For years, mental health has been viewed as something that begins and ends in the brain. But a growing field of research is revealing a key player in the history of our emotions: the gut. The trillions of bacteria living in our digestive tract do more than just help break down food. They appear to communicate directly with the brain, influencing everything from stress tolerance to symptoms of depression and anxiety. Scientists call this two-way connection the gut-brain axis, and it's rapidly changing the way we think about mood and mental well-being.
This connection is not just theoretical. Research has shown that people with depression often suffer less microbial diversitywhat are probiotics may relieve symptoms Anxiety and dietary changes may change brain chemistry associated with mood within days. “There is now more scientific evidence than ever of a direct link between gut health and brain health,” says Dr. Carl Kwok, a gastroenterologist at Kaiser Permanente in Southern California. “The gut microbiome or other gut bacteria can absolutely affect neuronal function.”
The question scientists are now rushing to answer is how far this influence extends and how we can use it to feel better from the inside out.
If you've ever had butterflies in your stomach before a big presentation, you've experienced the gut-brain connection in action. For decades, scientists assumed that the gut simply followed the brain's instructions. But research now suggests that the trillions of microbes in our gut may play a surprisingly active role in shaping our emotions and mental health.
“The lines of communication between the gut microbiome and the brain appear to involve waste products of gut bacteria, including neurotransmitters,” says Dr. Roy Ziegelstein, a cardiologist and researcher in depression and cardiovascular disease at Johns Hopkins University. “In addition, changes in fats or lipids in the blood and changes in chemicals that cause inflammation may be linked to the gut microbiome and 'talk' to the brain.”
This biochemical “talk” passes through multiple channels—the nervous system, hormones, and immune system—to form the gut-brain axis.
Dr. Catherine Ngo, a gastroenterologist and director of motility at the Hoag Institute for Digestive Health in California, likens it to a busy metropolis. “Imagine your gut is a bustling city and your brain is a town hall on a hill,” she says. “The two need to stay in constant contact to keep things running smoothly.”
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This communication occurs in several key ways. The vagus nerve acts as a direct line between the gut and the brain. The immune pathway transmits messages when inflammation increases or decreases. And the endocrine system broadcasts “updates” about hunger, stress and mood.
“Microbes help decide what programs will be broadcast – a calming channel when everything is balanced, or heavy programming when it is not,” says Ngo.
Research supports this opinion. One study describes how bacterial metabolites, neurotransmitters such as serotonin and GABA, and inflammatory molecules move from the gut to the brain, affecting mood, cognitive function, and the body's response to stress.
In fact, about 90% of the body's serotonin – a neurotransmitter often called the “happy chemical” –produced in the intestinesnot the brain. “It's not surprising that one of the most common classes of drugs used to treat mood disorders, SSRIs, can have gastrointestinal side effects,” Ngo says. “They are working on a system that is deeply interconnected.”
For years, researchers have suspected that microbiome disruptions may contribute to anxiety, depression, and even trauma-related disorders. Large-scale reviews found that people with depression often have lower microbial diversity and higher levels of pro-inflammatory species.
“The majority of the evidence so far links the gut microbiome to depression and anxiety,” says Ziegelstein. “However, other conditions, including bipolar disorder, schizophrenia, autism spectrum disorder, ADHD, post-traumatic stress disorder, anorexia and OCD were also examined.”
Animal studies provide some of the most intriguing clues. “Germ-free mice raised without gut microbes show more anxiety and hyperactivity than normal mice,” Ngo says. “When researchers transplant gut bacteria from people with depression into these animals, they begin to exhibit depression-like behavior.” This finding, supported by numerous studies, supports the fact that gut imbalance may play a role in regulating mood.
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However, human data is more nuanced. “While the evidence that the gut microbiome influences mood is relatively strong, the evidence that changing the gut microbiome improves mood is less compelling,” warns Ziegelstein. This is largely due to small sample sizes and differences in study methods and probiotic formulations.
