Essential functions of the large intestine
- Absorption of water and minerals
- Excretion of the non-utilizable food residues
- Energy production by degradation and absorption of energy-rich – not in the small intestine usable – food components, such as fiber and resistant starch, with the help of anaerobic intestinal bacteria.
Bacterial colonization of the colon
Bacterial colonization of the colon increases from the duodenum and jejunum to the ileum. Thus, the duodenum has the lowest bacterial count. The colon is the intestinal segment with the highest bacterial colonization density. Since the diversity of microorganisms increases with the increase in the number of germs, up to 400 different species of bacteria as well as fungi are found in the colon. The colon flora consists of 99% anaerobic bacteria – mainly Bacteroides, Eubacterium and Bifidobacterium – which do not require free oxygen to live. Their task is to break down and absorb energy-rich food components that cannot be broken down by the digestive secretions in the small intestine. Mainly complex carbohydrates – non-starch polysaccharides, which belong to dietary fiber, as well as starch -, endogenous carbohydrates – cervical secretions, tissue – and proteins or peptides – food components, pancreatic enzymes – are fermented in the colon. In addition to the gaseous substances carbon dioxide, methane and hydrogen, the end products of the bacterial degradation of carbohydrates and proteins include short-chain, low-molecular fatty acids – predominantly acetate, propionate and butyrate. The resulting lowering of the pH value particularly prevents the colonization of pathogenic germs [2.1]. On the other hand, a high pH value inside the intestine – in the case of butyrate deficiency – promotes the conversion of primary to secondary bile acids. High concentrations of secondary bile acids, in turn, increase the risk for the development of malignant tumors – carcinomas – in the colon, as they support mechanisms that promote the development of cancer cells. Butyrate thus has an antineoplastic effect, so that when sufficient butyrate is available, the development and proliferation of colon carcinomas is inhibited. By stimulating immunological defense mechanisms in the intestinal wall, as well as synthesizing bactericidal substances that inhibit the growth and proliferation of pathogenic bacteria, colonic microorganisms prevent the uptake of antigens and thus bacterial overgrowth in the colon
Importance of short-chain fatty acids
In contrast to the unreactive gaseous cleavage products, short-chain fatty acids, as essential organic acid anions, are essential for the functioning of the colonic mucosa. Short-chain fatty acids play an essential role in the metabolism of the colonic mucosa, serving as energy-providing substrates for the microflora of the colonic mucosa. Butyrate is the most important energy supplier of the mucosal cells. Together with propionate, butyrate stimulates physiological new cell formation in the mucosal invaginations (crypts) of the colon and maintains the activity of bacterial enzymes and thus the functional processes in the colon. Acetate promotes blood flow to the mucosa of the colon by supporting the relaxation or slackening of the finest branches of the arteries (arterioles). Consequently, the short-chain fatty acids counteract a reduction or shrinkage of the cells of the colonic mucosa (mucosal atrophy). Furthermore, low molecular weight fatty acids promote the absorption of sodium chloride and water in the colon. As a result of the coupled fatty acid, sodium chloride and water reabsorption, dissolved substances – osmotically active molecules, such as dissolved salts and glucose – are increasingly removed from the interior of the intestine. In this way, the tendency to diarrhea is significantly reduced. A high intake of carbohydrates as well as dietary fiber ensures a high content of short-chain fatty acids in the colon. Only if acetate, propionate and butyrate are available in sufficient quantities, an optimal activity of the colonic mucosa cells can be guaranteed. A fully functional energy metabolism of the cells is in turn a condition for maintaining the barrier function of the intestine. This barrier function (mucosal block) of the intestinal mucosa protects against the passage of bacteria and antibody-producing bacterial toxins from the interior of the intestine into the bloodstream and lymphatic system.In addition, the mucosal block prevents the uptake of bacteria and germs – antigens – into the intestine.
Importance of the amino acid glutamine
Another prerequisite for maintaining the barrier function of the colonic mucosa is an adequate supply of glutamine to the mucosal cells This substrate, which is ingested with food or formed in the organism, is – like the short-chain fatty acids – a decisive source of energy in the mucosa cells. The amino acid is thus of considerable importance for the energy metabolism of the cells of the small and large intestinal mucosa. Glutamine can also counteract damage to the inner wall of the digestive tract, such as ulcers or inflammation.
