A few years ago, doctors began noticing something unusual about many patients who were later diagnosed with Parkinson disease. Long before tremors or stiffness appeared, many of these individuals had been struggling with digestive problems. Some had lived with chronic constipation for years. Others noticed subtle changes in digestion that seemed unrelated to the brain. At first these symptoms were dismissed as minor complications of the disease. But over time scientists began asking different questions. What if these digestive changes were not simply consequences of Parkinson disease? What if the earliest stages of the disease might actually begin in the gut? That question has helped launch one of the most fascinating frontiers in modern neuroscience: understanding how the gut and the brain communicate. Researchers now refer to this communication system as the gut–brain axis, a complex biological network linking the nervous system, immune signaling, metabolism, and the trillions of microbes that live inside our digestive tract. What once sounded like a metaphor when people said 'trust your gut' is increasingly becoming a scientific reality. The scale of this hidden world inside us is astonishing. Scientists estimate that the human body hosts tens of trillions of microorganisms, many of them residing in the digestive tract. Large studies from the Human Microbiome Project, published in the journal Nature, revealed that the genetic material carried by these microbes vastly outnumbers roughly twenty thousand genes in the human genome. In many ways, our biology is shaped not only by our own DNA but also by the genetic potential of the microbes living within us. Hidden within the walls of the digestive tract is another extraordinary system. The enteric nervous system contains well over 100 million neurons, forming an intricate neural network that regulates digestion and communicates continuously with the brain. Because of its complexity, scientists often describe it as the body's 'second brain,' a concept popularized by neuroscientist Michael Gershon, whose research helped to revel deep neural circuitry embedded in the gut. The gut and brain communicate constantly through several pathways. One of the most important is the vagus nerve, a long neural highway connecting the brainstem to organs throughout the body, including the stomach and intestines. Through this pathway, signals travel back and forth between the digestive system and the brain, conveying information about digestion, inflammation, and internal bodily states. But the story becomes even more remarkable when we consider what microbes are doing inside the body. Scientists have discovered that many gut bacteria produce molecules that interact with the nervous system. Some generate compounds related to neurotransmitters such as serotonin and gamma-aminobutyric acid (GABA), which play critical roles in mood and emotional balance. In fact, researchers estimate that about 90 to 95 percent of the body's serotonin is produced in the gastrointestinal tract, highlighting the deep connection between digestion and brain signaling. A widely cited review in Nature Reviews Neuroscience described how microbes influence the brain through neural, immune, and hormonal pathways. Another major synthesis published in Physiological Reviews explained how microbial metabolites, immune signals, and neural circuits together form an integrated communication system linking the gut and brain. More recent discoveries suggest that this connection may be even more direct. Researchers reporting in the journal iScience recently demonstrated that specific microbial metabolites in the intestine can activate receptors that influence signaling in the vagus nerve. In simple terms, chemicals produced by gut microbes can alter nerve activity that ultimately reaches the brain. Animal experiments have provided further clues. Studies of mice raised in completely sterile environments without any gut microbes show that these animals exhibit differences in stress responses, brain development, and social behavior. When microbial communities are introduced into their systems, some of these behavioral differences begin to normalize. These findings suggest that microbes living in our digestive tract may play an important role in shaping how the brain develops and responds to the environment. One of the most intriguing areas where this research is gaining attention is Parkinson disease. Parkinson disease affects more than 10 million people worldwide, making it one of the most common neurodegenerative disorders. Global analyses published in The Lancet Neurology show that the number of people living with Parkinson disease has more than doubled over the past three decades. The disease is best known for movement-related symptoms such as tremors, muscle stiffness, slow movement, and problems with balance. These symptoms arise when dopamine-producing neurons in the substantia nigra gradually degenerate. Yet many patients experience digestive symptoms long before these motor problems appear. Research published in The Lancet Neurology has shown that gastrointestinal dysfunction, including chronic constipation, can appear years or even decades before the onset of motor symptoms. More than two decades ago, the German neuropathologist Heiko Braak proposed an influential hypothesis suggesting that Parkinson disease might begin outside the brain, possibly in the digestive tract or the olfactory system. According to this theory, abnormal forms of a protein called alpha-synuclein may originate in peripheral tissues and gradually spread through connected neural pathways into the brain. This idea gained further attention when scientists published a landmark study in the journal Cell. In that study, researchers found that gut microbes could influence motor symptoms and neuroinflammation in experimental models of Parkinson disease. Animals raised without gut microbes developed fewer neurological symptoms, while introducing microbial communities from Parkinson for patients worsened disease-related changes. Although these findings do not mean that gut microbes directly cause Parkinson disease, they suggest that microbial communities may influence how the disease develops or progresses. The gut–brain connection also raises fascinating questions about lifestyle and diet. These discoveries are particularly relevant for countries like Nepal, where traditional diets may strongly influence gut microbial diversity. Many Nepali meals are largely plant-based and rich in fiber, built around rice, lentils, vegetables, and seasonal produce. High-fiber diets provide nutrients that beneficial gut microbes use to produce molecules that influence metabolism and immune function. Fermented foods are also deeply embedded in Nepali culinary traditions. Foods such as gundruk, sinki, kinema, and fermented pickles contain natural microbial cultures that develop during fermentation. These foods have been part of Himalayan diets for generations, long before modern science began studying probiotics and microbial ecology. Research led by food microbiologist Dr. Jyoti Prakash Tamang has shown that traditional fermented foods from the Eastern Himalayas contain diverse microbial communities that may contribute to gut microbial diversity and metabolic health. Understanding these dietary traditions may become increasingly important as Nepal faces a growing burden of chronic disease. According to the World Health Organization, non-communicable diseases now account for roughly two-thirds of deaths in Nepal, highlighting the importance of studying factors that influence long-term brain and metabolic health. As a neuroscientist studying Parkinson disease, I find the connection between the gut and brain particularly compelling. In our research, we investigate how neural circuits linking the digestive system and the brain may influence vulnerability to neurodegeneration. By studying pathways such as the vagus nerve and related neural networks, scientists are beginning to understand how signals from the digestive system might shape the health of dopamine-producing neurons in the brain. These discoveries are changing how we think about neurological disease. The brain is not an isolated organ functioning alone inside the skull. Instead, it is part of a complex biological network that includes the immune system, metabolism, environmental exposures, and microbial communities living within us. The conversation between the gut and the brain is happening every moment of our lives. Learning to understand this hidden dialogue may help scientists better understand diseases such as Parkinson disease and may one day open new paths toward prevention and treatment. And perhaps the next time someone says they have a 'gut feeling,' science will remind us that this expression reflects one of the most remarkable communication systems in the human body. Pokharel is a PhD in Neurosciences and Neurological Disorders at the University of Toledo, USA.