Ganci, Michael (2021) Has Psychology Ignored Our Gut Feelings? Exploring the Relationship Between Gut Microbiota and Psychological Symptoms: A Call for a Paradigm Shift. PhD thesis, Victoria University.

Abstract

The profession of psychology has long been entrenched in a traditional central nervous system (CNS) centric framework. This specialisation has had its benefits and contributed to current knowledge of psychological symptoms and disorders. However, this reductionist approach has led to gaps in knowledge that will continue to persist without a broader appreciation of the complexity of the human body. Broader consideration of bodily systems may provide greater insight into the aetiology, diagnosis, treatment, and prevention of psychological symptoms and disorders. The enteric nervous system (ENS) and its resident gut microbiota (GM) has emerged as a peripheral influence on psychological functioning. The GM refers to the trillions of microorganisms residing in the gut including, but not limited to, bacteria, fungi, and protozoa. The GM has coevolved with its human hosts to share a highly complex multidirectional relationship. In a state of symbiosis, GM play a key role in protecting against pathogen colonisation, strengthening and maintaining the epithelial barrier, and nutrient absorption through metabolism, therefore promoting host health. On the other hand, in a state of dysbiosis (imbalances in the composition of GM), this mutually beneficial relationship between host and GM shifts towards a more antagonistic one. Dysbiosis of GM, as well as specific gut microbes themselves, have been associated with a wide range of psychological symptoms and disorders. To date, of the organisms that reside within the GM, bacteria have received the majority of attention in brain-gut-microbiota axis (BGMA) research. This thesis broadly aims to position the BGMA as falling within the purview of psychologists, while also exploring the concept of the microgenderome in a series of three papers. Paper 1 is a review paper which aimed to demonstrate that GM are intrinsically linked with each stage of psychological disorder, from aetiology through to treatment and prevention. The paper was framed around the Four P model of case formulation, often used in psychological practice. With the neglect of focus on other microorganisms, paper 2 was the first to investigate the effect of these protozoa on psychological symptom severity. Specifically, Paper 2 presents the results of a cross-sectional, retrospective study of the differences in Depressive, Neurocognitive, Stress and Anxiety, and Sleep and Fatigue symptom severity between individuals negative for intestinal protozoa (n= 563) compared to those positive for common protozoa Blastocystis sp. (n= 274), Dientamoeba fragilis (n= 69), or both (n= 73). The findings demonstrated that there was no statistically significant effect of either protozoan, or co-carriage, on psychological symptom severity for either males or females. Utilising correlational analyses, a retrospective cross-sectional exploration of the association between GM and Depressive, Neurocognitive, Stress and Anxiety, and Sleep and Fatigue symptom severity was carried out in Paper 3. While the overall sample was made up of 4610 clinically diverse participants, sample size for each correlational analysis was dependent on available data. The pattern of associations between several GM species and psychological symptom severity were distinctly different between males and females, providing support for the microgenderome. The results demonstrated that some bacterial species found in common probiotic supplements were positively correlated with symptom severity. The results provide support for the notion that, in future, modulation of GM may be appropriate as an ancillary treatment of psychological symptoms, however further research is needed before their implementation in treatment plans. Collectively, this thesis demonstrates that expanding the CNS-centric approach to include peripheral systems may revolutionise the way that psychological illness, and its prevention and treatment are conceptualised. Future directions for research and clinical practice are discussed which include methodological and practical challenges that must be overcome to substantiate the need for a paradigm shift for the discipline of psychology.

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