The Latest on the Microbiome-Gut-Brain Connection
The term microbiome refers to the bacteria, fungi, protozoa, archaea, and viruses that live inside the human body. The main areas of microbial colonisation in the body are the urogenital tract, airways, eyes, skin, and gastrointestinal tract, and of those, the microbiota are primarily concentrated in the gut.
The human microbiome is extraordinarily diverse and responsive to its environment. Dysbiosis or an imbalance in the microbiome can lead to inflammation, intestinal symptoms, and pathological changes.
Some recent research ties the health of the gut microbiome and brain health. A popular method of researching the microbiome-gut-brain connection is by studying animals that lack a microbiome, called germ-free (GF) animals.
Studies with GF animals show many changes in brain health, including a decrease in tight junctions between cells, an increase in the permeability of the blood-brain barrier (BBB) and a reduction in brain-derived neurotrophic factor (BDNF). BDNF is a protein that plays an essential role in synaptic plasticity, nervous system modulation, memory formation, and neurons’ growth, maintenance, and survival. Studies have also demonstrated that GF animals experience increases in anxiety-like behaviour, a decrease in sociability, and decreases in brain size.
Intestinal dysbiosis and Parkinson’s disease
A recent review article explored the association between intestinal dysbiosis and Parkinson’s disease (PD). PD is a neurodegenerative disorder characterised by motor dysfunctions that include rigidity, tremor, and postural instability, and other symptoms, such as sleep disturbance, anxiety, depression, constipation, along with gastrointestinal (GI) symptoms.
The authors of this study correlate the gut microbiome with certain aspects of PD progression. For example, reduced populations of species in the Prevotellaceae family in the gut microbiome have been linked to individuals with PD. Prevotellaceae synthesise neuroactive short-chain fatty acids (SCFA), including butyrate, propionate, and acetate, and the release of folate and thiamine.
butyrate, alpha-synuclein protein and neuroinflammation
Changes in butyrate concentrations influence the expression of occludin, which may, in turn, impact intestinal permeability. Intestinal permeability may then increase bacterial endotoxins, such as lipopolysaccharides (LPS), leading to the overexpression and aggregation of alpha-synuclein — a protein linked to neuroinflammation, motor deficits, and PD. In addition, SCFA decreases the BBB’s permeability and regulates microglia, the cleaning crew of the brain.
Dopamine and the microbiome.
Almost half of the dopamine in the body is produced in the gastrointestinal tract. Changes in the gut microbiome have been shown to affect dopamine production through the influence on ghrelin. SCFAs were recently shown to influence levels of ghrelin.
The human immune system communicates with its microbiome in the gastrointestinal tract. When dysbiosis occurs, toxic bacteria and other pathogens may cause inflammation and metabolic changes, resulting in pathologic processes affecting various aspects of brain health. Supplementation to support the health of the gastrointestinal microbiome may support cognitive processes.
By Colleen Ambrose ND, MAT