Disturbances in gut microbiota are associated with depletion of anti-inflammatory bacteria and proliferation of proinflammatory bacteria, a pattern tied to several major psychiatric disorders including depression, bipolar disorder (BD), schizophrenia, and anxiety, new research shows.
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A meta-analysis of 59 studies, encompassing roughly 2,600 patients with psychiatric conditions, showed a decrease in microbial richness in patients with psychiatric conditions versus controls.
In addition, those with depression, anxiety, BD, and psychosis had a similar set of abnormalities in the microbiota, particularly lower levels of Faecalibacterium and Coprococcus – two types of bacteria that have an anti-inflammatory effect in gut – and higher levels of Eggerthella, a bacterium with proinflammatory effects.
“The wealth of evidence we have summarized clearly demonstrates that the gut microbiota is vitally important to the wider mental health of individuals,” lead author Viktoriya Nikolova, MRes, Centre for Affective Disorders, King’s College London, said in an interview.
“While it is still too early to recommend specific interventions, it’s clear that clinicians need to place a greater awareness of gut health when considering the treatment of certain psychiatric disorders,” she said.
The study was published online Sept. 15, 2021, in JAMA Psychiatry.
Reliable biomarkers
“Evidence of gut microbiota perturbations has accumulated for multiple psychiatric disorders, with microbiota signatures proposed as potential biomarkers,” the authors wrote.
However, “while there is a wealth of evidence to suggest that abnormalities within the composition of the gut microbiota are connected to a number of psychiatric disorders, there haven’t been any attempts to evaluate the specificity of this evidence – that is, if these changes are unique to specific disorders or shared across many,” Ms. Nikolova said.
Previous research in individual disorders has identified “patterns that may be promising biomarker targets,” with the potential to “improve diagnostic accuracy, guide treatment, and assist the monitoring of treatment response,” the authors noted.
“We wanted to see if we could reliably establish biomarkers for individual conditions in an effort to further our understanding of the relationship between mental illness and gut microbiota,” said Ms. Nikolova.
The researchers wanted to “evaluate the specificity and reproducibility of gut microbiota alterations and delineate those with potential to become biomarkers.”
They identified 59 studies (64 case-control comparisons; n = 2,643 patients, 2,336 controls). Most (54.2%) were conducted in East Asia, followed by Westernized populations (40.7%) and Africa (1.7%).
These studies evaluated diversity or abundance of gut microbes in adult populations encompassing an array of psychiatric disorders: major depressive disorder (MDD), BD, psychosis and schizophrenia, eating disorders (anorexia nervosa and bulimia nervosa), anxiety, obsessive-compulsive disorder (OCD), PTSD, and ADHD.
Although studies were similar in exclusion criteria, few attempted to minimize dietary changes or control dietary intake. In addition, use of psychiatric medication also “varied substantially.”
The researchers conducted several analyses, with primary outcomes consisting of “community-level measures of gut microbiota composition (alpha and beta diversity) as well as taxonomic findings at the phylum, family, and genus levels (relative abundance).”
Alpha diversity provides a “summary of the microbial community in individual samples,” which “can be compared across groups to evaluate the role of a particular factor (in this case psychiatric diagnosis) on the richness (number of species) and evenness (how well each species is represented) in the sample.”
Beta diversity, on the other hand, “measures interindividual (between samples) diversity that assesses similarity of communities, compared with the other samples analyzed.”
Control samples consisted of participants without the relevant condition.