Your Gut Is Your "Second Brain" — Here's Why That Matters For Mental Health

Clinical neuroscientist psychiatrist By Daniel Amen, M.D.
Clinical neuroscientist psychiatrist
Daniel Amen, MD, is a clinical neuroscientist psychiatrist, physician, professor and 10-time New York Times bestselling author. He is a double board-certified child and adult psychiatrist and founder of Amen Clinics, Inc.
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Feeling down? Having trouble focusing? Dealing with anxiousness? You may think it's all in your head, but did you know these issues could actually be related to problems with your "second brain"? What's that? In scientific terms, it's the enteric nervous system—but in everyday language, it's your gut.

Why your gut is considered the "second brain."

The gut is often referred to as the second brain because it's lined with approximately 100 million neurons—essentially the same type of nerve cells found inside the 3-pound supercomputer between your ears. Surprisingly, your gastrointestinal tract contains more neurons than your spinal cord or peripheral nervous system.

The little brain in your gut is in direct communication with the big brain inside your skull. All these neurons "talk" to each other, sharing information about your moment-by-moment functioning. This gut-brain connection means emotional and psychological issues often emerge as gastrointestinal distress. It explains why you get butterflies in your stomach before a first date, feel queasy when you have to deliver bad news, or have loose stools when you're stressed.

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How gut health may affect brain & mental health.

A growing body of scientific evidence shows that the relationship between your two brains goes in the other direction as well. Here's how.

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The human GI tract is jam-packed with approximately 100 trillion microorganisms, including bacteria. However, not all bacteria are the same. Some promote physical and mental wellness (good guys), while others fuel disease and disorders (bad guys). As you go through your day, the good guys and bad guys are in a constant battle for control of your gut. To maintain overall health, the good guys need to make up an estimated 85% of the total gut bacteria. When the bad guys proliferate and comprise over about 15%, it spells trouble.

Increasingly, research is finding that gut bacteria have many influences on the brain, including the production of neurotransmitters, such as serotonin, dopamine, and GABA. Neurotransmitters are critical to emotional wellness, and abnormal production levels are commonly seen in a number of psychiatric disorders.

In particular, the following five conditions have been linked to poor gut health:

  • ADHD: A 2021 scientific review lays out evidence supporting a link between ADHD and increased levels of harmful gut bacteria. The good news is it's possible to improve gut health. For example, in a 2019 trial in Scientific Reports, kids with ADHD who took vitamins and minerals showed decreased levels of unhealthy bacteria associated with the condition. (Here are some more tips for supporting your gut.)
  • Depression: For a 2019 study, a team of European researchers analyzed the gut bacteria of depressed people and compared it to those who didn't have the mood disorder. The results? Having depression is associated with a lack of certain types of gut bacteria found in healthy people along with increased levels of harmful bacteria typically associated with Crohn's disease. 
  • Anxiety: When it comes to anxiety disorders, scientific research reveals that unhealthy gut bacteria may be partly to blame. Take the findings of a 2019 review of 21 studies that appeared in General Psychiatry, for example. In 11 of the studies, the researchers analyzed, regulating the gut bacteria with probiotics or other dietary changes decreased anxiety symptoms.
  • Bipolar disorder: People with bipolar disorder tend to have a higher incidence of gut-related conditions, such as inflammatory bowel disease. Researchers are ramping up investigations to explore the associations between gut health and this cyclical mood disorder 
  • Schizophrenia: This psychiatric disorder, which most people view as a problem with the mind, also appears to be associated with gut abnormalities. Individuals with schizophrenia have less gut bacteria diversity as well as unique types of bacteria that aren't found in their healthy counterparts, according to a 2019 study in Science Advances. In this particular trial, when scientists transplanted bacteria from the schizophrenic participants into mice, the lab animals began exhibiting behaviors similar to those seen in mouse models of schizophrenia.

These findings show that the "second brain" in your gut is either helping your brain health and mental health or hurting it. Because of this, it's critically important to eat a healthy diet and consider taking a probiotic to support gut health. (To help, here's everything you need to know about choosing a good probiotic supplement.)

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