Resistance of Bamboo Fibers to Gastrointestinal Digestion and Their Nutrient-Dependent Fermentation by Human Gut Microbiota

Bamboo fibers are increasingly incorporated into food products as sustainable dietary fiber ingredients, yet their gastrointestinal digestion resistance and fermentative behavior by human gut microbiota remain insufficiently characterized. This study combined an INFOGEST-based in vitro digestion protocol with colonic fermentation using fecal microbiota from three healthy donors as an exploratory donor panel. Bamboo fibers were resistant to salivary, gastric, and intestinal enzymatic hydrolysis, as demonstrated by the absence of structural modifications in FTIR spectra, preserved morphology observed by scanning electron microscopy, and negligible release of reducing sugars (<0.1 g/L across all digestive phases), in contrast to extensively hydrolyzed wheat starch used as a positive control. During in vitro colonic fermentation, microbial responses were strongly dependent on substrate availability. In nutrient-limited minimal medium, bamboo fiber supplementation increased gas production, acidification, and SCFA formation compared with control conditions. Concomitantly, SCFA concentrations increased under minimal-medium conditions, although the magnitude of the response varied among donors, with the strongest changes observed for donor 1 (acetate reaching 5.4 vs. 3.3 g/L and propionate 1.48 vs. 0.76 g/L). These effects were markedly attenuated in nutrient-rich medium, indicating competition with readily fermentable substrates. Beta-diversity analyses and metagenomic profiling revealed that microbial community composition clustered primarily according to donor identity rather than experimental conditions. SEM further showed surface erosion and microbial attachment on fermented fibers, supporting partial structural alteration. Overall, bamboo fibers are resistant to upper gastrointestinal digestion but display context-dependent fermentative activity by human gut microbiota under carbohydrate-limited conditions, highlighting the importance of inter-individual variability, dietary context and substrate availability in determining their fermentative potential.