What does the latest science tell us about our brain, our gut and our mental health?

Graphic representation in digestive system as an abacus.
(Patrick Hruby / Los Angeles Times)

Over the past few years, I’ve gotten a lot more interested in the mind-body connection and its implications for overall health. And that’s led me to pay closer attention to the gut — mine in particular.

I’ve noticed that I instinctively place my hand on my stomach when I’m anxious or upset. And while I can assure myself that there’s nothing to be nervous about when I’m entering an unfamiliar social situation, my gurgling stomach tells a different story.

These aren’t scientific observations, of course, just more intentional accounting of what we all feel intuitively — that our gut has something to do with mood, and our mood has something to do with our gut.

It seems like y’all are curious about this relationship, too. Danielle, a 32-year-old in Los Angeles, sent us this question: What is the latest research on the gut-brain connection and its relationship to mental health?

A brief history

Ancient healing traditions, including Ayurvedic and Chinese medicine, have long recognized the dialogue between the gut and the brain. More than 2,000 years ago, the Greek physician Hippocrates suggested that all disease begins in the gut.

Cut to the modern era, when in 1909 a British doctor reported that sour milk had cured a patient with “melancholia,” what they called depression at the time. Another physician around the same time alleged he could treat mental illness with kefir. This followed the discovery of bacteria in the gut in the mid-1850s. Some doctors theorized that when certain foods were broken down by microbes, harmful substances were created that could lead to stress and mental disorders, an ailment they called “autointoxication.”

But by the 1930s, Sigmund Freud’s psychoanalytic theory and its emphasis on the unconscious mind dominated modern thought on mental health. Then came the discovery of the first chemical antidepressant, imipramine, in the 1950s. Scientific interest in the gut-brain connection all but disappeared — until recently.

In 2004, the first modern-day study showing a potential link between stress and the gut microbiome — a complex ecosystem of bacteria, archaea, viruses and fungi — was published. Before then, the suggestion that our gut’s 100 trillion microbiota could be a factor in mental health was met with eye rolls from the Western medical community, according to experts I spoke with for this newsletter.


Now, hundreds of researchers around the world are trying to figure out how the connection works, and what it might mean for the treatment of mental health disorders.

“We’re in the middle of a paradigm shift in medicine, where the brain is being reintegrated back into the rest of the body,” Dr. Emeran A. Mayer, the director of UCLA’s G. Oppenheimer Center for Neurobiology of Stress and Resilience, told me.

What we know so far

Scientists agree that the mind and the gut — which spans from your mouth to the end of your digestive tract — are in constant communication. The gut has its own nervous system, which has been nicknamed “the second brain” because it can act independently of the brain in your head, and communicate directly with the central nervous system. The details of that relationship are still being sussed out, said Sarkis Mazmanian, a medical microbiologist at Caltech.

It’s likely that the gut-brain axis, as researchers call it, sends messages through a few pathways:

  • The longest cranial nerve in the body, the vagus nerve, sends information from the gut to the brain using neurotransmitters (like serotonin and dopamine), the body’s chemical messengers that help regulate sleep, mood, pain, stress and hunger.

    • Fun fact: A staggering 95% of serotonin, often dubbed your body’s “feel good” chemical or hormone, is made in the gut.
  • Scientists believe another form of communication might be when our gut microbiota make compounds that travel through our blood to the brain. Once there, these compounds interact with neurons, which might shift our mood, Mazmanian said.
  • Our immune system also likely plays a key role. Researchers speculate that when the gut’s delicate microbial balance is upset, it sends a message to the immune system, which may trigger gastrointestinal inflammation.

A different kind of dual diagnosis

Along with affecting our day-to-day mental health, the gut could also play a role in the development or severity of mental illness.

People with gastrointestinal disorders are much more likely to live with psychiatric conditions, including bipolar disorder and depression. And people who live with schizophrenia have higher rates of GI inflammation. “The patients I see with IBS [irritable bowel syndrome] almost always have mental and emotional symptoms, ranging from mild anxiety to an anxiety disorder,” Mayer said.

