Abstract
Dietary fibre (DF) intake is associated with the modulation of the gut microbiome and improving overall health and psychological wellbeing. However, there is a paucity of research on the impact of whole food DF interventions on these parameters in healthy individuals and those with constipation. This PhD thesis examined two DF interventions over four weeks: defatted rice bran fortified white toast bread (DRB bread) and green-fleshed kiwifruit. These DF interventions were chosen because bread is commonly consumed and serves as an ideal vehicle for increasing DF content by incorporating cereal bran, such as DRB, which may modulate the gut microbiota. Similarly, green kiwifruit has shown promising effects in alleviating constipation symptoms. However, more research is needed to confirm its long-term therapeutic efficacy and underlying mechanisms. It was therefore, hypothesised that: 1) consuming three to four slices of DRB bread increases the relative abundances of a composite of selected key genera and species of the gut microbiota that are relevant in DF degradation and metabolism in healthy individuals with low DF intake as compared to control white toast bread (control bread); 2) consuming two green-fleshed kiwifruit improves abdominal pain in individuals with functional constipation or constipation-predominant irritable bowel syndrome (IBS) as compared to calorie-matched maltodextrin. Other hypotheses were that DRB bread and green-fleshed kiwifruit interventions improve DF intake, change the relative abundance/concentration of faecal and/or plasma biological markers of host and microbial interactions (microbial taxa, bile acids, organic acids, neurotransmitters), improve gut symptoms and psychological wellbeing compared to control bread and maltodextrin, respectively. The daily consumption of three to four slices of DRB bread for four weeks did not affect the primary outcome of the microbial taxa composite. Still, there were biologically relevant (non-significant) increases in the relative abundance of beneficial microbial taxa involved in DF degradation and metabolism, Faecalibacterium prausnitzii, Bifidobacterium longum, and Bacteroides ovatus, but not other markers of host-microbial interactions. DRB bread intake also increased DF intake and was well tolerated but did not affect psychological wellbeing. The daily consumption of two green-fleshed kiwifruit for four weeks compared to maltodextrin did not change the primary outcome of abdominal pain. Still, there was a trend in improving constipation, which was clinically relevant, and IBS-Quality of Life, but not in other psychological wellbeing domains. Green-fleshed kiwifruit intake increased vitamin C intake but not DF intake. Faecal moisture content, faecal lithocholic acid, and isolithocholic acid concentrations increased, with a slight increasing trend or maintenance in the relative abundance of Akkermansia muciniphila and a slight decreasing trend in the Faecalibacterium prausnitzii following the intake of two green-fleshed kiwifruit compared to maltodextrin. Findings from this PhD thesis underscore the complex interplay between DF from DRB bread and green-fleshed kiwifruit, the gut microbiome, gut symptoms and psychological wellbeing in healthy individuals and those with constipation. These insights can pave the way for future research on whole food DF interventions aimed at promoting DF intake, modulating the gut microbiome, and improving overall gut health and psychological wellbeing in the adult population.