For years, mental health was seen as something that started and ended in the brain. But an expanding field of research is revealing a key player in the story of our emotions: the gut. The trillions of bacteria that live in our digestive tract don’t just help break down food. They appear to communicate directly with the brain, influencing everything from stress resilience to symptoms of depression and anxiety. Scientists call this two-way relationship the gut–brain axis, and it’s rapidly reshaping how we think about mood and mental well-being.

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That connection isn’t just theoretical. Studies have found that people with depression often have less microbial diversity, that probiotics may ease symptoms of anxiety, and that dietary shifts can alter mood-related brain chemistry within days. “There is now more scientific proof than ever of a direct link between gut health and brain health,” says Dr. Karl Kwok, a gastroenterologist with Kaiser Permanente in Southern California. “The gut microbiome or other gut bacteria can absolutely impact neuronal function.”

The question scientists are now racing to answer is how far that influence goes—and how we might harness it to feel better, from the inside out.

The science of the gut–brain connection

If you’ve ever had butterflies in your stomach before a big presentation, you’ve felt the gut–brain connection in action. For decades, scientists assumed the gut simply followed the brain’s lead. But research now suggests that the trillions of microbes in our intestines may play a surprisingly active role in shaping our emotions and mental health.

“The communication lines between the gut microbiome and the brain seem to involve products of intestinal bacteria, including neurotransmitters,” says Dr. Roy Ziegelstein, a cardiologist and researcher on depression and cardiovascular disease at Johns Hopkins. “In addition, changes in fats or lipids in the blood and changes in chemicals that cause inflammation may be related to the gut microbiome and ‘talk’ to the brain.”

These biochemical “conversations” travel through multiple channels—the nervous system, hormones, and the immune system—forming the gut–brain axis.

Dr. Catherine Ngo, a gastroenterologist and director of motility for the Hoag Digestive Health Institute in California, likens it to a busy metropolis. “Imagine your gut is a bustling city and your brain is town hall up on the hill,” she says. “The two have to stay in constant touch to keep everything running smoothly.”

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That communication happens along several key pathways. The vagus nerve acts as a direct line between the gut and the brain. The immune pathway relays messages when inflammation rises or falls. And the endocrine system broadcasts “updates” about hunger, stress, and mood.

“Microbes help decide which programs get aired—a soothing channel when things are balanced, or stress-heavy broadcasts when they’re not,” says Ngo.

Research backs this notion up. One study describes how bacterial metabolites, neurotransmitters like serotonin and GABA, and inflammatory molecules travel from the gut to the brain, influencing mood, cognition, and the body’s stress response.

In fact, about 90% of the body’s serotonin—the neurotransmitter often called the “happy chemical”—is made in the gut, not the brain. “It’s not surprising that one of the most common drug classes to treat mood disorders, SSRIs, can have gastrointestinal side effects,” says Ngo. “They’re working on a system that’s deeply interconnected.”

Mental-health implications

For years, researchers have suspected that disruptions in the microbiome might contribute to anxiety, depression, and even trauma-related disorders. Large-scale reviews have found that people with depression often have less microbial diversity and higher levels of pro-inflammatory species.

“Most of the evidence so far ties the gut microbiome to depression and anxiety,” says Ziegelstein. “However, other conditions, including bipolar disorder, schizophrenia, autism spectrum disorder, ADHD, PTSD, anorexia, and OCD have also been examined.”

Animal studies provide some of the most intriguing clues. “Germ-free mice—those raised without gut microbes—show more anxiety and hyperactivity than normal mice,” says Ngo. “When researchers transplant gut bacteria from people with depression into these animals, they start to exhibit depressive-like behaviors.” That finding, confirmed by multiple studies, strengthens the case that gut imbalances may play a role in mood regulation.

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Still, human data is more nuanced. “While the evidence that the gut microbiome affects mood is relatively strong, evidence that changing the gut microbiome improves mood is not as strong,” Ziegelstein cautions. That’s largely due to small sample sizes and differences in study methods and probiotic formulations.