Although there’s a clear correlation between gastrointestinal and mental health conditions, there’s no ironclad scientific proof (yet) that gut inflammation or microbial imbalance leads to psychological distress in humans or vice versa, Mazmanian said.


To date, most studies on the gut-brain connection have been conducted on animals. One study from the Chongqing Medical University in China found that the gut microbiomes of people diagnosed with major depressive disorder were very different from those of the general population. Researchers took fecal matter — yep! — from people living with depression and transplanted it into bacteria-free mice. Compared with a control group, those mice behaved in more anxious and depressive ways.

This study and others have motivated scientists to figure out whether a more balanced microbiome might lead to better mental health or play a role in the treatment of mental illness. Gut bacteria can be changed through the foods we eat, or probiotic and prebiotic supplements. So far, certain probiotics have been shown to reduce stress and anxiety behaviors in rodents, while prebiotics (essentially food for probiotics) may have an antidepressant effect on mice, reversing the impact of chronic stress.

Wild stuff. But needless to say, mice aren’t people. Some studies have been conducted on humans, but they’ve been pretty small, so the findings can’t be generalized. For example, researchers at the Sheppard Pratt Health System in Baltimore gave probiotics shown to have anti-inflammatory properties to 33 patients who had been hospitalized for mania; 33 others got a placebo. Participants took the probiotics or the placebo for a few months, along with their regular meds. Eight of the people taking the probiotics were re-hospitalized during that time, compared with 24 people in the placebo group.

And in two recent case studies, Australians diagnosed with bipolar disorder saw noticeable improvements in their mental health after they were given fecal transplants donated by people with more balanced microbiomes. One woman, who had tried more than a dozen medications to treat the disorder with no luck, reportedly became symptom-free over the next five years.

For us to have a true understanding of how altering the human microbiome could influence mental health, there will need to be more robust studies with much larger sample sizes, experts cautioned.

“What is the optimal diet? What is the optimal probiotic? I’m not sure we know what those interventions are yet,” Mazmanian said. “A lot of this is going to be trial and error, and very personalized. You and I might respond to stress differently, and our microbiomes are unique.”

Is the proof in the fermented pudding?

When it comes to boosting your probiotic intake, the general consensus among the researchers I interviewed was that it won’t hurt you. Fermented foods like yogurt, sauerkraut, kimchi and miso are rich in probiotics and, combined with other nutritious foods, may contribute to a healthy gut, and thus to a healthier brain. But probiotic supplements don’t require FDA approval, so it’s hard to know what’s inside those pills (talk with your doctor or naturopath if you want some guidance).

Though the research still has a long way to go, some doctors are already folding gut health into their treatment plans. The emerging field of nutritional psychiatry asks patients to consume more nutritious foods — namely grains and plants packed with vitamins, minerals, antioxidants, fiber and pre- and probiotics — potentially to reduce inflammation and boost mood.

Dr. Uma Naidoo, who directs the Department of Nutritional and Lifestyle Psychiatry at Massachusetts General Hospital, is a nutritional psychiatrist and trained chef. Naidoo told me that figuring out which kinds of foods might contribute to one person’s healthy microbiome is a “marathon, not a sprint.”

Naidoo’s patients most commonly come to her with depression and anxiety and problems with attention and sleep. She might recommend many different combinations of food to a patient, observing their reactions closely over time, before she sees improvements. Such interventions should go hand-in-hand with psychotherapy and aren’t meant to replace prescription drugs, Naidoo said.

So what can we do with all of this information, right now, with all the uncertainty that surrounds it? One tidbit from Dr. Glenn Treisman, a professor of psychiatry and medicine at Johns Hopkins University, really stuck with me: “A happy microbiome is a diverse microbiome.” That means eating a wide variety of vegetables, fruits, beans, nuts, whole grains, and maybe some of those fermented foods — but that all depends on what agrees with your own special gut.

Now if you’ll excuse me, I have a lunch of sauerkraut and yogurt calling my name. Just kidding.

Until next week,


If what you learned today from these experts spoke to you or you’d like to tell us about your own experiences, please email us and let us know if it is OK to share your thoughts with the larger Group Therapy community. The email gets right to our team.
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