Effects of Defatted Rice Bran-Fortified Bread on the GutMicrobiota Composition of Healthy Adults With Low Dietary FiberIntake: Protocol for a Crossover Randomized Controlled Trial

Background: Inadequate dietary fiber (DF) intake is associated with several human diseases. Bread is commonly consumed,and its DF content can be increased by incorporating defatted rice bran (DRB). Objective: This first human study on DRB-fortified bread primarily aims to assess the effect of DRB-fortified bread on therelative abundance of a composite of key microbial genera and species in fecal samples. Secondary outcomes include clinical(cardiovascular risk profile), patient-reported (daily bread consumption and bowel movement, gut comfort, general well-being,and total DF intake), biological (fecal microbiota gene abundances, and fecal and plasma metabolites), and physiome (whole-gutand regional transit time and gas fermentation profiles) outcomes in healthy adults with low DF intake. Methods: This is a 2-armed, placebo-controlled, double-blinded, crossover randomized controlled trial. The study duration is14 weeks: 2 weeks of lead-in, 4 weeks of intervention per phase, 2 weeks of washout, and 2 weeks of follow-up. Overall, 60healthy adults with low DF intake (<18 g [female individuals] or <22 g [male individuals] per day) were recruited in Christchurch,New Zealand, between June and December 2022. Randomly assigned participants consumed 3 (female individuals) or 4 (maleindividuals) slices of DRB-fortified bread per day and then placebo bread, and vice versa. The DRB-fortified bread provided 8g (female individuals) or 10.6 g (male individuals) of total DF, whereas the placebo (a matched commercial white toast bread)provided 2.7 g (female individuals) or 3.6 g (male individuals) of total DF. Before and after each intervention phase, participantsprovided fecal and blood samples to assess biological responses; completed a 3-day food diary to assess usual intakes andweb-based questionnaires to assess gut comfort, general and mental well-being, daily bread intake, and bowel movement via anapp; underwent anthropometry and blood pressure measurements; and drank blue food dye to assess whole-gut transit time. Additionally, 25% (15/60) of the participants ingested Atmo gas-sensing capsules to assess colonic gas fermentation profile andwhole-gut and regional transit time. Mean differences from baseline will be compared between the DRB and placebo groups, aswell as within groups (after the intervention vs baseline). For metabolome analyses, comparisons will be made within and betweengroups using postintervention values. Results: Preliminary analysis included 56 participants (n=33, 59% female; n=23, 41% male). Due to the large dataset, dataanalysis was planned to be fully completed by the last quarter of 2024, with full results expected to be published in peer-reviewedjournals by the end of 2024. Conclusions: This first human study offers insights into the prospect of