Proposed are innovative studies designed to investigate the biological basis of the increased risk for the development of colorectal cancer (CRC) independently associated with being African American (AA) or consuming a diet high in red meat and saturated fat. We hypothesize that the primary bile salt taurocholic acid (TCA) is a key diet-controlled metabolite whose use by the bacteria Bilophila wadsworthia and Clostridium scindens yields a carcinogen and tumor-promoter, respectively. The work is motivated by our extensive collection of published data indicating H2S and secondary bile acid production by colonic bacteria serve as key environment insults contributing to CRC risk. First we will examine differences between at-risk AAs and non-Hispanic whites (NHWs) in mucosal abundance of bacterial genes associated with sulfur and bile acid metabolism, markers of bile metabolism in serum and stool, and colonic inflammation. Rationale is provided by our intriguing observation that mucosal abundance of a bacterium (B. wadsworthia) that uses the sulfur amino acid taurine, which is abundant in red meat, is a strong predictor of CRC in AA but not NHW subjects. Bacterial deconjugation of the bile acid taurocholate provides another source of taurine, and once deconjugated, free primary bile acids are further metabolized by colonic bacteria to genotoxic and proinflammatory secondary bile acids. We will then test the hypothesis that a diet high in animal protein and saturated fat creates a metabolic milieu in the colon that promotes risk for CRC in at-risk AAs by increasing the abundance and activity of bacteria that generate genotoxic sulfide and secondary bile acids. Rationale for focusing the diet intervention on AAs comes from the observation regarding a taurine respiring bacterium distinguishing AA but not NHW CRC patients from healthy controls and our prior work in AAs that focused on mechanisms underlying the increased risk for CRC associated with consumption of a Western type diet. We will conduct a prospective randomized crossover feeding trial that examines two microbial mechanisms by which an animal-based diet may support the growth of bacteria that generate the genotoxic and proinflammatory end products, H2S and secondary bile acids, through the metabolism of TCA. An animal- based diet rich in taurine and saturated fat will be compared with a plant-based diet low in taurine and saturated fat. Subjects will receive each of the two diets in a crossover design thereby serving as their own control. A mediation model will be used to determine the extent to which the relationship between diet [independent variable] and mucosal markers of CRC risk and DNA damage [dependent variables] is explained by colonic bacteria and their functions [mediator variables]. This research will generate novel information on a mechanistically targeted nutrient aimed to develop effective cancer prevention interventions based simply on diet that may contribute to a reduction in the unequal CRC burden in AAs.