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Friday, November 21, 2014

The Gut Microbiome and the Brain


"Experimental studies with human volunteers and with small mammals demonstrate effects of commensal intestinal bacteria on behavior and brain function that are contextually meaningful and which appear to be biologically significant. Gut bacteria influence reactivity of the HPA axis and the
induction and maintenance of nREM sleep. They may influence mood, pain sensitivity and normal brain development.

Clinical studies have demonstrated distinct pathological CNS effects of commensal gut bacteria in hepatic cirrhosis and short bowel syndrome and have led researchers to speculate on possible adverse effects of gut microbes in alcohol dependence, CFS, fibromyalgia, RLS, ASD, schizophrenia, mood disorders, and degenerative or autoimmune neurologic disease. Adverse effects have been attributed to alterations in bacterial community structure (dysbiosis), SIBO, and increased intestinal permeability.

Several mechanisms, none mutually exclusive, may enable commensal gut bacteria to influence function or dysfunction in the CNS: (1) stimulation of host immune responses leading to diverse patterns of systemic cytokine activation; (2) synthesis of absorbable neuroactive metabolites, including neurotransmitters; and (3) alterations in neuronal circuitry by direct microbial effects on the ENS, with CNS transmission through vagal and other routes. The only mechanisms with a high level of proof in humans are the neurotoxic effects of ammonia in HE and of D-lactic acid in short bowel syndrome.

CNS and neuroendocrine activity, stress responses in particular, may, in turn, influence the composition of the gut microbiome by differentially altering the growth of bacterial species and the production of bacterial virulence factors. Enterobacteriaceae, a family that includes most of the
aerobic Gram-negative pathogens, is especially well tuned to exploiting host stress responses for enhancing bacterial growth and virulence.

Dietary patterns also modify microbiome composition and function, in complex ways that vary among individuals and cultures and are the subject of intense ongoing research. Prebiotics, probiotics, and fermented foods such as yogurt may influence the impact of the gut microbiome on the CNS and have shown significant effects on brain function in a number of experimental trials and clinical studies. Along with diet, these functional food components may offer future opportunities for altering the microbiome to enhance cognitive or emotive function and prevent or treat neurologic disorders."

J Med Food. 2014 Nov 17. [Epub ahead of print]
The Gut Microbiome and the Brain.
http://www.ncbi.nlm.nih.gov/pubmed/25402818

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