The small intestine consists of three successive sections – the duodenum (duodenum), the jejunum (jejunum), and the ileum (ileum). The duodenum is immediately adjacent to the stomach. At the transition of the colon into the large intestine, there is a valve that prevents the backflow of colon contents into the small intestine. The main function of the small intestine is the absorption of fluid, electrolytes, and nutrients and vital substances (macro- and micronutrients). Kerckring’s folds and intestinal villi, which increase the mucosal surface area by a factor of about 600, create a large absorbable surface area. This provides a resorption surface of approximately 120 square meters. In addition to absorption, the small intestine is also responsible for digestion. Inside the intestine, the absorbed food is usually in the form of complex molecular structures. For example, the disacharide molecules consisting of two monosaccharides cannot be absorbed or are insufficiently absorbed. Sufficient absorption of the two-saccharides occurs only through cleavage by specific disacharidases. Accordingly, disaccharides and peptides are cleaved to monosaccharides and amino acids, respectively, with the help of enzymes of the intestinal mucosa cells. In addition to the disaccharidases that cleave the disaccharides, which have the greatest practical-clinical significance of the enzymes, peptidases and lipases are found in the small intestinal mucosa, which are involved in the final cleavage of the corresponding nutrients and vital substances (macro- and micronutrients). The cleaved molecules are transported away with the portal blood and reabsorbed, for example, by proteins. Depending on their extent, diseases of the small intestine or surgical removal of sections of the small intestine are associated with greater or lesser impairment of nutrient utilization and absorption. Only in a healthy and optimally functioning intestine can large-molecule nutrients and vital substances (macro- and micronutrients) be broken down to absorbable cleavage products under the influence of bile and pancreatic juice [4.1. ].Since the duodenum and jejunum in particular have the highest villous density, absorption takes place primarily in these intestinal segments. However, the more centrally located (distal) sections of the small intestine also have a considerable absorption capacity, which becomes important, for example, in short bowel syndrome. Furthermore, the mucous membrane of the small intestine has the ability to prevent the uptake of bacteria, germs and antibody-producing bacterial toxins or the transfer of antigens from the intestines into the lymph and portal blood – barrier function of the intestine. The lymphoid tissue, IgA secreted by the mucosa, and the mechanical integrity of the epithelial layer with its kite ridges are involved in this important function.
Importance of the amino acid glutamine
An essential prerequisite for maintaining the barrier function of the small intestinal mucosa is an adequate supply of glutamine to the small intestinal mucosal cells. This substrate, which is ingested with food or formed in the organism, is a crucial source of energy in the cells and thus of considerable importance for the energy metabolism of the cells of the small intestinal as well as large intestinal mucosa. Glutamine can also counteract damage to the inner wall of the digestive tract, such as gastric ulcers or inflammation. In a state of starvation – exclusive parenteral nutrition or exclusive nutrition with chemically defined formula diets – or with increased consumption – burns, polytrauma, after operations – there is often a deficiency of glutamine in tissue and plasma. As a result, the barrier function of the mucous membrane of the small intestine is reduced due to the decreased cell activity, whereby more pathogenic bacteria and germs are absorbed into the intestine or pass from the inside of the intestine into the blood and lymphatic system and enter the peripheral circulation [4.2]. During periods of stress and starvation, it is therefore important to ensure a sufficient supply of glutamine in the diet in order to protect the intestinal mucosa from damage caused by antigens and from infections. Only optimal function of the intestinal mucosa – normal cell growth, normal immunological defense – protects against unphysiological bacterial overgrowth and the transfer of bacteria and endotoxins from the interior of the intestine into the lymph and portal blood [4.2].
Importance of the ileum
Vitamin B12 and bile salts are absorbed exclusively in the lower part of the small intestine (ileum). After surgical removal of the ileum or in the case of extensive intestinal wall damage, the intestinal-liver – enterohepatic circulation, which is essential for the regulation of vitamin B12 as well as bile acid balance, is interrupted. As a result, vitamin B12 and bile acids can no longer be reabsorbed by the ileum and thus cannot be absorbed into the bloodstream. Reabsorption of the bile acids – again via the liver into the bile, then into the intestine – does not occur The disturbed enterohepatic circulation eventually leads to reabsorption problems, resulting in vitamin B12 deficiency or bile acid loss syndrome – bile acids enter the colon and are excreted in the stool – with chologenic diarrhea. Resection of the segment of the small intestine located toward the trunk (proximal) is less dramatic, because in this case the distal small intestine takes over all absorption [4.1].
Importance of growth factors
The development of so-called growth factors – grow factors – is considered to be of special importance. Their main task is to improve nutrient and vital substance absorption (macro- and micronutrients) – especially after small bowel resection – by stimulating cell growth in the intestinal mucosa. In return, they ensure an increase in mucosal density and a certain growth in length of the remaining intestine. Growth molecules include epidermal growth factor, neurotensin, and insulin-like-growth factor.
