Introduction to Digestion

Alpha Nutrition Health Education

Stephen J. Gislason MD

Digestion is achieved by the Gastrointestinal Tract (GIT). Complex food structures are reduced to simpler, smaller molecules - nutrients. These nutrients are absorbed into the blood stream, both directly and indirectly via lymphatic circulation. Other names for GIT include "digestive tract, stomach, small and large intestines, bowel, colon, and gut".  

The gastrointestinal tract, GIT, is a food processor. This long muscular tube extracts all the molecular material required for the structure and function of human bodies. The GIT is the portal of entry into the body and must be selective about what materials it lets in. We can understand disease causing consequences of food ingestion by closely examining how the GIT works.   GIT makes numerous mistakes in food processing and absorption and has a very active immune function that can produce a wide range of symptoms and food allergic diseases.

Each person's food supply must be matched to the capacity of their GIT to process food. Dysfunction and disease arise when there is a mismatch. The GIT is not a passive processor of food, but, rather, it actively manages and responds to food. When all is well, we ignore the presence of  our GIT. Our awareness is limited to full or empty sensations. When there is trouble in the GIT, our attention is directed to our abdomen by a variety of discomforts, pain, noise, distention, and abnormal bowel movements.

Inflammation of various parts of the GIT is described by terms using the suffix 'itis' attached to the name of the GIT part; thus we have descriptive diagnostic names such as gastritis (stomach), colitis (colon), ileitis (ileum or small intestine), hepatitis (liver), cholecystitis (gallbladder).

Sensing and Reacting

The GIT's surface is sensitive, knowledgeable, and reactive. The sensing, deciding, and acting functions of the GIT are achieved by a complex system of nerves and cells of the immune-defense network, which extends along the surface. The velvety surface of GIT is described as mucosa. A continuous sensing apparatus in the mucosa is nicknamed MALT, short for its formal title, Mucosa Associated Lymphatic Tissue.

Immune sensors in MALT trigger immune responses causing symptoms such as nausea, vomiting, burning, pain, itching and swelling at the anal exit. The GIT knows when the wrong stuff has entered its space, and reacts defensively to get rid of it. Vomiting and diarrhea interrupt normal living in a distressing display of GIT's reactivity. Food allergy and other forms of food intolerance such as lactose (milk sugar) intolerance are capable of producing a wide range of digestive problems

The GIT lymphatic system is a fluid transport system that allows cells and fat molecules to flow through lymphatic filters (lymph nodes) scattered around GIT. Eventually lymph fluid returns to the blood via the large vein (superior vena cava) which returns to the heart. All food molecules are absorbed by GIT, mostly from the small intestine (duodenum, jejunum, ileum).

Digestion of food is accomplished by secretions from  the GIT surface and accessory glands. The liver and pancreas are the major secretory organs which contribute digestive factors. After food molecules are absorbed, they are carried in the venous blood (portal veins) from the GIT to the liver. The liver is a complex chemical factory whose function is to control the food supply for the rest of the body. The liver's main task is to take in newly absorbed food molecules for further chemical processing, storage, and slow release. Food molecules leaving the liver are distributed throughout the body in the bloodstream. The liver also acts as immune-filter, removing some of the "wrong-stuff" admitted by mistake through the GIT wall. If the efficiency of the liver filter is reduced, food allergy symptoms are increased.

When all is well we hardly notice the presence of  the GIT - the abdomen is quiet and awareness is limited to full or empty sensations. When there is trouble in the GIT, attention is directed to the abdomen by a variety of discomforts, pain, and noise.

Basic Ideas

Since the GIT is a food processor, we assume that anything that goes wrong with it is usually caused by the material passing through it - at least, until proven otherwise. When the system malfunctions, GIT input is altered in an intelligent manner to reduce or eliminate GIT dysfunction. Dysfunction in the rest of the body is often a result of GIT dysfunction (increased wrong input to the whole system), so that all downstream diseases require a similar close scrutiny of body input and GIT competence.

The GIT is not a passive processor of food, but rather it actively manages and responds to food with behavior for its own. A variety of sensors assess the composition of the incoming food and send local hormone messages to regulate GIT behavior accordingly. GIT is wired to the brain both by extensive autonomic efferents and afferents and also by hormonal, neurotransmitter-mediator chemical communication.

GIT Behavior

The GIT is muscular and contracts rhythmically to conduct food through its length (peristalsis). The timing, strength, and coordination of muscular contraction is influenced by the composition of food, and by state specific body-brain conditions. If the contractions are too vigorous, crampy pain is experienced. If the contractions are too frequent, diarrhea results. Constipation occurs when contractions are sluggish and slow, or if sustained contractions act to obstruct the tube.

Disorders of peristalsis are referred to as "Motility Disorders." Medical diagnoses sometimes refer to motility disorders as if they were the cause of symptoms. With a few exceptions, motility problems are not causes but they are effects of food-GIT interactions. The GIT surface is not deaf and dumb, but rather is sensitive, knowledgeable and reactive. The GIT knows when the wrong stuff has entered its space, and reacts defensively to get rid of it. Vomiting and diarrhea are reactive, defensive responses. Food allergy in the GIT is any discomfort, dysfunction and disease mediated by local immune responses to food. In our view, food allergy is a common cause of GIT dysfunction and disease.