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290 Orchard Road #06-06, Paragon Shopping Centre, Singapore 238859

Surgery is getting more advanced and “high technology” is in common everyday usage, both in speech and in practice. Surgical science is not just about how to do surgery but also about the molecular basis for surgery as well as basic physiology, biochemistry and of course anatomy. The colorectal surgeon of course deals mainly with surgical diseases of the lower gastro-intestinal tract. In recent years, there have been great advancements made in every aspect of the science and medicine of the lower gastro-intestinal tract. With all these advances there often arise situations where surgeons realize that realities in everyday practice do not tie in with conventional 'wisdom". Back to basics is still a good clarion call. Therefore whilst not wanting to make a mountain out of a mole hill, we will not do well if we can’t see the wood for the trees either. One very important understanding is the relationship between eating and defaecation.

The purpose of eating

One of the real hazards of living in the modern world is being constantly bombarded by information of all levels of trustworthiness. How and what to eat is something that we are being constantly manipulated by the media all around us. One problem with this sort of mass information dissemination is that many doctors are also swayed to believe them!

One example of this is "you must eat fibre to defaecate!" Before we can understand why this is a problem, we have to understand the purpose of eating.

Just why do we eat? There are four main reasons why humans eat. Firstly, we eat to get nutrition. Nutrition includes everything we need to grow and stay alive eg., proteins, fats, carbohydrate, minerals, vitamins. Secondly, we must eat to obtain our energy requirements for our daily work,play, reproduction, etc. Energy sources include carbohydrates, fats, etc. Thirdly we eat to satisfy our taste buds so that often times we something not because of nutritional needs or energy requirements but because we like the taste. Taste is individual and cultural. One man’s meat is another man’s poison. Lastly we also eat for social or community reasons. These sometimes include foods that taste bad to the eater but eaten because of the social environment. in-laws are not always good cooks! However, we should not be eating because we want to pass motion.

Passing motion is a side effect of and not the purpose of eating
  1. Illogical to pass more faeces
    Food to the human body is like petrol to the car. It allows the machinery to function. Why then do we have an anus? It is for the same reason that cars have exhaust pipes. The foods we take in cannot be completely digested and therefore cannot be completely absorbed. These substances rejected by the body have to be excreted. There are also dead bacteria, shed cells and secretions which have to be discharged. One does not put petrol into cars to increase exhaust pipe emission. Similarly it is not logical to eat foods just to increase anal emission. It is enough for the body to discharge whatever parts of the food eaten for the four reasons listed above that it cannot digest and does not want retained as waste.
  2. Fibre becomes faeces
    In any case, undigested and unwanted parts of the food which need to be evacuated will be a minor portion of food if indigestible fibre is not consumed in excess. However, excessive dietary fibre is the major cause of constipation, abdominal pain and bloatedness I see in my practice and yet it remains the most prescribed treatment for these symptoms! One of the most well documented effects of dietary fibre is bulky stool or increased stool production1. The additional stool mass that occurs with larger fiber intakes may either be unfermented plant material or bacteria. Stool moisture concentration is usually not affected by added fiber. 1 Indigestible fibre results in more faecal bacteria than other diets1. Highly digestible fibre does not stimulate microbial growth at least in young healthy volunteers1. The fact of the matter is that fibre has weight and volume. Fibre is also indigestible. Therefore consumption offibre leads to a heavier and bulkier volume of faecal matter forming in the rectum. Note the appearance of minimally digested vegetables and seeds in faeces after ingestion. The basis of the additional stool after eating more fibre is additional dietary fiber 2-4.
  3. Production of faeces and evacuation of faeces are two different processes.
    Many surgeons realize that eating fibre produces faeces. This is true but it is illogical when patients with constipation are asked to take more fibre. Confused? To understand this concept we have to take a step back. The major cause as well as effect of idiopathic constipation is the accumulation and difficulty in the evacuation of stool. The question is what is this stool? As we have said before, a large part would be undigested fibre. Now fibre bulks up stool. Highly indigestible fibre has a very low water holding capacity1. Therefore fibre causes hard bulky stool as implied by the words roughage and bulking agents. Hard bulky stools are more difficult to pass than soft thinner stools and hence the tendency to constipation results. Yes, fibre becomes faeces but in people who are prone to constipation, evacuation does not occur and therefore constipation results. The additional mass caused by high fiber diets only becomes prominent compared to the liquid in stools in the distal colon and is not helpful for distal propulsion of lumenal contents in the small intestine nor in the right colon where the volume of fluid overwhelms whatever dietary fibre might be present. Two things may happen when patients are fed more fibre. Firstly, many people develop abdominal discomfort, bloatedness and cramps and are then labeled as constipation associated irritable bowel diseases. Others by virtue of mass action of the larger bulky faeces manage to push the more distal bulky faeces out. This however becomes a vicious cycle of constantly needing large amounts of dietary fibre to evacuate. It is much easier to pass only the small thin amount of stool that a normal low fibre diet results in.
  4. Unfermented plant material dilutes but does not decrease the amount of faceal bacteria and does not decrease the absorption of noxious substances.
    Dietary fibre had been said to add bulk to the faecal stream which would then dilute colonic luminal contents, and therefore reducing the concentration of noxious materials2. Fibre in any part of the colon excepting the rectum however actually drifts within the fluid faeces and hence has no diluting effect on any bacteria or noxious substances. Whatever bacteria or noxious or for that matter nutritious substances would have long been absorbed by the small intestines and right colon. If fibre can prevent absorption of noxious substances then it will prevent absorption of nutrients as well! Of course this does not happen just as prevention of the absorption of noxious substances has never been shown to happen with fibre ingestion. Furthermore, even if dilution occurs, this is not equivalent to decreasing the absolute amount of noxious substances or bacteria. In the right colon, fluid faeces overwhelm the amount of fibre no matter how much is ingested and whatever can be absorbed would have been absorbed long before the solid sort of faeces which can actually dilute noxious substances reach the rectum.

Contrary to popular belief, fibre does not help ease constipation nor does it prevent the side effects of constipation. In fact it is one of the more important causes of constipation 5-6.

  1. Forsum E, Eriksson C, Goranzon H, Sohlstrom A. Composition of faeces from human subjects consuming diets based on conventional foods containing different kinds and amounts of dietary fibre. British Journal of Nutrition 1990: 64; 171-86.
  2. S Kurasawa, V S Haack, J A Marlett. Plant Residue and Bacteria as Bases for Increased Stool Weight Accompanying Consumption of Higher Dietary Fiber Diets. Journal of the American College of Nutrition, 2000: 19; 426-433.
  3. Stephen AM, Cummings JH: Mechanism of action of dietary fibre in the human colon. Nature 284: 283–284, 1980.
  4. Chen H-L, Haack VS, Janecky CW, Vollendorf NW, Marlett JA: Mechanisms by which wheat bran and oat bran increase stool weight in humans. Am J Clin Nutr 68: 711–719, 1998.
  5. Tan KY, Seow Choen F. Fiber and colorectal diseases: separating fact from fiction. World J Gastroenterology 2007;13: 4161-7
  6. Chuwa EWL, Seow Choen F. Dietary Fibre. Br J Surg 2006: 93: 3-4
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Seow-Choen Colorectal Centre Pte Ltd
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