Abstract/Summary

The purpose of this project was to determine the interaction between dietary protein and the gut microbiome in the production of genotoxic metabolites, with a particular focus on the poorly characterised metabolite 4-cresol. The thesis describes, in the first instance, data from a large human observation study (n=205 healthy Omani adults). In which dietary records and urinary nitrogen excretion were used to estimate protein consumption in relation to urinary 4-cresol excretion. The study observed positive correlations between excreted 4 cresol and protein intake and then sought to explain the inter-individual variance in this by evaluating the influence of the colonic microbiota. Then the study focused on predicting 4-cresol exposures in the colon using in vitro gut fermentation models. The microbiota composition and metabolic profiles from these models are evaluated against different substrates, including comparisons of animal and plant proteins. We show that the total production of 4-cresol is dependent both on the host microbiota and also upon the dietary nitrogen source. The metabolite profiles of these fermentations may be used to predict DNA damage, with 4-cresol emerging as the greatest correlate of fermentation supernatant mediated genotoxicity. Finally, the study explored whether specific tumour isolates of F. nucleatum produce 4-cresol, or other genotoxins, that could drive intestinal carcinogenesis. At this stage the study is unable to conclude whether or not these isolates are passengers or drivers of intestinal disease. This work suggests the need for better models of the effects of the tumour environment on microbial growth. The most significant aspect of this thesis is that it evidences both the potential genotoxic contribution of 4-cresol in the colonic milieu, but also that urinary 4-cresol sulfate may be used as a biomarker of genotoxic colonic fermentation and thus, may be of use as a cancer risk endpoint in future dietary intervention study.