is a term that refers to a group of food components that pass through
the stomach and small intestine undigested and reach the large intestine
virtually unchanged. Most other nutrients are digested and are being
used in other parts of the body by this stage. During its passage
through the large intestine some components of dietary fibre are broken
down to varying degrees and absorbed by the body; the remaining components
are excreted in the faeces.
To the extent
that products of dietary fibre breakdown in the large intestine are
absorbed, a small amount of energy is contributed, probably of the
order of 13 kilojoules (3 kilocalories) per gram of dietary fibre
ingested. Diets that are high in dietary fibre produce a slower rate
of stomach emptying, and bulky faeces which pass more quickly through
the large intestine.
The current attention
being given to the role of dietary fibre in prevention of certain
diseases is largely due to the observation that patterns of disease
observed in Africa and Asia were different from those in Western countries.
It was suggested that the dietary fibre content was associated with
this difference. Although it is not yet proven, there is evidence
to suggest that a diet high in dietary fibre can be of value for treating
or preventing such disorders as constipation, irritable bowel syndrome,
diverticular disease, hiatus hernia and haemorrhoids. Some components
of dietary fibre may also be of value in reducing the level of cholesterol
in blood and thereby decreasing a risk factor for coronary heart disease
and the development of gallstones. Dietary fibre is beneficial in
the treatment of some diabetics.
The actual role
of dietary fibre in many of these disorders is not known. It may be
that dietary fibre as such is not the major factor. When high dietary
fibre foods are eaten, other foods, which may be responsible for the
disease, are either reduced in quantity or completely excluded. For
example, diets that are high in fibre tend to be low in energy and
these diets can be useful in weight control. People who are grossly
overweight (more than 130 per cent of desirable weight or with a body
mass index of about 30 kilograms per square metre, see
p. 17) are more prone to heart disease and diabetes.
The analysis of
dietary fibre in food is very complex and only a limited number of
foods have been examined in detail. Dietary fibre from different foods,
and even different samples of the same food, contain varying quantities
of the components that collectively make up dietary fibre. Each of
these components has different biological properties and it is frequently
not clear which of these is most beneficial. The different components
of dietary fibre and their sources are shown in Figure
44.Foods of animal origin do not contain dietary fibre.
44: COMPONENTS OF DIETARY FIBRE
plants, especially cereal
OF DIETARY FIBRE
There is no recommended
dietary intake (RDI) for fibre as such in Australia. However, the
Australian Department of Health has among its dietary goals for Australians
an increased intake of fruit, vegetables, bread and cereals, all of
which are sources of dietary fibre. The greatest amount of dietary
fibre in wheat is in the outer layer or bran, of the wheat grain.
When white flour is produced, the bran layer is removed and the dietary-fibre
content of the flour is greatly reduced. Flour made from whole grains
contains about three times as much dietary fibre as white flour. It
is probably reasonable to aim for a dietary fibre intake of between
35 and 45 grams per day (see Figure
5), compared with current intakes in Western countries
of about 20 grams per day.