Not all complex carbohydrates/ polysaccharides are digested. This is important as the ones that cannot be digested make up ‘dietary fibre’ or ‘non-starch polysaccharide’ (NSP) as it is now scientifically termed.
Dietary fibre is a type of polysaccharide from plants that cannot be broken down by enzymes secreted in the human digestive tract. Instead fibre is fermented by microbes within the large intestine, releasing the gases hydrogen, methane and carbon dioxide which are responsible for flatulence. It plays an important role in bulking up the bowel contents, decreasing constipation, diverticulosis and even varicose veins.
Dietary fibre components are classified as insoluble or soluble.
Insoluble dietary fibre, found in wholegrain cereals and pulses (peas, beans and lentils) absorbs water in the large intestine to produce softer, bulkier stools. Insoluble fibre increases the time foods spend in the stomach, permitting digestion to occur at a slightly more leisurely pace and slowing the rate of rise in blood sugar. Transit time, the period between eating and defecation, is reduced by insoluble dietary fibre.
Soluble fibre (found in oats, fruits and vegetables) may act to lower cholesterol levels. A lot of research has been carried out on fibre since it was first enthusiastically promoted in the 1970s; work that has led to the proposed name change from ‘dietary fibre’ to ‘non-starch polysaccharides’ (NSP) which specifically describe the components of plant cell walls.