)
Lubricates the contents of the stomach preventing mechanical injury of its tissues. .
It also prevents chemical injury by acting as a barrier between the stomach wall and the highly corrosive gastric juice.
Hydrochloric acid (HCL) is present in potentially damaging concentrations and pepsins would digest the gastric tissues.
In general, there is always a small quantity of gastric juice present in the stomach, even when it contains no food.
In this case, when it contains no food, the pyloric sphincter – a muscular valve that regulates the flow of partially digested food from the stomach to the duodenum – is relaxed and open.
During digestion, it is closed in order to prevent the reflux of gastric acids into the eosophagus, which is not coated with mucus-secreting epithelial cells that offer protection against the corrosiveness of digestive acids.
The stomach is also an organ rich in cells that secrete various substances, such as hormones and neurotransmitters.
Numerous gastric glands are located below the surface of the stomach’s mucosal layer, containing special cells like neuroendocrine cells, chief and parietal cells that secrete the constituents of the digestive juices [7, 8].
When the peristaltic waves in the stomach wall propel the contents towards the pylorus – the last part of the stomach – it gradually opens, allowing the passage of food into the first part of the small intestine, the duodenum.
After the gastric contents are sufficiently liquefied (chyme), strong peristaltic contractions of the pylorus force them in small spurts through the pyloric sphincter into the first section of the small intestine (duodenum).
As long as the stomach remains active, the pyloric sphincter stays closed.
Parasympathetic stimulation increases the motility of the stomach and the secretion of gastric juices, while sympathetic stimulation has the opposite effect, impairing digestive function.
3. Intestinal Phase
When the food bolus enters the duodenum (first part of the small intestine) endocrine cells located in the intestinal mucosa produce two hormones, secretin and cholecystokinin (CCK).
These digestive hormones slow down the secretion of gastric juice and reduce gastric motility .
By slowing down the emptying rate of the stomach, the chyme (semi-digested, liquified food mixed with digestive secretions) in the duodenum becomes more thoroughly mixed with bile and pancreatic juice.
This phase is most noticeable following a high-fat meal, as fat is the most potent inhibitor of gastric emptying.
The rate by which the stomach empties its contents into the duodenum has to do with the type of food eaten.
A carbohydrate meal leaves the stomach in 2-3 hours, a protein meal stays for longer (depending on the protein source), and a fatty meal remains in the stomach for the longest.
The small intestine is about 2.5 cm long in diameter and a little over 6 meters long, and meets with the next section of the gastrointestinal tract, the large intestine (colon) at the ileocecal valve .
The small intestine is comprised of 3 parts:
- Duodenum: It is the first part of the small intestine, about 25 cm long and curves around the head of the pancreas. Secretions from the gallbladder and pancreas merge in a common structure- the hepatopancreatic ampulla, entering the duodenum at the duodenal papilla, which is guarded by the hepatopancreatic sphincter.
- Jejunum: It is the middle part of the small intestine with a length of about 2 meters. The transition from the duodenum to the jejunum happens at the duodenojejunal flexure. It has a critical role in the absorption of water, and the digestion and absorption of lipophilic nutrients, as it covers 40% of the whole small intestine.
- Ileum: It is the last section of the small intestine, about 3 meters long and terminates at the ileocecal valve, which controls the flow of material (and prevents backflow) from the ileum to the first part of the large intestine (caecum). The ileum is very rich in lymphoid follicles which are in many ways similar to lymph nodes, called Peyer’s patches, because it deals with potential pathogens coming from the colon.
You may learn more about the small intestine by interacting with the 3D model below:
When acid chyme passes into the small intestine it is mixed with pancreatic juice, bile, and intestinal juice, and comes in contact with the absorptive enterocytes of the villi.
The villi are tiny, finger-like projections of the mucosal layer into the intestinal lumen, about 0.5-1 mm long.
Their covering consists of columnar epithelial cells or enterocytes, with tiny microvilli (1 μm long) on their free border.
This is where in normal healthy conditions the digestion of all nutrients is completed:
- Carbohydrates are broken down to monosaccharides.
- Proteins are broken down to amino acids.
- Fats are broken down to fatty acids and glycerol.
The Pancreas and its Functions
Digestion in the duodenum is a complex process, regulated by the neuroendocrine system, with the participation of both neuronal communication and hormone secretion.
In this intestinal phase of digestion, the organ/gland known as the pancreas, and its enzymes become active.
