| dc.description.abstract | Background: Studies have shown an inverse relationship between dietary fiber intake and risk of colorectal cancer (CRC). More recent studies suggest that the gut microbiome may also have an impact on CRC risk. The aim of this study was to explore the relationship between fiber intake and colorectal neoplasia on two levels: non-advanced adenoma (NAA) and advanced neoplasia (AN). We also aimed to explore the possible relationship between microbial Alpha diversity in faecal microbiome and AN, and the relationship between dietary fiber and Alpha diversity. Methods: This was a cross-sectional study where we invited men and women aged 50-74 years, who tested positive for faecal occult blood in the Norwegian Bowel Cancer Screening pilot study, to participate in a gut microbiome sub-study. We asked participants to fill out a food frequency questionnaire and a lifestyle questionnaire prior to their follow-up colonoscopy examination. Colonoscopy results were collected from the screening database. Faecal samples in participants without any neoplastic findings and with AN were selected for microbiome analysis using metagenomic shotgun sequencing. Microbiome Alpha diversity in the faecal samples was presented with Shannon index. We used multivariate logistic regression (odds ratio, OR, and 95% confidence intervals, CI) to analyse associations between quartiles of fiber intake, and NAA and AN. We investigated differences in Alpha diversity with Wilcoxon signed rank tests, and explored correlations between fiber intake and Alpha diversity with Pearson correlation analysis. Results: In total, 910 men and women had sufficient data quality for analysis. We had sufficient data quality and metagenome data on 232 participants (131 without neoplastic findings and 101 with AN). The basic regression model showed a significant negative OR in the Q3 of fiber intake for NAA compared to Q1 (OR 0.57, 95% CI 0.37-0.86), but not in Q2 and Q4. The negative association remained significant after multivariate adjustment (OR 0.57, 95% CI 0.34-0.97). No association was observed between fiber intake and AN risk. There was no difference in Alpha diversity between subjects without colorectal neoplasia and AN, and no relationship was found between fiber intake and faecal microbiome Alpha diversity. Conclusions: We did not find an association between dietary fiber intake, colorectal neoplasia and gut microbiome Alpha diversity. Future studies on dietary fiber intake and neoplasia should distinguish more between specific sources of fiber and different stages of neoplasia. Fiber may also be preventative on a secondary level by protecting against metabolic disorders. Finally, more studies are needed to investigate specific bacterial species and the composition of the microbiome, regarding both neoplasia and dietary fiber. | nob |