Inhibitory effects of soybean oligosaccharides and water-soluble soybean fibre on formation of putrefactive compounds from soy protein by gut microbiota

•Indole was produced from soy protein (SP) by gut microbiota.•Soy oligosaccharides and dietary fibre inhibited indole production in human faecal culture.•Compared with the SP diet, the soybean flour (SFL) diet reduced the caecal indole in rats.•The indole content was also decreased by soybean water-soluble fibre (SFB).•Prevottela was detected as dominants in caecum of rats fed SFL or SFB diet. Soybeans are part of the traditional food consumed in Asia countries. In this study, we investigated inhibitory effects of soybean oligosaccharides and water-soluble soybean fibre (Soyafibe) on putrefactive compounds from soy protein by gut microbiota in rats. Caecal microbial fermentation products and microbiota in rats fed 20% soy protein (SP-1) and whole soybean flour (SFL: protein content was 20%) diets were determined. The caecal environment in rats fed 20% soy protein without dietary fibre (SP-2) or with 2% Soyafibe (SFB) was also determined. Compared to SP-1 and SP-2 group, low indole content with high lactic acid was shown in SFL and SFB group, respectively. Using the 16S rRNA genes polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and pyrosequencing. Prevotella, Gram-negative anaerobic rods, were detected as dominant in both SFL and SFB groups. Our findings indicated that fermentable polysaccharides in soybeans have inhibitory effects on the formation of putrefactive compounds generated from soy protein by the microbiota.