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Over the past decade, researchers have made strides in understanding the link between two seemingly unrelated organs: the gut and the brain. Findings from microbiologists, neurologists, and nutritionists who study the gut-brain axis, or the interaction between these two essential organs, all point to the profound influence the gut has on the brain, and vice versa.
But how exactly does this connection work?
It starts with what's called the gut microbiome, or the trillions of microorganisms harbored by the human gastrointestinal tract. These include bacteria, fungi, and viruses. Some are harmful, but many are quite the opposite—they have health benefits and help the body function.
Gut microbiome composition can rapidly change in response to stress. These changes in turn affect communication between the gut and the brain, which occurs through chemical messengers sent from the endocrine system to the nervous system. Studies show that the recent rise in gastrointestinal conditions such as irritable bowel syndrome and ulcers may be linked to stress, which costs $300 billion per year in lost work and related medical costs.
For instance, a 2024 study in the journal Nature Mental Health conducted by researchers at the University of California, Los Angeles, found that people resilient to stress had a gut microbiome characterized by microbial diversity. That means their guts produce substances that reduce inflammation while also preventing harmful substances like pathogens and toxins from entering. This helps to maintain what's known as gut barrier integrity. Resilience to stress was also associated with fewer symptoms of depression and anxiety, according to the study.
Top Nutrition Coaching reviewed scientific studies and news coverage to find out how our understanding of the interaction of the brain and gut has changed over time.
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How our understanding of the gut-brain axis evolved over time
Scientists have long been curious about the gut-brain axis. Early- to mid-19th-century Britons believed the stomach played a significant role in physical and mental well-being. Doctors published material urging people to eat healthy foods, abstain from alcohol, and encouraged them to eat moderate portions and stick to regular mealtimes.
Excessive amounts of tea were associated with nervous disorders. In 1883, the Dean of Bangor claimed that excessive tea drinking affected the stomach, causing nervousness and other mental health conditions. Daily tea consumption was a convenient explanation for Victorian-era diagnoses such as female hysteria; a pot of tea kept on the stove all day long and being drunk continuously was considered the root of the problem.
Later in the 19th century, however, researchers began to focus on individual organs and tissues rather than taking a more holistic, or interconnected, view of the body. In the 20th century, they began to study the effects of genes and genetics on health. This so-called reductionist view, however, soon proved inadequate.
In the early 20th century, as new psychological sciences emerged, the concept of the gut-brain axis once again became popular. Gastrointestinal conditions were often thought to be stress-related. In the 1970s, the discovery of histamine-2 receptor antagonists like Pepcid and Tagamet used to treat heartburn, acid reflux, and ulcers raised new questions about psychological factors as causes of GI conditions. The unexpected discovery in the 1980s that gastric ulcers were caused by bacteria also raised questions about the gut-brain axis.
Later studies established the extent of the integration between the gut and the brain. In 2022, researchers from Melbourne and Edinburgh, Australia, concluded that the gut microbiome is linked to the regions of the brain responsible for memory, visual processing, and other cognitive functions. Manipulating the gut microbiome seems to be a promising way to enhance emotional well-being in healthy adults in stressful and nonstressful situations.
How stress impacts the GI tract
Today, the relationship between stress and the gut is well-established. When the body is under stress, cells in the GI tract secrete hormones such as norepinephrine and dopamine in response. This occurs in reaction to many well-documented causes of stress, including psychological stressors such as fear, environmental stressors such as noise or pollutants, and physical stressors such as inadequate nutrition or sleep deprivation.
Stress also induces changes in signaling from the vagus nerve, the primary nerve that transmits signals between the heart, brain, and digestive system. These changes affect how food moves through the digestive system and can reduce functionality. This manifests in different ways, including indigestion, loss of appetite, and constipation. During the stress response, blood is also directed away from the GI tract, particularly during vigorous exercise or exposure to extreme heat. This can lead to oxidative stress (i.e., excessive free radicals and a lack of antioxidants in the body) and inflammation in the gut.
As the relationship between the digestive system and thoughts, moods, and emotions becomes increasingly clear, more recent research focuses on how our diets impact mental health issues like anxiety and depression. A 2024 study in the journal Nature Mental Health revealed the potential for dietary modifications or the use of prebiotics, foods for healthy gut microbes, or probiotics, foods or supplements containing live microorganisms that help maintain healthy gut microbes, to improve resilience to stress.
Reducing stress is an important way to support a healthy gut microbiome. Practicing stress management techniques—such as getting regular exercise, practicing meditation and deep breathing, and getting at least seven to eight hours of sleep per night—can all help to reduce stress. Eating a healthy diet high in fiber, whole grains, fruits, and vegetables, and low in refined carbohydrates, processed foods, and saturated fats is also crucial. Fermented foods like sauerkraut, yogurt, and kefir contain probiotics and help introduce healthy bacteria into the gut.
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Story editing by Alizah Salario. Additional editing by Kelly Glass. Copy editing by Tim Bruns. Photo selection by Clarese Moller.
