By Joseph E. Scherger, MD, MPH
Vice President, Primary Care, Eisenhower Medical Center; Clinical Professor, Keck School of Medicine, University of Southern California
Dr. Scherger reports no financial relationships relevant to this field of study.
SYNOPSIS: The human gut microbiome regulates intestinal function and health. There is mounting evidence that the gut microbiome influences the immune system and the central and peripheral nervous systems. This article reviews the bidirectional relationship between the gut microbiome and brain disorders.
SOURCE: Petra AI, et al. Gut-microbiota-brain axis and its effect on neuropsychiatric disorders with suspected immune dysfunction. Clin Ther 2015;37:984-995.
These authors reviewed articles on Medline starting in 1980 for a wide range of neurologic disorders and two systems, the gut-microbiota-brain axis and the hypothalamic-pituitary-adrenal axis. Bidirectional influences exist between the brain and the gut flora that are associated with mood disorders, autism spectrum disorders, attention deficit hypersensitivity disorder, multiple sclerosis, and obesity. This article joins a growing list of other studies illuminating these relationships.1-4
Bacterial dysbiosis, small intestinal bacterial overgrowth, and increased intestinal permeability may produce numerous immunologic effects, including central nervous system (CNS) inflammation. Our mood is affected by these changes. Bacterial proteins cross-react with human antigens to stimulate dysfunctional responses of the immune system that may lead to neurodegenerative disorders.
Communication between the gut and the brain goes both ways. Antibiotics, environmental and infectious agents, intestinal neurotransmitters, sensory vagal fibers, cytokines, and essential metabolites all convey information to the CNS about the intestinal state. The hypothalamic-pituitary-adrenal axis is the CNS regulatory area of satiety, and neuropeptides released from sensory nerve fibers affect the gut microbiota composition directly or through nutrient availability. Such interactions appear to influence the pathogenesis of a number of nervous system disorders, from mood disorders to autoimmune and neurodegenerative conditions to obesity.
COMMENTARY
“You are what you eat” is an age old expression highlighting that we are organisms that depend on food for growth and survival. The title even became a popular diet and TV program in the United Kingdom from 2004-2007. With the emphasis in modern medicine on pharmacologic therapies and procedures, the vital importance of nutrition has been downplayed in human health and disease. Many people eat whatever they want, and healthcare does little to intervene. We continue to have debates on what constitutes a healthy diet.
The recent appreciation of the gut microbiome, the 100 trillion organisms that reside within us, has added a new dimension to this expression. These gut bacteria together weigh about 10 pounds and would occupy a half gallon container. They are a new vital organ to the human species. They completely depend on us for sustenance.
The gut microbiome is an important intermediary between what we eat and our health. The gut bacteria get first crack at what we eat and play a vital role in what gets into our bodies and what happens to these nutrients. A healthy gut microbiome is critical for good health, and an unhealthy gut microbiome assures that we will not be well.
The science around the gut microbiome is in its infancy. The Human Microbiome Project at the NIH was established in 2008.5 The emerging knowledge from this “new organ” is a paradigm shift for medicine. Hopefully, it will usher in renewed interest in human nutrition and its impact on our health.
REFERENCES
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Galland L. The gut microbiome and the brain. J Med Food 2014;17:1261-1272.
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O’Mahony SM, et al. Serotonin, tryptophan metabolism and the brain-gut-microbiome axis. Behavioural Brain Research 2015;277:32-48.
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Perlmuter D. Brain Maker. New York: Little, Brown and Co. 2015.
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Mayer EA, et al. Gut/brain axis and the microbiota. J Clin Invest 2015;125:926-938.
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NIH Human Microbiome Project. Available at: http://hmpdacc.org/. Accessed Aug. 12, 2015.
