If you thought the gut was all about digestion then you might be surprised to read that it plays an important role in the immune system. Furthermore, the intestinal microflora is fundamental to the operations of both systems.
There are many hundreds different bacterial species, which are predominately situated in the colon. The gastrointestinal tract is sterile at birth, but rapidly develops an intestinal microflora that varies according to factors such as mode of birth, infant nutrition, antibiotic use, diet and age.
Microflora and digestion
The primary function of the gut is uptake of water and nutrients. The specific role of the resident colonic microflora in digestion is to ferment substances provided in the diet (e.g. dietary fibre), which cannot be digested by the host in the small intestine. This fermentation produces among other molecules lactic acid and short chain fatty acids (acetic, propionic and butyric). The latter provide energy to the cell wall lining of the colon, improve the absorption of minerals and beneficially influence lipid and glucose metabolism in the liver.
Microflora and immune function
There are three main routes through which the gut acts as a defence system. The first is via the resident microflora, which protects against invading bacteria. Proposed mechanisms include competing for nutrients and for receptor sites on the gut wall and generation of an adverse environment for pathogens (e.g. low pH). Secondly, the intestinal wall cells not only absorb nutrients, but also provide a protective barrier to the entry of harmful substances. Thirdly, the gut immune system is made up of specialised immune cells. These immune cells can react through their own innate response whilst also triggering production of antibodies, proteins that specifically bind to another proteins called antigens -in this case the invading pathogen- to deactivate it and remove it from the body. Our gut bacteria communicate with cells of the gastrointestinal immune system and the liver to co-ordinate an immune response to food antigens and harmful micro-organisms.
Thus, the intestinal microflora is essential for the protection mechanism to work optimally. In fact, not having the correct balance of bacteria in the gut has been associated with a number of conditions including irritable bowel syndrome, inflammatory bowel disease, colon cancer and gastroenteritis. Changes to diet and eating patterns, and the use of antibiotics can have a harmful effect on the balance of the gut microflora. These can combine to shift the balance of the gut microflora away from potentially beneficial or health-promoting bacteria such as the lactobacilli and bifidobacteria, towards an increase in the harmful or pathogenic micro-organisms (e.g: certain clostridia and enteroccocci).
A number of food products have been developed that can modify the intestinal microflora and possibly benefit health. These contain probiotics, prebiotics and synbiotics (a combination of probiotics and prebiotics). Many clinical studies have shown promising effects.
Probiotics (e.g. specific lactic acid bacteria) are living micro-organisms which upon ingestion in sufficient quantities, exert health benefits on the host beyond basic nutrition. They are most commonly found in milk fermented products.
Prebiotics are non-digestible food components belonging to the fibre family that stimulate the growth or activity a number of micro-organisms of the intestinal flora (i.e. Bifidobacterium species and/or lactobacilli). They are found naturally in some foods (e.g: onions, artichokes, bananas, chicory, leeks) or can be added in other foods (e.g: breads, biscuits).
The full extent of the importance of the colonic microflora to health and well being needs further investigation. However, research indicates an influence on constipation, diarrhoea, the immune system, cancer and the absorption of minerals. Thus, the approach of improving the gut microflora for better health is very relevant for today’s population.
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- Teitelbaum JE and Walker WA (2002) Nutritional impact of pre- and probiotics as protective gastrointestinal organisms. Annual Review of Nutrition. 22, 107-38.