The links between human breath methane, dietary fibre digestion, and the gut microbiota : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Nutrition Science at Massey University, Manawatu, New Zealand

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The concentration of methane that is exhaled in human breath has been associated with the composition and fibre-fermenting function of the human colonic microbiota. The current research aimed to investigate whether composition and fibre fermentation function of the colonic microbiota differ in individuals who are low breath methane emitters (LE) or high breath methane emitters (HE). Healthy adult individuals (18) were recruited and breath tested. Unexpectedly, they showed positive correlations between breath hydrogen and methane. Then, the highest and lowest breath methane emitters provided faecal samples used for shotgun metagenomic sequencing and as faecal inocula for in vitro colonic fermentations with dietary fibres (β-glucan and lignocellulose). Individuals who were LE reported higher dietary vitamin E, fibre, and fat intakes and a Bacteroides-driven microbiota composition compared to HE individuals who reported a greater starch intake and a Prevotella-driven microbiota composition. The faecal microbiota from individuals who were HE had a greater abundance of taxa from the Methanobrevibacter genus and more methane gas production during in vitro colonic fibre fermentation compared to the microbiota of individuals who were LE; however, the results were variable within the HE group. There was a greater rate and extent of dietary fibre fermentation in LE compared to HE individuals during in vitro colonic fermentation, which aligned with the greater fibre intakes of LE individuals. Furthermore, the faecal microbiota of LE individuals showed increased beneficial organic acid production and a greater abundance of functional pathways related to amino acid metabolism compared to HE during in vitro colonic fermentation. These results did not align with published research on human breath methane and the gut microbiota. However, there was consensus with emerging hypotheses suggesting that there are important pathways involved in hydrogen sulphide production and hydrogen utilisation that are largely unexplored. Further investigations in these areas could help redefine our understanding of fibre fermentation by the human colonic microbiota.
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Gastrointestinal system, Microbiology, Fiber in human nutrition, Methane, Respiration