Modelling the effect of undigested dietary carbohydrate on the health and function of the large bowel

Consuming dietary fibre confers a number of large bowel health benefits through various interacting mechanisms. Here we describe and mathematically model how these key mechanisms respond to changes in the concentration of fibre consumed. The key markers of bowel health and function, short chain fatty acid (SCFA) and branched chain fatty acid (BCFA) concentrations, faecal bulk, transit time and pH are interrelated and captured in a model with just four parameters. The model was parameterised using animal (rat) data from the literature for faecal bulking, BCFA production, SCFA production and increasing acidity by the gut microbiota. Surprisingly, the model replicated the data in the absence of explicit representation of both the microbiota and their interactions with mucins. Interestingly, the model identified that there was an optimal fibre concentration at which SCFA was maximised, and further increases in fibre actually caused SCFA to decline. Extrapolating beyond the range of fibre content in the dataset predicted interesting results in terms of very short transit times for digesta and high faecal bulk, suggesting the need for new experiments to verify the range of fibre concentrations for which the model is valid. The parameterised and validated model may have value as a tool for predicting the potential health benefits arising from consuming varying fibre concentrations.