Statistical analysis
Accumulated data will be processed by SPSS (version 25) statistical software. All data will be checked for normality. Continuous variables will be expressed by mean with standard deviation or median with quartiles 1 and 3. Discrete variables will be expressed as proportions and percentages. Changes in the PCI, CEA, Ca72-4, and EORTC QLQ-C30 and EORTC QLQ-STO22 questionnaires score will be assessed by using paired sample t-test, Wilcoxon rank-sum test. For statistical analysis of gut microbiome compositions, the web-based application Calypso (version 8.84) will be used. Alpha diversity will be quantified by the Shannon index. Beta diversity will be quantified by principal coordinate analysis (PCoA) based on a Bray–Curtis dissimilarity matrix with analysis of similarity (ANOSIM), as well as redundancy analysis (RDA) with one or multiple explanatory variables. Additional analyses will be performed if necessary. P values < 0.05 will be considered statistically significant in all statistical analyses.