The “human microbiota” describes all microorganisms within the human body, including bacteria, viruses, and eukaryotes. The related term “microbiome” refers to the complete catalog of these microbes and their genes.1 There is a growing awareness that the human microbiota plays an important role in maintaining mental health, and that a disruption in its composition can contribute to manifestations of psychiatric disorders. A growing body of evidence has also linked mental health outcomes to the gut microbiome, suggesting that the gut microbiota can modulate the gut-brain axis.2
Numerous neurotransmitters, including dopamine, serotonin, gamma-aminobutyric acid, and acetylcholine, are produced in the gastrointestinal (GI) tract, and our diet is vital in sustaining and replenishing them. At the same time, our brain regulates our GI tract by secretion of hormones such as oxytocin, leptin, ghrelin, neuropeptide Y, corticotrophin-releasing factor, and a plethora of others. Dysregulation of this microbiome can lead to both physical and mental illnesses. Symptoms of psychiatric disorders, such as depression, psychosis, anxiety, and autism, can be a consequence of this dysregulation.2
Our diet can also modify the gut microorganisms and therefore many of its metabolic pathways. More attention has been given to pre- and probiotics and their effects on DNA by epigenetic changes. One can quickly start to appreciate how this intricate crosstalk can lead to a variety of pathologic and psychiatric problems that have an adverse effect on autoimmune, inflammatory, metabolic, cognitive, and behavioral processes.2,3
Thus far, links have mostly been reported in animal models, and human studies are limited.4 Researchers are just beginning to elucidate how the microbiota affect gut-brain signaling in humans. Such mechanisms may include alterations in microbial composition, immune activation, vagus nerve signaling, alterations in tryptophan metabolism, production of specific microbial neuroactive metabolites, and bacterial cell wall sugars.5 The microbiota-gut-brain axis plays a part in regulating/programming the hypothalamic-pituitary-adrenal (HPA) axis throughout the life span.3 The interactions between the gut microbiome, the immune system, and the CNS are regulated through pathways that involve endocrine functions (HPA axis), the immune system, and metabolic factors.3,4 Recent research focusing on the gut microbiome has also given rise to international projects such as the Human Microbiome Project (Human Microbiome Project Consortium, 2012).3
Several studies have looked into psychiatry and inflammatory/immune pathways. Here we review 3 recent studies that have focused on the gut-brain axis (Table6-8).
1. Rudzki L, Pawlak D, Pawlak K, et al. Immune suppression of IgG response against dairy proteins in major depression. BMC Psychiatry. 2017;17(1):268.
The aim of this study was to evaluate immunoglobulin G (IgG) response against 40 food products in patients with depression vs those in a control group, along with changes in inflammatory markers, psychological stress, and dietary variables.6
- N = 63, IgG levels against 44 food products, cortisol levels, tumor necrosis factor (TNF)-alpha, interleukin 6 (IL-6), and IL-1 beta levels were recorded. The psychological parameters of 34 participants with depression and 29 controls were compared using the Hamilton Depression Rating scale, (HAM-D-17), Perceived Stress scale, and Symptom Checklist scale. The study was conducted in Poland.
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