Dietary fibre is defined as food material, particularly plant material, that is not hydrolysed by enzymes secreted by the human digestive tract, but that may be digested my microflora in the gut. It can be either insoluble or soluble. Dietary fibre from plant material includes non-starch polysaccharides eg celluloses, some hemi-celluloses, gums and pectins as well as resistant starches.
Foods high in fibre, because of their consistency, encourage mastication and stimulate the secretion of digestive juices. In the colon, dietary fibre tends to increase faecal bulking due to increased water retention, and the insoluble dietary fibre reduces transit time. Transit time is particularly important because the conversion of sterol to carcinogenic polycyclic aromatic hydrocarbons is known to happen over time.
Resistant starches comprise approximately 10 percent of the dietary intake of starch. Resistant starches are the starch that is not digested in the small intestine and that passes into the large intestine where it is a substrate for bacterial fermentation. The bacteria produce short chain fatty acids from the resistant starches that may help maintain the health of the cells lining the colon and prevent bowel cancer.
These fatty acids are also absorbed into the bloodstream and may play a role in lowering blood cholesterol levels.
Resistant starches are found in intact whole grain cereals, nuts and seeds, legumes and in unripe fruit especially bananas.