You have a big presentation in two hours and your stomach is doing something unpleasant. Or you have been anxious about something for weeks and noticed that your digestion has been completely unpredictable. Bloating for no dietary reason. Cramps that come and go. An urgency you cannot explain. You eat the same things, but your gut is behaving differently.
This is not imagined. The gut and the brain are in constant, real-time communication, and stress is one of the loudest signals that crosses between them.
The gut has its own nervous system, called the enteric nervous system, which contains roughly 100 million nerve cells lining the gastrointestinal tract. Scientists sometimes call it the “second brain,” not because it thinks the way the brain does, but because it operates with a remarkable degree of independence and is deeply sensitive to emotional states. The vagus nerve is the primary highway between the two systems, running from the brainstem down through the chest and into the abdomen.
When anxiety activates the body’s fight-or-flight response, it triggers a cascade that affects the gut almost immediately. Blood flow is diverted away from the digestive system toward the muscles. Gut motility, the speed at which food moves through the intestines, changes. In some people it speeds up, leading to diarrhoea. In others it slows down, causing constipation and bloating. Neither response is a coincidence.
Dr. Emeran Mayer, a gastroenterologist and professor at UCLA who has spent decades studying the gut-brain axis, describes the relationship as bidirectional. The gut is not just receiving stress signals from the brain. It is also sending them back. About 90 percent of the information travelling along the vagus nerve flows upward, from gut to brain, not the other way around. This means a distressed gut can amplify anxiety, creating a feedback loop that is frustrating to break.
Irritable bowel syndrome is one of the clearest examples of this connection in clinical practice. Studies consistently show that people with IBS have higher rates of anxiety and depression, and that psychological interventions like cognitive behavioural therapy can meaningfully reduce gastrointestinal symptoms without any change in diet.
What does this mean practically? It means that if your stomach issues worsen during stressful periods, treating only the stomach is incomplete. Managing the nervous system is part of the treatment. Diaphragmatic breathing, where you breathe slowly and deeply using the belly rather than the chest, directly stimulates the vagus nerve and shifts the body toward a calmer state. Even five minutes before meals can improve digestion.
Probiotic research also points toward a connection between gut microbiome composition and anxiety levels, though this area is still developing and no single probiotic is a proven fix.
What helps most, according to the evidence, is recognising the pattern. When your gut flares, ask what is happening emotionally. Not because it is all in your head, but because your gut and your head are, quite literally, in this together.