fibre, dietary

Plant material (outer husks and peel) that cannot be digested by human digestive enzymes. It consists largely of cellulose, a carbohydrate found in plant cell walls. Fibre adds bulk to the gut contents because it can absorb water, assisting the muscular contractions that force food along the intestine. A diet low in fibre causes constipation and is believed to increase the risk of developing diverticulitis, diabetes, gall-bladder disease, and cancer of the large bowel – conditions that are rare in non-industrialized countries, where the diet contains a high proportion of unrefined cereals. It is also known as NSP (non-starch polysaccharide).

Animals are unable to produce the enzyme necessary to break down cellulose. However, cellulose still forms a necessary part of the human diet as fibre (roughage) – because it is not broken down it acts as a signal that the gut needs to contract to move the contents on. Any undigested plant material acts as fibre.

Soluble fibre consists of indigestible plant carbohydrates (such as pectins, hemicelluloses, and gums) that dissolve in water. A high proportion of the fibre in such foods as oat bran, pulses, and vegetables is of this sort. Its presence in the diet has been found to reduce the amount of cholesterol in blood over the short term, although the mechanism for its effect is disputed.

In April 2000 a US study of the effect of a high-fibre diet on cancerous polyps in the bowel contradicted the theory that dietary fibre significantly decreases the incidence of bowel cancer.