The pancreas functions as both an exocrine and endocrine gland, meaning that it secretes chemical substances both directly into the bloodstream (endocrine), as well as through various ducts (exocrine) .
In adults, the pancreas is about 10-15 cm in length (5-7 inch.), with a widened head, a narrowing body and a tapering tail.
The head of the pancreas fits snugly in the C-shaped duodenum, the first part of the small intestine.
Microscopically, the tissue of the pancreas consists of two distinct cell types: the endocrine or hormone-secreting cells, and the exocrine or digestive-enzyme-producing cells.
The endocrine cells are scattered in clusters throughout the pancreas and are known as the islets of Langerhans.
The islets consist of three basic cell types which are distinctive under the microscope:
Alpha cells produce and secrete glucagon, which the pancreas releases in response to low blood sugar levels.
Glucagon circulates in the bloodstream and stimulates both muscle and liver cells to break down glycogen, a storage form of sugar, into glucose, which is released into the bloodstream to serve as a source of cellular energy.
Beta cells of the pancreas secrete insulin in response to high blood sugar levels, as happens after a meal.
Traditionally, scientists believed that insulin only served to drive glucose, the main blood sugar, into muscle and liver cells, where it can be stored as glycogen.
In recent years, however, scientists have learned that insulin is a far more complex hormone than originally thought, involved with processes other than sugar metabolism.
Cancer researchers, for example, now believe insulin is a growth factor that in excess can stimulate certain tumors to grow uncontrollably .
The delta cells of the pancreas produce and secrete somatostatin, a complex hormone that is also produced by the cells lining the intestine, as well as certain neurons in the brain.
In the pancreas, somatostatin can inhibit the release of either glucagon or insulin, to help maintain blood sugars in a very narrow range.
Scientists do not know how one hormone, somatostatin, is able to inhibit either glucagon or insulin release, in order to drive blood sugar levels up or down, as needed by the body .
These endocrine cells of the pancreas- the alpha, beta, and delta cells, are dispersed in clusters throughout the entire pancreas, like stars in the sky.
They release their hormone products directly into the bloodstream for action at distant sites, such as the liver and muscles.
The cells of the exocrine pancreas, which produce the various digestive enzymes, are arranged in a very distinctive pattern.
These cells cluster in what are called acinus glands or acini.
Acinus glands resemble cul-de-sacs of a housing development, with a single layer of elongated cells forming the house lots around a common space that leads into a duct, the equivalent of a roadway for secreted pancreatic enzymes.
The cells lining this common opening are very active cells, producing copious amounts of enzymes needed by the body every day.
In the past, scientists had identified only the three basic pancreatic digestive enzymes: the proteolytic or protein-digesting enzyme trypsin, the starch-digesting enzyme amylase, and the fat-digesting enzyme lipase.
We now know that the pancreas produces many more enzymes in each of these three classes.
For example, several dozen proteolytic enzymes have been identified now in addition to trypsin, such as chymotrypsin, carboxypeptidase A, carboxypeptidase B, elastase, aminopeptidase, dipeptidases, tripeptidases, and another form of pepsin, the enzyme also produced by stomach cells .
Each of these enzymes has a very specific and precise function.
Trypsin will degrade a protein molecule only at amino acid linkages that contain arginine or lysine.
Chymotrypsin attacks proteins only at points with tryptophan, phenylalanine, or tyrosine.
The dipeptidases break down protein fragments consisting of two amino acids, and tripeptidases cleave three-amino-acid fragments.
The acinar cells also secrete multiple lipases and esterases, both fat-digesting enzymes with very specific target molecules.
And the family of pancreatic amylases, like those released with saliva in the mouth, break down starch into simpler sugar linkages.
The proteolytic enzymes, such as trypsin and chymotrypsin are very powerful molecules and will attack any protein- including the protein of the pancreas itself.
This problem has been neatly resolved in the acinar cells, which produce the various proteolytic enzymes in their inactive or precursor forms.
For example, the acinar cells manufacture trypsin initially as trypsinogen, which is completely inactive with no digestive ability whatsoever.
Trypsin in itself, like all enzymes, is a protein consisting of 255 amino acids arranged in a very complex three-dimensional pattern.
Trypsinogen is chemically just like trypsin, but with an additional six amino acids added to the end of the molecule like a tail.
These six amino acids make all the difference between an active and a totally inactive enzyme.
