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How the Microbes Living in Your Gut Might Be Making You Anxious or Depressed

Researchers have made some surprising findings.

Photo Credit: Robert Kneschke /


Microbes are in the news these days. Specifically, the microbes that live in and on the human body, making up our “microbiome.” Michael Pollan made a splash with a column titled “ Some of My Best Friends are Germs” about a year ago, and now Martin Blaser, director of the Human Microbiome Project at NYU, has published a book called Missing Microbes: How the Overuse of Antibiotics Is Fueling Our Modern Plagues.

In a short period of time, bacteria, fungi and other microbes have gone from enemy to friend in the public consciousness. 

But in addition to the many studies finding out about the numbers and diversity of the microbes with whom we share our bodies and their roles in our nutrition and immune function, some researchers have made some surprising findings: the bugs in your gut might actually impact your emotions.

The bidirectional connection between our brains and our guts is not news. When we are hungry, full, queasy, or suffering from gas or constipation, our guts let our brains know. And our emotions can easily impact how we feel in our guts, like when one has “butterflies in the stomach.” The link between emotions and the gut is so strong that we talk about “gut instinct” or gut feelings.

What’s more, the human gut is connected to the brain by the vagus nerve. Within our guts, we have what is called the “enteric nervous system” (ENS), which is so significant it is often referred to as a “second brain.” 

Premysl Bercik, an associate professor of gastroenterology at McMaster University, is one scientist on the forefront of researching the link between gut and emotions. He began by studying low-grade gut inflammation in patients with bowel disorders like irritable bowel syndrome (IBS) and functional dyspepsia. “These are basically patients who do not have any structural abnormalities but their gut misbehaves,” he explains. Over the years, his research drifted to studying how microbiota impact gut function and “from there it was only a step” to look at how they impact emotions.

Researchers like Bercik made the leap because a significant portion of their patients suffer from psychiatric co-morbidities like anxiety and depression. Was there a link between what was happening in their guts and what was happening in their brains? And, if so, what was it? Could it be that a person with a constant tummy ache feels extra anxiety simply because he or she feels lousy all the time? Or is it something more than that? 

As early as 2004, scientists questioned whether probiotics might benefit patients suffering from major depression. As a 2009 journal article put it, “While evidence is still limited in psychiatric illnesses, there are rapidly coalescing clusters of evidence which point to the possibility that variations in the composition of gut microbes may be associated with changes in the normal functioning of the nervous system.” In other words, we know there might be something there, but we don’t know enough about it just yet.   

Bercik was among those who wanted to know more. He and his team “began looking at communication between gut and brain, how it can impact function of host and what is the role that microbiota plays in all of this.”  

Several animal studies by Bercik and others showed that acute and chronic gut inflammation resulted in changes in behavior and central nervous system biochemistry, although scientists could not yet explain how or why. In 2010, Bercik and others published a study (funded in part by the food conglomerate Nestlé) that found that “mild gut inflammation caused by chronic parasitic infection… induces anxiety-like behavior in mice.”