Dietary fibre is the portion of fruits, vegetables, nuts and grains we eat that is resistant to digestion and absorption in the small intestine. However, bacteria that live in the large intestine may digest part of it.

Inclusion of fibre in the diet has proven health benefits, including decreasing blood cholesterol levels, prevention and treatment of constipation and reducing the time potential cancer-producing compounds, derived from our food, spend in the bowel.

Soluble or insoluble

There are many types of fibre found in the plant foods that we eat, and they are mostly carbohydrates. They include water-soluble fibres like pectin and gum, found in plant cells, as well as water-insoluble fibres such as cellulose, a component of plant cell walls.

Water-soluble fibres help to slow the passage of food through the gut, whereas water-insoluble fibres increase faecal bulk and speed up the passage of food through the gut.

Benefits of fibre

Insoluble fibre binds water, and this has the effect of softening and bulking out faecal matter. Whole grain products are high in insoluble fibre, and eating these is helpful in the treatment and prevention of constipation, haemorrhoids and diverticulitis. (Diverticula are pouches in the intestinal wall that can become inflamed and lead to abdominal pain and discomfort.)

Water-soluble fibre such as pectins and gums can bind cholesterol taken in through the consumption of certain foods. This helps to lower cholesterol levels by sweeping cholesterol out of the body before it gets to the bloodstream. Foods such as oat bran, beans, peas, rice bran and citrus fruits are especially high in soluble fibre.

High fibre diets may be useful for people who wish to lose weight since it does provide a ‘full’ feeling because of its water absorbing ability.

The indigestibility of cellulose

Cellulose is a carbohydrate thought to be the most abundant organic chemical on Earth. It consists of linear molecules, each made up of at least 3,000 glucose units joined together by β 1–4 linkages, which cannot be digested in the small intestine.

Compare this with starch, where the glucose units are joined by slightly different α 1–4 linkages. These linkages are capable of being broken down by digestive enzymes present in the gut, unlike the linkages in cellulose.

Although these linkage differences do not appear to be significant, they are in biochemical terms. Unlike starch, cellulose cannot be digested in our small intestine, as the system of enzymes needed is not present, so humans cannot effectively use the planet’s most abundant carbohydrate as a dietary energy source.

Large intestine digestion of dietary fibre

When dietary fibre reaches the large intestine, the bacteria present release enzymes that cause the fibre to be broken down into smaller molecules such as butyric acid (CH3CH2CH2COOH). The intestinal bacteria and the cells lining the large intestine can then use these smaller molecules as an energy source.

In addition to the production of these smaller molecules, the bacterial fermentation process releases gases such as carbon dioxidehydrogen, methane and hydrogen sulfide. If not used by the bacteria, these gases can build up and are finally excreted as flatus through the anus.

Nature of science

New observations in dietary fibre science have challenged prevailing theories about the role dietary fibre plays in the human diet. This highlights an enduring feature of the scientific process: scientific ideas are subject to change in light of new evidence

Useful link

The New Zealand Nutrition Foundation website has more information on fibre in the diet.

    Published 18 March 2011