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3:  DIETARY FIBRE:

Dietary fibre 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.

FIGURE 44: COMPONENTS OF DIETARY FIBRE

COMPONENT	SOURCE
Cellulose	All food plants
Hemicellulose	All food plants, especially cereal bran
Pectin	Mainly fruit
Lignin	Mainly cereals and 'woody' vegetables
Gums and some food thickeners	Food additives in processed foods

INTAKE 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.