High-Protein Diets Have the Potential to Reduce Gut Barrier Function in a Sex-Dependent Manner ^†

Abstract

Increased intestinal permeability is linked to low-grade systemic inflammation associated with chronic diseases. Undigested dietary proteins reach the colon, where they are fermented by components of the gut microbiota to produce metabolites shown to increase intestinal permeability in vitro. As evidence for sex differences in the microbiota grows, we hypothesised that the effects of the microbial fermentation of protein would also be sex-dependent. Thus, our objective was to determine whether there were sexual dimorphisms in microbial composition and metabolic output following the fermentation of different proteins using in vitro human gut model systems. Faeces from healthy male (n = 5) and female (n = 5) donors were used to inoculate gut fermentation systems supplemented with non-hydrolysed proteins (0.9 g) derived from whey, fish, milk, soya, mycoprotein, egg or pea. At 0, 8, 24 and 48 h, the microbiota composition was quantified using fluorescence in situ hybridisation coupled with flow cytometry, while bacterial-derived metabolite production was assessed via gas chromatography/mass spectroscopy and an ELISA. Increased protein availability resulted in significant increases in proteolytic Bacteroides spp. (p < 0.01) and Clostridium coccoides (p < 0.01) and significant increases in the production of potentially detrimental metabolites including phenol (p < 0.01), p-cresol (p < 0.01), indole (p = 0.018) and ammonia (p < 0.01), all of which were highly dependent on protein type. Furthermore, we showed higher abundances of Clostridium cluster IX (p = 0.03) and concentrations of p-cresol (p = 0.025) at 24 h in males, while females produced more ammonia (p = 0.02) irrespective of the protein source. The fermentation of mycoprotein resulted in significantly higher abundances of Clostridium cluster IX in males at 8 and 24 h compared to females (p < 0.01). There were also significant interactions between sex, protein source, bacterial populations and bacterial-derived metabolic-end-product concentrations. Our study provides new evidence that the effects of the microbial fermentation of dietary proteins in vitro are highly dependent on the source of the protein and the sex of the donor. Consequently, we suggest that different proteins are likely to have differential impacts on intestinal barrier function in vivo, and these effects may be different in males and females. If corroborated in human studies, our results would have important implications for dietary recommendations to limit chronic diseases.