How does high fibre food assist in digestion and what are the benefits of a high fibre diet?

This answer is brought to you by many of the Australian nutrition professionals who regularly contribute to a nutrition email discussion group.

Dietary fibre is only found in plant foods. Fibre has several effects on digestion, some beneficial, and some detrimental. One beneficial effect is that fibre tends to slow down the rate of digestion of food, leading to a more gradual emptying of the food from the stomach into the small intestine. This means that there is less likelihood of large quantities of glucose (the major breakdown product of carbohydrate) being absorbed rapidly from the small intestine into the blood and therefore a lower chance of an 'insulin surge'. Insulin is the hormone that is released when glucose is absorbed from the small intestine. It is possible that by slowing stomach emptying, fibre helps avoid the situation where the body has to produce large quantities of insulin (as a result of repeated rapid release of glucose into the intestine). In turn this may help protect against diabetes in susceptible people.

However, fibre does interfere with the absorption of some nutrients. For example, up to 5% of the fat in a moderately high fibre diet is not absorbed because of this interference. This may even be a good thing in Australia, given that 63% of men and 47% of women were overweight in 1995 (with no sign that these levels of overweight and obesity will decrease in Australia). High fibre foods also interfere to some extent with the absorption of some essential minerals and trace elements, but a high fibre diet is also likely to provide you with extra minerals and trace elements, so the effect is not believed to be very significant for normal Western diets.

Despite these minor detrimental effects, a high fibre intake is believed to be significantly beneficial overall. Low intake of fibre (particularly of the insoluble forms of fibre such as those in bread and other wheat products) is one of the major causes of constipation. Low fibre intakes are also strongly associated with an increased risk of diverticulitis. Although the evidence is less compelling, lack of fibre in the diet may also contribute to the incidence of rectal cancer, haemorrhoids, obesity, appendicitis and ulcerative colitis. High intake of soluble fibres such as the pectins and gums (found in fruits, vegetables, rolled oats) and saponins (found in legumes) is associated with reduced blood cholesterol. High intake of foods of plant origin (all of which contain some fibre) is associated with a reduced risk of heart disease, cancer, and an increased life expectancy.

Another benefit, and one that may assist with weight control, is the feeling of satiety (that is, a feeling of fullness) that follows a meal rich in fibre. It is also true that high fibre foods are almost invariably low in fat, so a high-fibre diet will usually be a low-fat diet.

Of course, it may not always be the fibre as such that is helpful. Plant foods contain many vitamins, phytochemicals, trace elements, non-nutritional antioxidants and other substances that may be just as valuable as fibre, or even more valuable. But the fact remains - to get all these other potentially beneficial chemicals you have to eat plant foods.

The current daily intake of fibre is probably 20-25g on average for Australian adults. Increasing this average intake to 30-35 g would very likely lead to improved health outcome. This is achievable by simply eating more fruits, vegetables and cereal foods (preferably whole grain).

For more information Nutrition Australia recommends:

• The Fibre in Your Food
• Healthy Food for Families (cook book)

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[Date issued: February 1999]

Disclaimer: This material is provided on the basis that it constitutes advice of a general nature only. It is not intended to replace the advice of a physician or a dietitian.