Background Generalized anxiety disorder (GAD) is a common psychiatric condition, with unknown etiology and pathophysiology. Recent studies have suggested alterations in the microbiota-gut-brain axis may be involved in the development of GAD. We aimed to explore the interactions between the gut microbiota, gastrointestinal and psychiatric symptoms, neuroimmune markers and dietary patterns in patients with GAD.

Methods We recruited 83 GAD patients and 98 age- and sex-matched healthy controls (HC) and assessed their psychiatric and gastrointestinal symptoms, and long-term diet using validated questionnaires. We measured serum and stool neuroimmune markers and metabolites by ELISA and LC-MS, microbiota was analyzed using 16S rRNA gene sequencing with functional predictions by PICRUSt2. Microbial carbohydrate degradation capacity was assessed ex vivo. The data was analyzed using classical statistics and machine learning (XGBoost).

Results GAD patients exhibited higher BMI, gastrointestinal symptoms and inflammatory markers, while reporting reduced intake of fiber and other macro- and micronutrients compared to HC. Gastrointestinal symptoms were the most predictive feature separating GAD from HC. GAD patients had a distinct microbiota profile, dominated by Bacteroides, compared with a Prevotella-dominated microbiota in HC. Carbohydrate degradation pathways were enriched in GAD and strongly associated with Bacteroides abundance. Anxiety scores correlated with Bacteroides abundance, carbohydrate degradation pathways and gastrointestinal symptoms, while negatively correlating with dietary fiber intake. Ex vivo mucin-to-inulin degradation ratio was higher in GAD and correlated with inflammatory markers.

Conclusions GAD patients exhibited marked gastrointestinal symptoms, elevated immune markers, reduced fiber intake and a Bacteroides-dominated microbiota that preferentially degrades mucin. These data suggest that their microbiota adapted to utilize host-derived carbohydrates that may affect the mucus barrier, altering immune homeostasis and leading to gastrointestinal symptoms and anxiety. Dietary interventions, such as gradually increasing fiber intake, could reprogram bacterial carbohydrate metabolism, thus ameliorating gut barrier function and alleviating anxiety and gastrointestinal symptoms.

The authors have declared no competing interest.

This study was funded by the Canadian Institutes of Health Research (CIHR) Foundation grant #143253 (PB, SMC), the National Institute of Health (NIH) grant #R33 MH108167 (PB, MB) and the Chilean National Agency for Research and Development (ANID)/Scholarship Program: DOCTORADO BECAS CHILE EXTRANJERO (2019).

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The study followed the Helsinki Declaration and was approved by the Hamilton Integrated Research Ethics Board on April 1, 2015 (#14-249).

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