The whole subject of food residue is a complicated one because of the variety of indigestible compounds and the different ways in which they influence the digestive and absorption processes in the body. In the simplest terms, dietary fibre acts in the digestive tract by binding or absorbing water and other substances, thus causing 'bulking' of the contents of the gut. It influences the speed with which food moves through the various parts of the digestive tract. However, the ways in which fibre affects digestion, absorption and the movement of food in the digestive tract are diverse, far-reaching and not always well understood. To illustrate this, fibre in the diet may affect blood sugar levels, the production of bile, the transport of cholesterol in the body and your chances of developing colon cancer. It can even affect the hormonal responses to a meal.
A variety of factors influence the effect of a particular fibre or high- fibre food. The chemical composition of the fibre, its particle size, the age of the plant source and methods used in the processing of the food, all affect the physical properties of the fibre. It is these characteristics which account for the different functions of fibre in addition to its main effect of causing increased bulk formation in the digestive tract.
Fibre effects are also influenced by the total composition of the meal and the amount of fibre in it. Short-term effects - for example on the blood sugar level - can be seen immediately after a person has eaten a single high-fibre meal. Long-term effects, such as lowering of blood cholesterol levels, occur only after a regular and prolonged intake of fibre. The effect of fibre in the different parts of the digestive tract (stomach, small and large intestine) also differs and may vary between individuals.
Fibre and the diseases of affluence
As already mentioned, research on dietary fibre was initially stimulated by studies which showed that population groups who follow high-fibre diets suffer far less from the 'diseases of affluence' - the medical term used to describe degenerative diseases and disorders which have been linked to a prosperous Western lifestyle. Among these are colon cancer, appendicitis, gall stones, varicose veins and constipation. Their causes are many; what has been established is that a variety of environmental factors, including dietary habits, can trigger these diseases in people who are genetically 'vulnerable' to them. This explains why they tend to run in families and why some people, even though they follow a Western lifestyle, do not develop a particular disease.
Let's look at those diseases which are known to be associated with the amount of fibre in our daily diets:
Constipation
Constipation is probably one of the most common disorders in the world and is also the cause of other diseases (see diverticular disease and colon cancer below.) It can be defined as the relatively slow movement of unduly firm waste matter through the large bowel, which results in the infrequent passing of small, hard and dry stools - often accompanied by straining.
Including enough fibre in your diet prevents constipation in various ways. Firstly, fibre is not digested in the small intestine. It therefore moves to the large bowel where it has a bulking effect, increasing the volume of the bowel content. Because it binds water, fibre prevents the absorption of water from the large bowel to some extent, ensuring that the contents remain large in volume and soft in consistency. Fibre also stimulates the growth of microbes in the large bowel, which contributes to larger, softer stools. The larger volume in the bowel stimulates peristaltic movements and eases the process of defecation so that straining is not necessary. In addition, during the partial fermentation of fibre in the large bowel by microbial enzymes, substances such as methane and hydrogen gasses, water and the short-chain fatty acids are produced. There is now convincing evidence that these acids play an important role in maintaining a healthy bowel.
Diverticular disease
It is now recognised that constipation - and the resultant increase in pressure in the colon (large bowel) because of strained bowel movements - is the underlying cause of diverticular disease. In this disorder, small, blown-out pouches or sac-like swellings (diverticula) form in the wall of the colon and project into the abdominal cavity. Inflammation of these pouches is known as diverticulitis and may cause abdominal discomfort and pain.
Diverticular disease is very common in affluent societies, especially among elderly people. The treatment of constipation with high-fibre diets will not only help prevent the development of the disease, but will also improve - and may even 'cure' - an existing condition.
Colon cancer
Although the role of diet in the development of this disease is still unclear, some scientists believe that high energy and fat and low fibre intakes are important contributing factors (see Cancer - can Your Diet Make the Difference?, page 181). The way in which fibre may protect against colon cancer is probably very complex. It is thought that the rapid movement of a bigger volume through the large bowel shortens the bowel wall's exposure time to substances which may induce cancer (carcinogens).
Because of its bulking and water-binding effects, fibre dilutes the concentration of carcinogens in the gut. It may even bind these substances and help with their excretion in the faeces. New research has showed that butyric acid, one of the short-chain fatty acids produced from fibre fermentation in the large bowel, also protects the bowel against the development of cancer. In addition, changes in colonic pH as a result of the fermentation process may be of importance.
