The association of dietary factors and bacterial alpha diversity in the colorectal tumor-associated microbiome

Background: Aspects of the gut microbiome, such as microbial diversity, are related to the development and prognosis of colorectal cancer (CRC). Diet has been linked to the gut microbiome but no studies have considered the association of diet and microbial diversity in the tumor-associated microbiome. The goal of this study was to determine if dietary factors are associated with bacterial alpha diversity in CRC tumor tissue. Methods: Patient-matched CRC tumor and normal tissue samples were obtained from a subset of participants in the Puget Sound Colorectal Cancer Cohort study (n=446). Self-reported dietary factors included vegetable, fruit, red meat, and alcohol intake. Tumor tissue alpha diversity was measured using the Shannon index and was dichotomized two ways: “low” (tumor tissue alpha diversity less than the sample mean) and “depleted” (tumor tissue alpha diversity less than patient-matched adjacent normal tissue alpha diversity). Logistic regression was used to estimate odds ratios (OR) and 95% confidence intervals (CI) for the association between dietary factors and tumor tissue alpha diversity adjusted for age, sex, and smoking status. Results: About three-fifths of the sample had depleted tumor tissue alpha diversity. Participants who reported that they had <1 serving of vegetables/day had 2.0 times the odds of having a low tumor alpha diversity (95% CI 1.00, 4.16; p=.05). Higher red meat intake tended to be associated with higher odds of low and depleted tumor tissue alpha diversity, although results were not statistically significant. Fruit intake and alcohol intake were not associated with tumor tissue alpha diversity. Conclusions: Our study failed to detect an association between self-reported diet and bacterial alpha diversity in the CRC tumor tissue, which may have resulted from the crude measure of diet. Regardless, diet is an important factor in CRC risk and further work is needed to verify the findings of this study using more robust measures of diet.