Page 33 - To keep a balance in disease specific intestinal insufficiency. Diagnostics and practical nutritional aspects - Nicolette Wierdsma

Diagnostic tests for malabsorption and maldigestion
digestive juices, motility and other pro-secretory functions. Hormonal regulation
usually takes place via feedback mechanisms. Additionally, the so-called gut-
brain axis refers to the biochemical signaling taking place between the GI tract
and the nervous system. This interaction of the gut with the brain is essential
for optimal functioning of the GI tract. It comprises symphatic, parasymphatic,
and other neuro-intestinal networks. Moreover, biologically active peptides are
likely to emerge as neural and endocrine messengers in order to orchestrate the 3 microbiota-gut-brain axis in health and in disease.
Digestion reflects enzymatic conversion of complex dietary substances to an absorbable form, which is initiated by sight, smell or taste of food. In humans, this includes two processes; a mechanical breakdown (oral mastication, chewing, grinding and stomach churning, mixing) and chemical breakdown (action of digestive enzymes, bile acids). The digestive process starts with the production of saliva in the mouth (for complex carbohydrates). It continues in the stomach for proteins by producing hydrochloric acid, which is the catalyst for the conversion of pro-enzym pepsinogen into the active peptide pepsin. Nonetheless, it is primarily the (proximal) intestine that releases digestive enzymes, produced in intestinal organs and the brush border, into the lumen. Absorption comprises the transportation of fluids and (predigested) nutrients across the intestinal mucosa into the body tissues by active transportation, passive or facilitated diffusion, or phagocytosis. It mainly occurs in the villi of the jejunum and ileum.
(Gastrointestinal) Motility, refers to the movement of food from the mouth through the GI tract and per anally out of the body. This coordinated intestinal transport is critically required for a normal functioning GI tract.
Biological assimilation, or bio-assimilation, is the combination of two processes to supply cells with nutrients after digestion and absorption. The first is the process of absorbing vitamins, minerals and other chemicals from food within the GI tract. The second process of bio-assimilation, is the chemical alteration of substances in the bloodstream by liver cells (hepatocytes) or by cellular secretions. Assimilation occurs when the food molecules become part of the body tissues (cells) and therefore it follows the absorption process.
The intestinal barrier function is, next to its function as (semi-)selective barrier in the process of assimilation, critical in immune defense since the gut forms an immunological barrier to protect the body against potentially harmful, antigenic luminal compounds. The GI tract has a single contiguous layer of
31