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Systemic inflammation, gut microbiota, and anxiety in emerging adulthood: biological, methodological, and conceptual complexity
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Date
2026-07-01
Authors
Cooney-Quane, Jennifer
Journal Title
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Publisher
University College Cork
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Abstract
Anxiety disorders are the most common type of mental health disorder, with prevalence peaks observed during adolescence and emerging adulthood. Emerging adulthood, a developmental stage occurring between the ages of 18 and 25 years, may present as a time of increased anxiety risk due to changes in psychosocial factors, lifestyle behaviours, and ongoing brain development and plasticity. Psychosocial factors such as stress, and lifestyle factors such as diet and physical activity, can influence peripheral systemic inflammation, gut microbiota, and gut microbiota metabolites, which may in turn influence neuroinflammation and alter neurotransmission with the potential to impact on mood. Anxiety can present alongside physical symptoms, such as pain and fatigue, which may be also influenced by inflammatory mechanisms. The overarching aim of this thesis was to examine the relationship between mental health, systemic inflammation, and gut microbiota in emerging adulthood, through a psychoneuroimmunology-grounded conceptual framework. This framework postulates that lifestyle and psychosocial factors influence systemic inflammation and the gut microbiota, and their metabolites, which in turn influence neurobiological processes in the brain involved in mood and pain processing. These relationships are conceptualised as bidirectional, with mental health, such as anxiety, and physical symptoms, such as pain, being both influenced by, and influencing, these pathophysiological processes. Further to this, this thesis also sought to examine how psychobiological processes can be studied in a methodologically rigorous and feasible way, in emerging adult cohorts.
This thesis approached these aims through a series of distinct, but related, studies in both pre-clinical and human research. Chapter 2 presents excerpts of an Encyclopedia of Adolescence chapter, providing an expert guide to the assessment of psychobiological processes and systems, specifically the immune system and the microbiota-gut-brain axis, and how they may play a role in development and wellbeing in adolescence.
Chapter 3 presents a systematic review to synthesise evidence for the impact of antibiotic-induced gut microbiota depletion on anxiety and social behaviours in pre-clinical studies, finding that results were inconsistent across the included studies, and highlighting that methodological limitations were common. Chapter 4, using data from the Avon Longitudinal Study of Parents and Children, explored the relationship between systemic inflammation, mental health, and pain-type somatic symptoms in adolescence, finding no association between anxiety nor depression and c-reactive protein (CRP, a marker of inflammation). While associations were observed between depression and anxiety and pain symptoms, CRP was associated with pain symptoms independently of depression and anxiety symptoms, suggesting that inflammatory processes may be associated with pain-type somatic symptoms independent of mental health symptoms. Chapter 5 was a study designed to explore the relationship between mental health, somatic symptoms, systemic inflammation, and gut microbiota and their metabolites in young adults. This study was discontinued due to significant challenges conducting a research study exploring inflammatory markers in the context of the COVID-19 pandemic, including lockdown restrictions, infection-related biological confounding, and recruitment challenges. Chapter 6 presents an exploration of the barriers and facilitators to young adults participating in research studies that collect biological samples, and via thematic analysis finds that a range of psychosocial, health, and practical factors, such as trust and transparency, motivational and social factors, and accessibility factors, could influence participation.
Collectively, these chapters make empirical, methodological, and conceptual contributions to psychoneuroimmunology, and wider psychobiological research.
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Keywords
Emerging adulthood , Adolescence , Systemic inflammation , Anxiety , Depression , Somatic symptoms , Pain , Gut microbiota
Citation
Cooney-Quane, J. 2026. Systemic inflammation, gut microbiota, and anxiety in emerging adulthood: biological, methodological, and conceptual complexity. PhD Thesis, University College Cork.