2021 review V Advances in nutrition concluded that while the gut-brain axis is a promising target for improving mental health, more rigorous research is needed to confirm whether interventions such as probiotics, diet, or prebiotics can reliably prevent or treat mental health disorders.
But the first signs are encouraging. “Recent analysis from the University of Oxford analyzed randomized controlled trials in which some participants received interventions aimed at changing the gut microbiome, says Ziegelstein. “They found that probiotics reduced symptoms of depression and anxiety.” But even the authors of the study admitted that this is far from the last word and more research is needed.
As science advances, one fact is already clear: lifestyle habits have a major impact on the interaction between the gut and the brain. What do you eat, how do you cope with stressAnd how long do you sleep all of which can change the microscopic balance inside your gut and possibly your mood.
“The foods you eat feed your gut microbes, shaping their composition and the metabolites they produce,” Ngo says. “Diet is one of the largest factors influencing the gut microbiome.”
Ziegelstein agrees, noting that “dietary changes seem to have the greatest impact. A healthier gut microbiome is promoted by foods that don't spike blood sugar too much, as well as foods rich in omega-3 fatty acids, such as fish, chia seeds, flaxseeds and walnuts.” He adds that such a diet “is likely to have many other health benefits beyond any potential effects on the gut microbiome or mood.”
“Whole grains and cruciferous fruits and vegetables, such as kale, are good for increasing healthy gut bacteria, which in turn can improve brain health and mood,” says Kwok. “But more than half the average American's diet is based on… ultra-processed foodsand they can, over time, weaken the lining of the digestive tract and cause damage to the brain.”
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US Centers for Disease Control and Prevention (CDC) reports that 57% of the calories in the average American's diet come from ultra-processed foods, which may contribute to chronic inflammation and decreased microbial diversity—key risk factors for mood disorders.
It turns out that the gut microbiome also responds quickly. “It can change over a period of days or even hours depending on diet, stress or medications,” says Ngo. “But the effects on mood are a little more complex and slower.” Rapid changes, such as eating high-fat or high-sugar foods, can alter microbial composition within 24 to 48 hours, although the emotional effects often take days or weeks to become apparent, she adds.
Other lifestyle factors also matter. Chronic stress and poor sleep can reduce beneficial bacteria and increase inflammation, and regular exercise can help restore balance. “It's a symphony of synergy that improves gut bacteria associated with mood,” says Kwok.
With so much buzz surrounding the microbiome, it's easy to see it as the next frontier in mental health care. But experts caution against treating it as a miracle cure.
“The gut-brain connection is real,” says Ngo, “and good nutrition, stress management and gut support can help improve your mood. But these strategies are not a cure for depression or anxiety.” For most people, gut-focused approaches should complement, rather than replace, proven treatments such as therapy or medications.
However, the gut-brain axis opens new doors. In 2024 review V Frontiers in Pharmacology Scientists have found that probiotics can reduce inflammation and pathogens in the gut, and potentially improve cognitive outcomes. “There is evidence that altering the gut microbiome through probiotics, prebiotics, dietary changes, or even fecal microbiota transplants can improve mental health,” says Ziegelstein. “The evidence for the effects of probiotics on depressive symptoms appears strong, although individual studies are too small to be sure.”
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Kwok sees dietary changes as a practical starting point. “If you start taking steps to eat healthier, you'll quickly notice changes in your gut microbiome—within days,” he says. “Major changes will take several months, but the point is to adjust your diet as soon as possible. With these changes, your mood can improve step by step.”
The future of mental health treatment may include both psychotherapy and nutrition. But for now, experts emphasize moderation and scientific rigor. “It is very important for clinicians to remember that the evidence for manipulating the gut-brain connection must be carefully and critically assessed,” says Ziegelstein. “This is a rapidly evolving field, and the strength and limitations of the evidence must be assessed by experts.”
In other words, taking care of your gut is not a replacement for professional care, but may simply be one of the most effective ways to support it. “Health is not one size fits all,” Ngo says. “Guidance from a trusted physician can help you optimize your health and wellness to speed up your healing process.”
