Lactate and buyrate proportions, methanogen growth and gas production during in vitro dietary fibre fermentation all depend on fibre concentration

Dietary fibre (DF) concentration plays a critical role in the gas kinetics and short-chain fatty acid production profiles during in vitro human faecal fermentation. However, the mechanisms leading to concentration-dependent fermentation outcomes are unknown. In this study, fructo-oligosaccharides and pectin at three concentrations (0.2, 0.4 and 0.6g/44 mL), were selected to investigate the effect of DF type and concentration on human gut microbiota. An absolute quantification method was applied to determine the microbial biomass and load for each genus. Additionally, the gas kinetics and fermentation metabolites (acetate, propionate, butyrate, i-butyrate, valerate, i-valerate, formate, succinate, methanol, ethanol and lactate) were measured. Results showed that DF type and concentration affected the microbial diversity, microbial loads and metabolite profiles to different extents. At high DF concentrations Methanobrevibacter, a genus of Archaea, was found to increase by ∼28 fold for FOS, and ∼93 fold for pectin. We propose three main pathways, dictated by DF type and concentration, driving the observed alterations in microbial loads and metabolite profiles. Firstly, the lactate cross-feeding pathway could lead to increased butyrate production. Secondly, methanogenesis from H2 and CO2, and thirdly, reductive acetyl-CoA pathways resulted in the conversion of gases to increase microbial biomass. Understanding these underlying mechanisms allows for tailoring DF fermentation by varying their concentrations to control SCFA and gas production.