Whole grain and the gut-brain axis : The role of microbiota composition, dietary fiber, and metabolites

Abstract

Whole grain (WG) is rich in dietary fiber, other nutrients, and phytochemicals, and has potential effects on the brain through the gut-brain axis and non-microbiotarelated routes. This thesis explored the interplay between gut microbiota, dietary fiber, and microbiota-derived metabolites during in vitro fermentation of WG oat bread, WG rye bread, and refined bread. The effects of WG rye bread on the gutbrain axis were studied in a three-week dietary intervention study. Differences in short-chain fatty acid (SCFA) production and dietary fiber degradation were observed between two donors with different fecal microbiota composition and between different breads during the 24-h in vitro fermentation experiments. High levels of butyrate were observed especially in fermentation samples with rye substrate in one donor, where a high relative abundance of Subdoligranulum genus was also observed. The metabolite profile of fermentation samples with WG rye differed from that of fermentation samples with WG oats and refined bread. Metabolites showing significant changes included several suggested microbiota-derived metabolites. The three-week intervention with high intake of WG rye bread did not reveal any significant effects on fecal microbiota, SCFA levels, blood markers related to the gut-brain axis, intestinal permeability, stress responses, cognitive performance, or perceived long-term stress and well-being in healthy subjects. However, the abundance of two butyrate-producing taxa, Anaerobutyricum hallii and Eubacterium ventriosum group increased within the group consuming WG rye bread. Altogether, these results indicate that WG rye may have butyrate-promoting effects depending on gut microbiota composition, but effects on the gut-brain axis were not observed.