A 2021 review in Advances in Nutrition concluded that while the gut–brain axis is a promising target for improving mental health, more rigorous trials are needed to confirm whether interventions—such as probiotics, diet, or prebiotics—can reliably prevent or treat mental-health disorders.

But early signals are encouraging. “A recent analysis from the University of Oxford reviewed randomized controlled trials in which some participants received interventions to alter the gut microbiome,” says Ziegelstein. “They found that probiotics reduced symptoms of depression and anxiety.” But even the study authors acknowledged that this is hardly the last word, and more research is needed.

Everyday influences on the microbiome

As science evolves, one fact is already clear: lifestyle habits have a major impact on the gut–brain conversation. What you eat, how you handle stress, and how much you sleep can all change the microscopic balance inside your intestines—and, potentially, your mood.

“The foods you eat feed your gut microbes, shaping their composition and the metabolites they produce,” says Ngo. “Diet is one of the biggest influencers of your gut microbiome.”

Ziegelstein agrees, noting that “dietary changes seem to have the greatest influence. A healthier gut microbiome is fostered by foods that do not raise blood sugar much and by foods rich in omega-3 fatty acids—like fish, chia seeds, flaxseed, and walnuts.” He adds that this kind of diet “is likely to have many other health benefits apart from any potential effect on the gut microbiome or mood.”

“Whole grains, fruits, and cruciferous vegetables like kale are helpful for amplifying healthy gut bacteria, which in turn can improve brain health and mood,” says Kwok. “But more than half of the average American diet is based on ultra-processed foods, and those can weaken the mucus lining of the digestive tract over time and wreak havoc on the brain.”

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The U.S. Centers for Disease Control and Prevention (CDC) reports that 57% of calories in the average American diet come from ultra-processed foods, which can contribute to chronic inflammation and reduce microbial diversity—key risk factors for mood disorders.

The gut microbiome, it turns out, is also quick to respond. “It can shift in days or even hours in response to diet, stress, or medication,” Ngo says. “But the effect on mood is a bit more complex and slower.” Rapid changes, like a high-fat or high-sugar meal, can alter microbial composition within 24–48 hours, though emotional effects often take days or weeks to surface, she adds.

Other lifestyle factors matter too. Chronic stress and poor sleep can reduce beneficial bacteria and heighten inflammation, while regular exercise helps restore balance. “It’s a symphony of activities working together that improves gut bacteria linked to mood,” says Kwok. 

What this means for treatment and prevention

With so much buzz around the microbiome, it’s easy to see it as the next frontier of mental-health care. But experts warn against viewing it as a magic bullet.

“The connection between your gut and brain is real,” says Ngo, “and eating well, managing stress, and supporting your gut can help boost your mood. But these strategies are not a cure for depression or anxiety.” For most people, gut-focused approaches should complement—not replace—proven treatments like therapy or medication.

Still, the gut–brain axis is opening new doors. In a 2024 review in Frontiers in Pharmacology scientists found probiotics may reduce inflammation and pathogenic microbes in the gut, as well as potentially improve outcomes for cognition. “There is evidence that changing the gut microbiome by probiotics, prebiotics, dietary changes, or even fecal microbiota transplantation can improve mental health,” says Ziegelstein. “The evidence seems strongest for probiotics on symptoms of depression, though individual studies are too small to be certain.”

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Kwok sees dietary change as a practical entry point. “If you start taking steps to eat healthier, you’ll notice gut microbiome changes quickly—within a few days,” he says. “Significant shifts take a few months, but that’s the point of adjusting your diet as soon as possible. Mood can improve step-by-step with those changes.”

The future of mental-health treatment may involve both psychotherapy and nutrition. But for now, experts emphasize moderation and scientific rigor. “It is critical for clinicians to remember that 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 need to be evaluated by experts.”

In other words, nurturing your gut is no replacement for professional care, but it may just be one of the most powerful ways to support it. “Health is not one-size-fits-all,” says Ngo. “The guidance of a trusted physician can help you optimize your health and wellness to get you on your healing journey faster.”