Assessment of the Gut Microbiome in Patients with Coexisting Irritable Bowel Syndrome and Chronic Fatigue Syndrome

Background: The gut microbiome is a key modulator of the gut–brain axis and may contribute to the pathophysiology of both gastrointestinal and systemic disorders. This study aimed to evaluate gut microbiota composition and tryptophan/phenylalanine metabolism in women with unclassified irritable bowel syndrome (IBS-U), with or without coexisting chronic fatigue syndrome (CFS). Methods: Eighty women were enrolled and divided into two groups: IBS-U without CFS (Group I, n = 40) and IBS-U with coexisting CFS (Group II, n = 40). Microbial composition and diversity were assessed using the GA-map™ Dysbiosis Test, including the dysbiosis index (DI) and Shannon Diversity Index (SDI). Hydrogen and methane levels were measured in breath samples. Urinary concentrations of selected microbial and neuroactive metabolites—homovanillic acid (HVA), 5-hydroxyindoleacetic acid (5-HIAA), kynurenine (KYN), kynurenic acid (KYNA), xanthurenic acid (XA), quinolinic acid (QA), hydroxyphenylacetic acid (HPA), and 3-indoxyl sulfate (3-IS)—were quantified using LC-MS/MS. Fatigue severity was assessed using the Chalder Fatigue Questionnaire (CFQ-11) and the fatigue severity scale (FSS). Results: Compared to Group I, patients with IBS-CFS showed significantly greater microbial diversity, higher breath methane levels, and elevated urinary concentrations of QA, XA, 3-IS, and HVA, alongside lower concentrations of 5-HIAA and KYN. Fatigue severity was positively correlated with urinary XA and QA levels. Conclusions: Women with IBS and coexisting CFS exhibit distinct gut microbiota and tryptophan metabolite profiles compared to those without fatigue. The observed metabolite–symptom associations, particularly involving neuroactive kynurenine derivatives, warrant further investigation. These preliminary findings should be interpreted as hypothesis-generating and require validation through high-resolution microbiome analyses, functional pathway profiling, and longitudinal or interventional studies to clarify causality and clinical significance.