Fibre in the diet promotes more frequent bowel movements and softer stools having increased weight. The softness of stools is largely due to the presence of emulsified gas, which is produced by the bacterial action on the fibre. A high fibre intake results in greater efficiency in the peristaltic movement of the colon. This helps in relieving the constipation, which is the main cause of several acute and chronic diseases.

Recent studies suggest that increasing the dietary fibre intake may be beneficial for patients with irritated bowel syndrome who have diarrhoea and rapid colonic transit, as well as to those who have constipation and slow transit. The high fibre diet, like bran, thus regulates the condition inside the colon so as to avoid both extremes - constipation and diarrhoea.

Investigations have shown that several potential carcinogens are produced in the faeces. Their production is related to the acidity of the gut content. The greater the acidity in the bowel content, the less is the production of these carcinogens. The breaking down of the fibre by bacteria renders the faeces more acidic. This reduces the amount of possible carcinogenic substances. Fibre also reduces the possibility of formation of harmful toxins in the large intestine by reducing the intestinal transit time of the food contents.

Dietary fibre increases the bacteria in the large intestines, which require nitrogen for their growth. This in turn reduces the chances of cancerous changes in cells by reducing the amount of ammonia in the large bowel. Fibre reduces the absorption of cholesterol in the diet. It also slows down the rate of absorption of sugars from the food in the digestive system. Certain types of fibre increase the viscosity of the food content. This increased viscosity indirectly reduces the need for insulin secreted by the pancreas. Thus a fibre-rich diet can help in diabetes mollitus

Introduction

Although a great deal of research was stimulated throughout the world by the Burkitt and Trowell's hypothesis (84), it is still early to assign clear health claims to dietary fibre. This difficulty is due in great part to the fact that dietary fibre includes many complex substances, each having unique chemical structure and physical properties. Moreover, dietary fibre is often intimately associated in the plant cell structure with other organic compounds, such as vitamins, phyto-oestrogens, flavonoids, etc., displaying their own biological activity. Nevertheless, numerous prospective and well-designed experimental studies have highlighted several physiological and metabolic effects of dietary fibre which may be important for human health.

Digestive fate of dietary fibre

It is now well-established that dietary fibre reaches the large intestine and is fermented by the colonic microflora with the production of short chain fatty acids (SCFA), hydrogen, carbon dioxide and biomass. This fermentative process dominates human large bowel function and provides a means whereby energy is obtained from carbohydrates not digested in the small bowel, through absorption of SCFA.

Fermentation of fibre in the colon

Polysaccharides cannot penetrate in the bacterial cells. They are first hydrolysed in monosaccharides, by membranous or extra-cellular enzymes secreted by bacteria. Metabolism of these monomeric sugars continue in the bacterial cells using the Embden-Meyerhoff pathway which leads to pyruvate. Pyruvate does not appear in the large bowel because it is immediately converted in end-products. These are SCFA, mainly acetate, propionate and butyrate, and gases: carbon dioxide , hydrogen, and methane.

Colonic fermentation is an efficient digestive process since starch is almost totally degraded, as well as lactose, alcohol-sugars and fructans if the intake of these sugars is not too high. More than half of the usually consumed fibres are degraded in the large intestine, the rest being excreted in the stool (see Table 9). A number of factors are likely to affect the utilization of fermentable carbohydrates in the colon. Among these is solubility. The more soluble substrates, being more accessible to hydrolytic enzymes, are likely to be degraded more rapidly. Nevertheless, some soluble fibres such as alginates or carragheenans are poorly fermented. Other factors involving digestive motility and individual differences in microflora could also modulate fermentation. Furthermore, certain metabolic pathways can be modified by the repeated occurrence of some sugars (lactose, lactulose, fructans) in the colon. The mechanisms and the physiological consequences of this adaptation are not completely identified.