PROJECT SUMMARY / ABSTRACT Diet is an established etiologic factor for colorectal cancer (CRC) and is estimated to account for more than 40% of CRC cases and deaths. Despite several healthy dietary indices were suggested to be CRC-preventive, their associations with CRC risk remain moderate. A dietary index guiding dietary modulation for an efficacious CRC prevention remains lacking so far. One important reason is that all those dietary indices were developed based on their associations with cardiometabolic risk or certain general biological pathways, rather than mechanisms specifically targeting the colon. Growing evidence indicates an important role of gut microbial metabolites in mediating dietary effects on CRC risk. Among the metabolites, secondary bile acids (SBA) are suggested by extensive evidence to trigger a plethora of tumorigenic effects in colon, pointing to the gut microbiome-SBA axis as an emerging colon-specific pathway for CRC prevention. High-fat intake increases SBA production, but its SBA-stimulating property depends on fat type. Also, carbohydrates, proteins, and some other nutrients/foods also influence SBA but remain understudied. These data support the hypothesis that a dietary index specifically targeting the gut microbiome-SBA axis improves the currently weak CRC dietary prevention. To test this hypothesis, we will conduct an observational study in Aim 1 by leveraging the blood metabolomics and repeatedly measured diet and CRC diagnosis over 30 years in a racially/ethnically diverse population from four large cohorts: the Nurses’ Health Study I and II, Health Professionals Follow-up Study, and Southern Community Cohort Study. In Aim 2, we will conduct a single-arm intervention study among 40 patients who have recently undergone resection of colorectal adenoma and consume a habitual Western diet to assess the effect of a dietary intervention for more plant-based and less animal-based food over 4 weeks on SBA production, gut microbiota, and colonic gene expression via detailed food diary and repeated stool, blood, and colonic tissue collection. We will also guide patients to consume more foods, if there is any, that are found to substantially reduce SBA in Aim 1. I am well suited to perform this research based on 1) my expertise in epidemiology, quantitative methods, and biomarker research; 2) the exceptional multidisciplinary mentoring team comprised of leaders in their respective fields; and 3) the unparalleled research environment to support my career development. A team of outstanding scientists will mentor me to help me achieve this goal: Dr. Andrew Chan, a leader in clinical intervention and colorectal cancer prevention; Dr. Curtis Huttenhower, a leader in gut microbiome and bioinformatics; Dr. Mingyang Song, a leader in clinical epidemiology and nutritional intervention; and Dr. Edward Giovannucci, a leader in dietary effects on cancer. Through this study, I will expand my expertise in several new areas, including nutritional strategy for cancer prevention, the gut microbiome and bioinformatics, and clinical trial. The proposed research and training will help achieve my long-term career goal to become an independent investigator and develop a transdisciplinary research program in human nutrition and the gut microbiome for cancer prevention.

PROJECT NARRATIVE This project seeks to interrogate the interplay between diet, the gut microbiome-derived secondary bile acids, and colorectal cancer risk by conducting an observation study in four large prospective cohorts and a single-arm pilot clinical trial. The proposed research will enable unprecedented perspectives on how the gut microbial metabolism of bile acids mediates the dietary influence on colorectal cancer development and provide compelling data to develop a bile acid/gut microbiome axis-targeted diet for the prevention of colorectal cancer.