Sourdough Bread and Baker's Yeast Bread Affect Gut Symptoms and Targeted Gut Microbial Taxa Differently: A Randomised Controlled Trial.

Background/Objectives: The intake of bread has been associated with gastrointestinal discomfort in some individuals. The primary aim of this study was to investigate gut symptoms upon consuming sourdough bread compared to baker's yeast bread. Secondary endpoints were changes in targeted gut microbial taxa, micronutrient levels, and metabolic markers. Methods: This randomised, controlled, double-blind cross-over study included twenty healthy participants. Sourdough or baker's yeast bread was consumed for one week before crossing over. Before and after the interventions, participants reported gut symptoms, and they also provided blood samples to analyse micronutrients, metabolic markers, and faecal samples for targeted gut microbial taxa analyses. Results: The breads were made of whole grain wheat with similar macronutrients content. The sourdough bread contained less fructose than the baker's yeast bread. The consumption of sourdough bread caused a significant reduction in the diarrhoea sub-dimension score (p = 0.016) and a borderline significant reduction in the overall GSRS-IBS score compared to baker's yeast (p = 0.051). Several gut bacteria were significantly increased after intake of sourdough bread compared to baker's yeast bread: Alistipes putredinis (p = 0.029), Alistipes shahii (p = 0.015), Bifidobacterium adolescentis (p = 0.042), Bifidobacterium longum (p = 0.010) and Bifidobacterium longum subspecies longum (p = 0.021). There was a significant correlation between changes in diarrhoea score and changes in Alistipes shahii (r= -0.353, p = 0.025). Conclusions: This exploratory cross-over study did not demonstrate a statistically significant difference in the overall GSRS-IBS score between the two breads. A nominal difference in the diarrhoea sub-dimension and selected targeted microbial taxa was observed between the two breads, but these findings require further investigations.