The term microbiome refers to the bacteria, fungi, protozoa, archaea, and viruses that live inside the human body. The main areas of microbial colonization in the body are the urogenital tract, airways, eyes, skin, and gastrointestinal tract, and of those, the microbiota are mostly concentrated in the gut. The gut microbiome is believed to contain more than 10 trillion cells and approximately 1,000 different microbial species. Over 99% of the genes in the human body are microbial and number greater than 10 million. 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 through the study of animals that lack a microbiome, called germ-free (GF) animals. Studies with GF animals show many changes in brain health as compared with animals that have an intact microbiome, including a decrease in tight junctions between cells, an increase in the permeability of the blood-brain barrier (BBB), and a decrease in brain-derived neurotrophic factor (BDNF). BDNF is a protein that plays an important role in synaptic plasticity, nervous system modulation, memory formation, and the growth, maintenance, and survival of neurons. Studies have also demonstrated that GF animals experience increases in anxiety-like behavior, a decrease in sociability, and decreases in brain size. 

A recent review article explored the association between intestinal dysbiosis and Parkinson’s disease (PD). PD is a neurodegenerative disorder characterized 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 play a role in the synthesis of neuroactive short-chain fatty acids (SCFA), including butyrate, propionate, and acetate, and the release of folate and thiamine. 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), which can then lead to the overexpression and aggregation of alpha-synuclein — a protein linked to neuroinflammation, motor deficits, and PD. In addition, SCFA decrease the permeability of the BBB and regulate microglia, the cleaning crew of the brain. 

Changes in dopamine levels are also associated with the gut 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 the production of dopamine 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, the presence of toxic bacteria and other pathogens may cause inflammation and metabolic changes which could result 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