Understanding Digestion

The gastro-intestinal (GI) system extends from the brain to the rectum; it is one that is critical to health, but poorly appreciated. It communicates with the external environment, and actually has its own brain! The surface of the intestines, which is greater than that of a tennis court, is where most of the action occurs. This is where the body decides what to digest and absorb and what to excrete. It is where the outside world meets the inside of the body, so it is an area of battle between our body and other life forms that want to feed on it. The GI system contains the majority of immune cells in the body for good reason. As a result of its interaction with food, living organisms and toxins, the GI system is the major system where things go wrong. The main functions of the GI system are; digestion of fat, carbohydrate and protein; absorption of their constituent parts, excretion of waste and toxins and to act as a barrier to keep out toxins, poisons, bacteria, viruses and parasites.

Digestion has components that include neurological, chemical, mechanical and hormonal influences. The GI tract may be broken down into distinct areas, which are: mouth (sensing toxins (taste), chewing, salivary digestive enzymes, swallowing), esophagus (transport from mouth to stomach), stomach (production of stomach acid for digestion and pepsin- a digestive enzyme, food storage and mechanical breakdown), small intestine (digestion by mechanical means, pancreatic enzymes and bile), hepato-biliary (bile formation and secretion, neutralization of toxins) and the large intestine which absorbs water and eliminates waste. Let’s talk about each section in more detail.

The most important functions of the mouth are mastication and saliva. Chewing food increases the surface area on which mechanical and enzymatic actions increase digestibility of the food. Saliva consists of a mix of digestive enzymes, lubricants to facilitate swallowing and immune protective molecules. The main enzyme, amylase, digests complex sugars. It only works in the mouth between a pH of 7.0-7.4. Outside those ranges it becomes inactive. Once it reaches the stomach, normal acid production inhibits its action. You can check your mouth pH with litmus paper. Check just prior to consuming food or fluids. If it is consistently outside this range, you may already have suboptimal digestion. The mouth produces agents that protect against invaders, including: lactoferrin (an iron containing protein with bactericidal and fungicidal activity), lysozyme (an enzyme that destroys bacterial walls), hypothiocyanate (oxidizes bacterial walls), and Secretory IgA, the most prevalent immunoglobulin in the body.

The esophagus is protected by mucosal secretions. Its major job is transporting food. The major problem with the esophagus is erosion from Gastro-esophageal reflux.

The stomach is the main area of protein digestion, which must occur in an acidic environment. One of the truly amazing feats of the body, one that is little appreciated, is that, to go from the average body pH of about 7.2 to the harsh acidic environment of the stomach, pH 1-2.5 requires that the body concentrate hydrogen ions (acid) over 3,000,000 times that of the rest of the body! To create a liter of stomach acid requires 1,500 calories! Another article, Disorders of the Stomach, discusses low stomach acid, which, despite all the pharmaceutical ads, is much more common than too much.

Digestion continues in the small intestine. Food from the stomach stimulates the release of two hormones, secretin and cholecystokinin. At the same time, food, acid peptides and mucosal irritation of the upper duodenum due to acid, cause the stomach to stop producing the hormone gastrin, which shuts off acid production and delays stomach emptying. Secretin causes the pancreas to produce bicarbonate, (baking soda) which neutralizes the acid and activates pancreatic digestive enzymes. At the same time Secretin causes release of pancreatic digestive enzymes, trypsin, chymotrypsin and amylase, which must all work in an alkaline environment or they are not active. Cholecystokinin release is stimulated by the presence of fat. It causes the release of the fat digesting enzymes lipase, cholesterol esterase and phospholipase. Additionally it causes the gall bladder to secrete bile. In a typical day the liver produces a whopping 600-1200 ml of bile, which is stored in the gall bladder for future use. Bile acids functions include: a “detergent” action that decreases the surface tension of fat particles thereby emulsifying large fat particles (Just like soap does); breaking them down into smaller size so digestive enzymes can work. Bile also aids in transportation and absorption of fatty acid “micelles” across the intestinal wall into the body. Inadequate production of bile leads to significant loss of fat in the stool. If you have greasy stools that float, you have serious problems with your liver and gall bladder. Bile also facilitates excretion of Bilirubin (end product of red blood cell destruction), excess cholesterol and fat soluble toxins, while assisting in the absorption of fat soluble vitamins.

The intestines normally move food through the system in one direction in a process called peristalsis. This is under control of the enteric (GI) nervous system, which runs mostly independently from the brain. There are more nerve cells (about 100,000,000) in the enteric nervous system than in the rest of the peripheral nervous system combined. It is connected to the brain only by about 2,000 nerve cells of the vagus nerve. Most of the communication is one-way going from the enteric nervous system to the brain. Cutting the vagus nerve has little effect on the function of the enteric nervous system. Interestingly, the primary neurotransmitter, serotonin is also a neurotransmitter found in the brain. Most of the serotonin in the body is located in the enteric nervous system. Anti-depressants, which work on affecting serotonin levels, can have a significant effect on GI function.

Peristalsis is controlled by the enteric nervous system. Dietary fiber plays an important part in stabilizing GI motility. Fiber consists of carbohydrate that is indigestible by humans, but not by bacteria. Fiber provides bulk to the stool. Its presence shortens transit time through the intestines. It binds bile acids and toxins to aid in their excretion. Most conditions that affect the GI system also affect motility, either increasing (diarrhea) or decreasing (constipation).