4: Functional Components and Mechanisms of Action of ‘Dietary Fibre’ in the Upper Gastrointestinal Tract: Implications for Health

Dietary fibre is a generic term for a chemically diverse group of carbohydrates that are resistant to endogenous enzymes of the human digestive tract. The major component of dietary fibre consists of plant cell walls, which are supramolecular structures, composed of complex heterogeneous networks of cellulose, hemicelluloses and pectic substances. The amounts and relative proportions of these carbohydrates vary depending on the type and maturity of the plant tissue. Some plant cell walls, especially those from leguminous seeds, are rich in water-soluble non-starch polysaccharides (NSP).

Intact plant cell walls and NSP affect the rate and extent of nutrient digestion, with important implications for health and disease. Certain types of fibre reduce the rate of starch digestion, which in turn can significantly attenuate the postprandial rise in blood glucose and insulin concentrations. This is potentially beneficial in the prevention and treatment of diseases, including diabetes mellitus and cardiovascular disease. However, the mechanisms of action of NSP in relation to the digestive process are still not well understood. They are thought to include formation of viscous solutions, encapsulation of nutrients and inhibition of digestive enzymes.

These mechanisms are illustrated using specific examples. Oat β-glucan is used to show the effects of various processing techniques on β-glucan molecular weight, and hence viscosity, on risk factors for diabetes and cardiovascular disease. Evidence for guar galactomannan acting as an inhibitor of α-amylase, in addition to forming viscous solutions, is presented. Finally, the effect of intact plant cell walls on the bioaccessibility of nutrients is discussed.