Chymotrypsin is initially produced in the acinar cells as chymotrypsinogen, and carboxypeptidase A and B are manufactured as procarboxypeptidase A and B- all inactive precursor forms of the enzymes.
The acinar cells store the inactive precursors in little sacs or vacuoles in their cytoplasm, until they are needed for digestion.
As long as these pre-enzymes remain in their inactive form, they pose no threat to the pancreas cells themselves.
The body has a remarkable mechanism for signaling the acinar cells of the pancreas to begin manufacturing, as well as secreting, stored precursor enzymes.
First, the presence of food in the mouth and in the stomach stimulates the vagus nerve to release its neurotransmitter, acetylcholine.
All the pancreatic acinar cells have on their membranes receptors for this parasympathetic neurotransmitter.
When the acetylcholine from the vagus nerve attaches to these receptors, the acinar cells begin releasing the stored pre-enzymes into the common space of the cul-de-sac mentioned earlier.
In addition, when the digestive products of the stomach make their way into the duodenum, the presence of food stimulates the cell lining there to secrete the hormone cholecystokinin (CCK) into the bloodstream.
This hormone, like the vagus nerve, stimulates the acinar cells to produce and release stored enzymes.
After the cells secrete the precursor molecules, these pre-enzymes make their way down the small ducts or enzyme roadways, until they reach the main pancreatic duct (duct of Wirsung), which traverses the length of the pancreas and then joins the common bile duct to empty into the duodenum, at the hepatopancreatic duct (ampulla of Vater).
When these pre-enzymes, such as trypsinogen and chymotrypsinogen, first make their way into the small intestine, they are still in their inactive form, thus useless for digestion.
But the body has a very efficient process for activating the digestive enzymes.
The cells lining the small intestine produce a proteolytic enzyme called enteropeptidase, whose function is specifically to cleave off the six-amino-acid tail of trypsinogen, leaving the very active and very powerful trypsin.
Trypsin itself can then activate other trypsinogen molecules, as well as chymotrypsinogen and the other proteolytic enzymes.
Scientists describe this process as a cascade, which speeds up as more trypsin molecules are produced.
But there’s an added complication.
These semiliquid food boluses that make their way from the stomach into the small intestine are extremely acidic from all the hydrochloric acid secreted by the stomach earlier in order to start the digestive process.
Pancreatic enzymes, such as trypsin and chymotrypsin can perform their digestive function only in an alkaline environment.
In the presence of acid, even the active forms of the enzymes do nothing.
To solve this problem, when the lining cells of the small intestine sense the acid load of the incoming food, they release yet another hormone, secretin, into the bloodstream.
Secretin stimulates the cells lining the ducts of the pancreas to produce copious amounts of bicarbonate-rich water that quickly empties along with the enzymes into the duodenum through the main pancreatic duct (duct of Wirsung).
Bicarbonate is a powerful antacid, known commonly as baking soda.
The bicarbonate very quickly neutralizes the acid products coming into the duodenum from the stomach.
Now, the activated enzymes have the ideal environment in which to begin their digestive work.
If the major pancreatic ducts become blocked- by a tumor, a gallstone, or scarring from drugs or infection, the pancreas and its owner are in serious trouble.
In response to the usual stimuli- such as the sight or smell of food, the acinar glands will continue to secrete the inactive enzyme precursors into the smaller pancreatic ducts.
At the same time, the lining cells across the main pancreatic duct and other ducts will continue to secrete bicarbonate.
In case that the ducts obstruct, the enzymes will sit there unable to move, in an ideal alkaline environment optimal for their conversion to their active, protein-digesting form.
With time, trypsinogen will spontaneously convert into trypsin, and the activation cascade will proceed very rapidly.
The enzymes will then begin to attack the pancreas tissue itself, producing pancreatitis, which can be a life-threatening emergency.
In addition, as the enzymes back up, the small blood vessels that circulate through the organ/gland will begin absorbing the overflow of enzymes.
This is why physicians routinely monitor blood levels of the two major pancreatic enzymes- lipase and amylase, in order to assess the progress with regards to pancreatitis .
Pancreatic Enzyme Products
Pancreatic enzyme supplements are usually obtained from the pancreas of pigs or cows.
The pancreas is the primary digestive organ in both animals and humans, that secretes and produces specific proteins, called enzymes – amylase, lipase, and protease – that are needed for proper digestion.
Pancreatic enzyme supplements don’t require a prescription to be obtained.
