What is dietary fibre?

Over the years, many terms have been used to describe the indigestible residue that remains in the digestive tract after all the available nutrients have been absorbed. In fact, when research on the subject began in earnest, one of the first problems the researchers faced was agreement on terminology and definition. It was decided that the terms 'crude fibre', 'roughage' and 'fibre' were inappropriate because some components of dietary fibre are water soluble (see below) and not fibrous or 'rough' at all. The term 'dietary fibre' is however firmly established in the history of nutrition and in the public mind and it was agreed that this should continue to be used.

Hugh Trowel and Denis Burkitt, two British doctors working in East Africa and pioneers in dietary fibre research, recognised early on that fibre comprises a group of substances, all either polysaccharides or lignin. Polysaccharides consist of a multiple grouping of glucose units (as in starch) which are however impossible for humans to digest, while lignin is a coarse, 'woody' compound found in the gritty part of pears and vegetable stalks. Unfortunately, the method they used to determine the quantity of this material also measures a variety of other substances (including some starch) which resist digestion in the small intestine and are only handled by the large intestine later.

Researchers from the Dunn Clinical Nutrition Centre in Cambridge England, developed a new and more appropiate method (the Eglyst method) of analysis, in which starch is completely removed and dietary fibre measured as the non-starch polysaccharides. This has led to a redefinition of dietary fibre as NSP (Non-Starch Polysaccharides). Lignin, although not a polysaccharide, is also regarded as dietary fibre; it is bound to the cell wall and probably contributes to the effects of NSP on body functions. It is however very difficult to measure and amounts present in edible foods are believed to be small.

Soluble vs insoluble fibre

A more practical classification, based on solubility and differences in effects in the body, is that of water soluble and insoluble dietary fibre. Insoluble fibre binds water and has a 'bulking' effect, whereas soluble components are inclined to form gels in water and are therefore also referred to as 'gel-forming' dietary fibre. Interestingly, recent research has shown that water soluble fibre (found in oats, oat bran, dried legumes and some fruit) has a significant cholesterol-lowering effect.

Extraordinary oats

The well-known quip that oats is 'a grain which in England is generally given to horses but in Scotland supports the people' gives no clue as to the diverse nutritional benefits of this cereal product. Firstly, the protein present in oats is of good quality and, coupled as it is with fibre and carbohydrate, it provides a breakfast with a high satiety value at a low price. Foods which - like oats - banish hunger for lengthy periods are particularly useful for schoolchildren, who won't feel the need to dash to the tuckshop at first break!

But this does not mean that oats is strictly for the kids, since the type of fibre which oats contains is of particular significance. Dietary fibre, or roughage, is made up of insoluble fibre (which forms bulk in the digestive tract and plays an important role in promoting peristalsis and preventing constipation), and soluble or gel-forming fibre. The fibre in oats is predominantly of the latter type, which has been shown to play a useful role in lowering blood cholesterol levels. It does this by binding with cholesterol in the digestive tract and preventing its absorption into the bloodstream. In addition, it slows down digestion and, at the same time, the release of glucose into the bloodstream, which is of special benefit to diabetics. Soluble fibre also promotes bacterial action in the digestive tract, helping to prevent constipation, and may well play a role in reducing the risk of bowel cancer.

How fibre affects the body

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 in^creased 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.