Gel-forming fibres differentially modulate inulin fermentation: A comparison of psyllium and methylcellulose in in vitro colonic models

1.Fermentable fibres such as inulin can support metabolic health but may exacerbate gastrointestinal symptoms in individuals with irritable bowel syndrome (IBS) due to rapid fermentation and gas production. The gel-forming fibre psyllium improves IBS symptoms, although the underlying mechanisms remain unclear. We hypothesised that fibre gelation alters fermentation by modulating microbial access to substrates. To test this, we compared psyllium with methylcellulose, a chemically modified, gel-forming fibre, to determine the effects of gelation on inulin fermentation. Inulin alone or combined with psyllium or methylcellulose was fermented for 48 hrs in a colonic fermentation model inoculated with healthy human faeces. Gas production, metabolite profiles, microbial community composition and microbial localisation within fibre gels were assessed. Bioactivity of fermentation products was evaluated in STC-1 cells. Psyllium co-fermentation significantly accelerated fermentation and enhanced production of metabolites, while methylcellulose had minimal effects. Psyllium maintained higher diversity and enriched polysaccharide-degrading taxa including Bacteroides and Phoecaeicola species, which were strongly associated with metabolic activity. Bacterial penetration into the psyllium matrix was observed but not into methylcellulose. Fermentation products from psyllium but not methylcellulose stimulated GLP-1 and 5-HT secretion in STC-1 cells. These findings demonstrate that delayed-onset fermentable gel-forming fibres enhance microbial access to entrapped substrates, driving metabolic and hormonal responses.