Listen to Your Gut; It’s Your Second Brain

Gut flora, gut microbiota, or gastrointestinal microbiota is the complex community of microorganisms that live in the digestive tracts of all vertebrates. The gut microbiota has the largest numbers of bacteria. This microbiome also contains the greatest number of species compared to other areas of the body. An associated term, sometimes used interchangeably with gut microbiota, is gut microbiome, which refers to the aggregate of all gut microbiota outnumbering human cells by 10 to 1. Because of their small size, however, microorganisms make up only about 1 to 3 percent of the body's mass (in a 200-pound adult, that's 2 to 6 pounds of bacteria), but play a vital role in human health.
These bacteria make up a sort of organ whose beneficial functions have only begun to be discovered. Put simply, everything about our health – how we feel both emotionally and physically – hinges on the condition of your microbiome.

Given the extent to which bacteria are now understood to influence human physiology, it is not surprising that considerable research indicates that brain health is dictated by the condition of your microbiome. This biochemical signaling taking place between the gastrointestinal tract and the nervous system is repeatedly linked to intestinal microbiota, and is being referred to as the gut-brain axis and/or your second brain. How gut microbes communicate with the nervous system is by using some of the same neurochemicals that relay messages in the brain. Specifically, The gut microbiota influences the brain by communicating with the central and enteric nervous system through different pathways (neural, immune, and endocrine) that influence the brain’s function and its behavior. Thus, this communication influences the brains’ motor, sensory, autonomic, and secretory functions of the gastrointestinal tract.

This communication allows the microbiota to modulate brain function. Similarly, some of your gut’s microbes can release chemical messengers, or metabolites, that speak to the brain through the vagus nerve. Among these chemicals are the same substances used by our neurons to communicate and regulate mood, like dopamine, serotonin and gamma-aminobutyric acid (GABA). Remarkably, 80-90 percent of the serotonin is manufactured by the nerve cells in your gut; your gut makes more serotonin – the master happiness molecule – than the brain makes in your head.

The gut-brain axis – the brain’s influence on the gastrointestinal tract and the other way around – is now attracting enormous budgets. For instance, the US National Institute of Mental Health (NIMH) in Bethesda, Maryland, has funded seven pilot studies with up to $1 million each to examine the 'microbiome–gut–brain axis'. In 2015, the US Office of Naval Research in Arlington, Virginia, agreed to pump around $14.5 million over the next 6–7 years into work examining the gut's role in cognitive function and stress responses. Pharmaceutical firms, hungry for new leads in treating neurological disorders, are beginning to invest in research related to gut microbes and the molecules that they produce. Without a doubt, the gut and the brain are keeping tabs on each other. The task facing scientists is to delve deeper into the molecular actions that allow this communication to take place, helping us to better understand the huge importance of the gut-brain axis in human health.


  • Perlmutter, David MD; Loberg, Kristin. “Brain Maker: The Power of Gut Microbes to Heal and Protect Your Brain – for Life”, (New York: Hachette Book Group, Inc. 2015), pgs: 9-32, 59-64.
  • Montiel-Castro, A. J.; González-Cervantes, R. M.; Bravo-Ruiseco, G.; Pacheco-López, G. (2013). "The microbiota-gut-brain axis: Neurobehavioral correlates, health and sociality". Frontiers in Integrative Neuroscience 7.
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  • Cryan, J. F.; Dinan, T. G. (2012). "Mind-altering microorganisms: The impact of the gut microbiota on brain and behaviour". Nature Reviews Neuroscience 13 (10): 701–712.
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