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Basic concepts / Intestinal transit
Intestinal transit
Intestinal transit is affected by the intestinal flora. ]
The bacteria present in the colon transform carbohydrates that have not been digested in the small intestine into SCFA ( Short Chain Fatty Acids ) , which, then, are released into the colon. SCFA are beneficial for both the intestinal microflora (lowering of pH, causes the intestine to be more acidic) and the intestinal cells (which need SCFA for energy supply)(1). SCFA not only reduces colonic pH, but also increases the medium osmolarity. These two mechanisms, found essentially in the proximal colon, result in stool volume and weight increment, while reducing the stool consistency.
When intestinal transit is slowed, the intestinal flora imbalance enhances production of secondary amines, the toxic role of which is now well recognized(1).
If the transit along the small intestine is delayed, there is more time available for absorption of the contents in the small intestine, resulting in decreased chyme within the colon. This reduction in nutrient substrate results in a fall of bacterial mass. As result, decreased bacterial mass leads to lower production of SCFA, leading to harder stools and reduced stool weight.
Maintaining a regular intestinal transit is essential for health and general well being. Delayed intestinal transit is a source of discomfort for a large portion of the population in Western countries, notably affecting elderly people(1). The physical and psychological consequences of delayed transit may considerably diminish these subjects' life quality.
The food bolus passes through the small intestine allowing absorption of water, electrolytes, nutrients and micro-nutrients. In the duodenum, the splitting action of the digestive pancreatic enzymes contributes to extract monoglycerides and fatty acids from lipids, sugars from digestible carbohydrates, and amino acids from proteins. The gallbladder secrets bile, a fluid which provides emulsification to fatty acids, thereby helping to their absorption.
At the end of the ileum, any substance that has not been absorbed in the small intestine passes into the large intestine, and is then referred to as chyme. The colon represents the last stretch wherein the gastrointestinal tube may reduce the faecal volume. Via fermentation of substrates that have not been digested in the small intestine and effective re-absorption of water and electrolytes, the colon limits the loss of energy and water/electrolytes.
In a healthy adult, the transit time from mouth to anus takes up to 72 hours, and 80% of the total transit occurs in the colon.]
Transit time varies significantly between individuals, even if they have the same dietary habits. This variation may be explained to some extent by psychological factors. On the other hand, it is commonly accepted that transit time is longer in women than in men additionally; transit time is known to increase with age.
Normal transit time is between 24 and 72 hours according to the methodology used . Slow transit is not necessarily pathological in nature when it approaches the upper limit of the normal interval.
Constipation is considered as a pathology since it is characterized by a transit time longer than "physiological values" (~120h) and others clinical symptoms associated (i.e. difficulty at evacuation, sensation of anorectal blockage, etc.)
Intestinal transit is dependant on gastrointestinal tract motility, particularly that of the colon. Intestinal transit regulation mechanisms are the following(1):
· Quality of diet (volume of gastric distension, caloric load greater than 800 calories, and type of food).
· Nervous control of colonic motility ( central nervous system, autonomic peripheral nervous system and enteric nervous system).
· Hormones released following a meal, such as CCK (cholecystokinin r serotonin;
· Bacterial fermentation.
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