Since they are classified as dietary supplements and not pharmaceuticals, the FDA does not regulate their distribution.
Prescription pancreatic enzyme products are most commonly used to treat digestion problems that result when the pancreas has been removed or is malfunctioning due to disease.
Cystic fibrosis or long-term swelling of the pancreas (i.e., due to chronic pancreatitis) are two of the conditions that can cause the pancreas to function poorly.
While pancreatic enzyme supplements are also available, official health authorities do not recommend their use for this or any other medical condition.
That of course doesn’t mean that pancreatic supplements are not potent or effective.
Pancreatic supplements contain bovine / porcine / ovine pancreatic tissue, and thus the naturally occurring pancreatic enzymes- amylases, proteases, trypsins and lipases – in the correct physiological amounts that exist in nature.
They do not comprise a processed, standardized USP extract, but a wholefood dietary supplement.
Consuming pancreas, whether through food or as a supplement, may strengthen and support your own pancreas.
A pancreatic glandular is a wholefood extract that provides all the nutrients, co-factors, bioactive compounds, and enzymes that may support your own pancreatic health.
High-quality, ethically-sourced glandular extracts, including pancreas, can be found by Ancestral Supplements.
Since the pancreas is an endo- / exocrine gland, pancreatic supplements are considered glandular extracts.
Glandular extracts are extracts originating from the endocrine glands of cows, pigs, lamb or sheep.
They may contain lyophilized raw tissue from various organs, like brain, adrenals, liver, thyroid, thymus, pancreas, testicles, etc.
All glandular extracts have organ-specific therapeutic benefits and are used in glandular therapy along with other animal organ components.
According to the concept of glandular therapy, all endocrine organs/glands contain vitamins, minerals, enzymes, peptides, nucleotides, and other nutrients specific to that organ/gland.
Their consumption supports or promotes the healthy functioning of that particular organ or gland.
Put simply, glandular therapy is based on the concept of “homostimulation” or “like supports like.”
For example, an animal that consumes a piece of liver is taking in nutrients that closely resemble its own liver, providing its body with corresponding building blocks for liver repair and regeneration.
A damaged liver needs a specific and complete combination of amino acids, nucleic acids, and other materials to rebuild functional liver cells, which can be found in another animal’s liver.
According to glandular therapy, the most complete, bioavailable source of raw materials for liver repair and regeneration would be healthy liver cells.
Similarly, beef or lamb brain, which comprises a rich source of fats (phospholipids, sphingolipids, Omega-3s, and other fatty acids), can support the repair and maintenance of a healthy brain.
Another fine example of the “like heals like” principle is bovine/porcine trachea and cartilage, which contain glycosaminoglycans and other vital compounds (i.e., hyaluronic acid, chondroitin sulfate), that have been scientifically proven to promote joint health.
Nevertheless, the concept of glandular therapy is not exactly new.
Since the early 1900s, scientists have been actively trying to break foods apart, looking for the “active principles” responsible for their health benefits.
However, once isolated, a compound could be synthetically manufactured and concentrated at a lower cost.
In this way, commercial incentives led to the isolation of many “active principles”, including thyroxine (1926), estrogen (1941) and cortisone (1936).
Predictably, the practice of glandular therapy rapidly lost favor, and by the mid-1940s, glandular therapy had largely disappeared from use in medical practice.
Additionally, from 1940 to the 1980s, little was done to advance the therapeutic use or clinical research of animal glandular tissue extracts.
With the exception of whole thyroid extracts (Armour Thyroid and others), most other glandular therapy products disappeared from the market.
Additionally, double-blinded, cross-over studies on glandular therapy have not been performed yet, and data supporting the therapeutic benefits of glandular extracts are primarily based on historical and anecdotal use.
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About George Kelly
George Kelly M.Sc. is a Registered Dietitian Nutritionist that specializes in chronic and autoimmune conditions. He is the CEO and lead author of Metabolic Body.
This content is for informational and educational purposes only. It is not intended to provide medical advice or to take the place of such advice or treatment from a personal physician. All readers/viewers of this content are advised to consult their doctors or qualified health professionals regarding specific health questions. Neither Metabolic Body nor the publisher of this content takes responsibility for possible health consequences of any person or persons reading or following the information in this educational content. All viewers of this content, especially those taking prescription or over-the-counter medications, should consult their physicians before beginning any nutrition, supplement or lifestyle program.