The bi-directional crosstalk between the gut microbiome and the brain, called the gut-brain axis, is a complex process that influences many body systems. While psychopathology in the brain is still not fully understood, clinical evidence suggests that a healthy gut microbiome may help support many aspects of mood health and brain function. 

A recently published review article by Varesi and colleagues explored new findings regarding the role of the gut microbiome and the gut-brain axis on overall mood health. The authors describe the potential biochemistry involved. In healthy individuals, the metabolites from commensal gut microbes, including short-chain fatty acids (SCFAs), vitamins, and amino acids, help support intestinal epithelial health and brain homeostasis. In individuals who may suffer from mood disorders like depression, changes to intestinal immunity and increased proinflammatory cytokine activity (e.g., interleukin [IL]-6, TFN-α, and interferon-γ) have been observed. Neuroinflammation, decreased neurotrophins (e.g., brain-derived neurotrophic factor [BDNF]), and increased intestinal permeability may potentially play a role in this patient population. 

In several studies, certain parameters associated with inflammation and increased gut permeability have been associated with the incidence of depression. These include IL-6, zonulin, lipopolysaccharide (LPS) binding protein, C-reactive protein (CRP), and intestinal fatty acid binding protein (I-FABP). I-FABP is released during gut mucosal damage. 

The microbial composition of the gut may be correlated with certain aspects of brain health and mood. For instance, reductions in Firmicutes populations and increases in Bacteroidetes and Proteobacteria have been observed in instances of major depressive disorder (MDD). 

Clinical studies indicate that probiotic administration may help promote a healthy gut microbial composition and may help support the body’s response to mood disorders like general anxiety and depression. Probiotics may influence mood health and neurological and brain function through neural, metabolic, neuroendocrine, and immune biochemical pathways. They may also support the production of neurotransmitters, a healthy inflammatory response, and healthy expression of BDNF levels. Furthermore, SCFA metabolites play a critical role in supporting normal inflammation and the gut-brain axis.

Several randomized clinical studies outlined in the publication by Varesi and colleagues have explored the efficacy of strains from the Bifidobacterium and Lactobacillus genera, or a combination of the two, with a typical study length between four and nine weeks. As compared to placebo, improvements in depression and quality of life scores and parameters associated with inflammation, brain function, and gut health were observed. These include improvements in BDNF levels, IL-6 activity, limbic reactivity, and high-sensitivity CRP. However, more data is needed before clinical conclusions can be made. Particularly, larger studies with repeatable primary outcomes and standardized dosing are needed. Evidence suggests that probiotics may help certain aspects of brain health. They may also help support the inflammatory response, intestinal barrier health, and cognitive function. 

By Dr. C. Ambrose, ND, MAT