Functional categorisation of dietary fibre in foods: Beyond 'soluble' vs 'insoluble'

Background: Diets rich in dietary fibre are associated with multiple health benefits, but there is often only a restricted understanding of the mechanisms underlying these associations. This limits the ability to select or design foods for specific nutritional purposes. Traditionally, the diverse physical and chemical forms of dietary fibre have only been categorised as either soluble or insoluble. Scope and approach: In this review, the physicochemical properties that have been proposed to be responsible for the biological functionality of dietary fibres in the digestive tract are summarised and classified. The extent to which these properties follow naturally from categorisation into soluble vs insoluble forms are then assessed. Based on this analysis, a new approach to functional categorisation of dietary fibres is proposed. Key findings and conclusions: The physicochemical properties of dietary fibre components that are relevant to digestive tract functionality can be classified under the headings of binding, structuring, and transport barriers. Major nutritional outcomes such as control of macronutrient digestion or the nature of residual digesta that are available for fermentation by the large intestinal microbiota depend on combinations of these physicochemical properties in ways which are not readily reflected by a soluble vs insoluble fibre definition. An alternative approach is proposed based on 2D mapping of dietary fibre materials as a function of molecule/particle size and local density. This effectively separates diverse fibre materials and can be linked semi-quantitatively with functionally-important properties.