Effects of high-protein rice bran fiber on intestinal microbiota composition in healthy older Chinese adults based on in vitro fermentation and a pilot intervention study

Rice bran, a by-product of rice processing, is rich in dietary fiber (DF) and other bioactive compounds, offering considerable potential for shaping gut microbiota and improving metabolic health, yet its functional properties remain insufficiently explored. This study combined in vitro fermentation and a 4-week intervention in elderly subjects to evaluate the effects of high-protein rice bran fiber (RBF) on gut microbial activity, intestinal physiology, and cardiometabolic markers. In vitro, RBF selectively influenced microbial metabolism, characterized by reduced CO₂ and H₂ production and a marked increase in acetic acid, while butyric acid exhibited a modest upward trend. These metabolic shifts were accompanied by suppression of Escherichia–Shigella and enrichment of beneficial genera, such as Clostridium, Lachnoclostridium, Faecalibacterium and Bifidobacterium. In a subsequent human intervention trial involving healthy older adults with habitually low DF intake, RBF supplementation did not drastically alter the overall gut microbiota composition or fecal short-chain fatty acids (SCFAs) levels. However, RBF supplementation selectively enriched ErysipelotrichaceaeUCG-003, a taxon associated with butyrate production, and modestly increased Bifidobacterium and Roseburia, genera linked to barrier integrity and anti-inflammatory metabolism. Meanwhile, DRB intake significantly reduced serum triglycerides (TG) and apolipoprotein E (ApoE) concentrations without adverse gastrointestinal effects, with high adherence and tolerability. Collectively, these results suggest that DRB increases DF intake, improves serum lipid profiles, and promotes favorable modulation of gut microbiota, highlighting its potential as a functional dietary ingredient for supporting nutritional balance and metabolic regulation. • In vitro, RBF enhanced acetate and propionate production, and selectively enriched beneficial gut bacteria. • Four-week RBF intake lowered serum triglycerides and apolipoprotein E in older adults with good tolerability. • RBF may modulate gut microbial composition and contributes to improved host lipid regulation.