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.
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