Particle size of dietary fibre has diverse effects on in vitro gut fermentation rate and end-products depending on food source

Dietary fibre (DF) is commonly categorised according to its solubility, but this is not necessarily related to functional traits such as fermentability, particularly for structurally complex plant-based food particles that survive digestion in the small intestine. In this study. examples of DF from a broad range of foods, including cereals, brans, legumes, nuts, vegetables and fruits, were selected to investigate relationships between particle size, composition and fermentability. These DF were either washed, or digested and washed to remove all accessible starch, protein and/or lipid, and then characterized chemically and microscopically prior to being fermented in vitro using a pooled human faecal inoculum. Results showed that: i) different substrates irrespective of particle size had diverse fermentation outcomes (gas kinetics, short chain fatty acid and ammonia profiles); ii) particle size differences did not have consistent effects on the fermentation outcomes for different food types, and iii) the nature of the particles and their major chemical components together determined particle size influences. To explain how particle size affects in vitro fermentation outcomes for diverse DF we propose that, in general, a combination of particle nature and the fermentability of major chemical components determine both the rate and end-products of fermentation.