People often feel a "knot" in their stomach when they're nervous, whether they're stepping on stage to deliver a speech or onto the field for a championship game. Stress and anxiousness can also lead to nausea, painful bloating, constipation and diarrhea.
This phenomenon is often referred to as "nervous stomach" — but what causes it, exactly?
Nervous stomach happens because of the close connection between the nervous system and digestive system, Melissa Hunt, a clinical psychologist at the University of Pennsylvania, told Live Science in an email.
"From the earliest stages of embryonic development, the brain, spinal cord and digestive tract are all tightly wired to each other," she said. "Millions of neurons send information from the gut back to the brain, and just as many neurons send signals back to the gut." This link is often referred to as the gut-brain axis.
The neurons, or nerve cells, that line the gastrointestinal tract make up the enteric nervous system. They are part of the "autonomic" nervous system, which regulates involuntary bodily functions, such as breathing, heartbeat and digestion. When food enters the gut, for example, enteric neurons prompt muscle cells to contract and push the food through the intestines, according to Harvard Health.
The autonomic nervous system is divided into two branches: the parasympathetic and sympathetic. These systems, respectively nicknamed the "rest and digest" and "fight or flight" systems, balance each other out. In general, the parasympathetic nervous system relaxes the body, while the sympathetic nervous system bolsters its response to danger.
"Digestion is controlled by the parasympathetic branch," Hunt said. "When we're stressed, the sympathetic branch is activated and suppresses the functions of the parasympathetic system." In this state, the body releases stress-related hormones, such as cortisol, that suppress digestion in the stomach and small intestine; meanwhile, other hormones actually stimulate the large intestine.
"So instead of engaging in a mellow state associated with resting and digesting, the stomach and intestines might spasm or contract suddenly as the person prepares to fight or flee," Hunt said.
Just as stress can trigger a nervous stomach, frequent gastrointestinal (GI) problems can also cause stress, Harvard Health notes.
What's more, stress can increase the frequency or severity of symptoms in "disorders of gut-brain interaction," or DGBIs. These include conditions like irritable bowel syndrome (IBS), in which a variety of symptoms, such as abdominal pain, diarrhea or constipation, occur without clear cause, or functional dyspepsia, stomach aches that occur during or after eating without known cause.
DGBIs are thought to be characterized by ongoing disturbances in communication between the gut and the brain. Over time, people can become anxious and hypervigilant about their GI symptoms, Hunt noted.
"This leads to visceral hypersensitivity, which becomes a vicious feedback loop of anxious arousal, scanning the body for uncomfortable sensations, catastrophizing, amplification of those sensations, which increases anxiety and then leads to increased GI discomfort and distress," she said.
That's why behavioral therapy is sometimes incorporated into patients' treatment plans for DGBIs, according to Harvard Health.
This article is for informational purposes only and is not meant to offer medical advice.
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Anna Gora is a health writer at Live Science, having previously worked across Coach, Fit&Well, T3, TechRadar and Tom's Guide. She is a certified personal trainer, nutritionist and health coach with nearly 10 years of professional experience. Anna holds a Bachelor's degree in Nutrition from the Warsaw University of Life Sciences, a Master’s degree in Nutrition, Physical Activity & Public Health from the University of Bristol, as well as various health coaching certificates. She is passionate about empowering people to live a healthy lifestyle and promoting the benefits of a plant-based diet.