What causes the feeling of 'butterflies' in your stomach?

It's a familiar feeling: Before a test, a first date, or a public speaking engagement, your stomach starts to "flutter" as if butterflies were flying around inside your gut. Stomach sensations are a common side effect of being nervous, and they may also manifest as a feeling of a "knot" or even as more severe digestive symptoms.

But where do these feelings actually come from?

It turns out, a "nervous stomach" is one of the best examples of the two-way connection between the digestive system and the nervous system.

"From the earliest stages of embryonic development, the brain, spinal cord and digestive tract are all tightly wired to each other," Melissa Hunt, a clinical psychologist at the University of Pennsylvania, told Live Science in an email. "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. It's modulated by hormones and neurotransmitters — chemical messengers that respectively have broad effects via the bloodstream or act locally between nerve cells. It is also controlled by direct nerve connections between the brain and gut? and even by bacteria, and it's one reason your mood can so often affect the rest of your body, and vice versa.

"When we feel "butterflies" in our stomach, it's a vivid reminder that our emotions are deeply embodied," John Cryan, a professor of anatomy and neuroscience at University College Cork in Ireland, told Live Science in an email. "Ultimately, butterflies in the stomach illustrate the gut-brain axis in action: a continuous, bidirectional conversation between the central nervous system and the gut through neural, hormonal, and microbial pathways."

How the gut-brain axis creates butterflies in your stomach

The nerve cells that line the gastrointestinal tract 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, its resident nerve cells 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 nervous system and sympathetic nervous system. 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.

When you're feeling anxious, the fight-or-flight response is activated. In this state, the body releases stress-related hormones, such as cortisol, that suppress digestion processes in the stomach and small intestine; meanwhile, other hormones actually stimulate the large intestine. These simultaneous changes cause muscle contractions that can feel like "butterflies" in the stomach, and they can also cause more severe digestive distress, such as nausea, bloating, constipation or diarrhea.

While these feelings often seem like an annoyance or inconvenience today, they may have once played a pivotal role in human life.

"From an evolutionary standpoint, this reaction likely helped our ancestors survive," Cryan explained. "Shutting down digestion and diverting resources toward immediate physical readiness would have improved chances of escape or confrontation. The gut sensations that accompanied these shifts also served as internal cues, highlighting moments of high importance or uncertainty."

You may have heard that the microbiome — the community of microorganisms that live in and on our bodies — plays a big role in the gut-brain axis. And that holds true for the sensation of a nervous stomach, too.

"Although the gut microbiome doesn't cause the flutter directly, it helps shape how strongly we experience and recover from such sensations," Cryan said. The bacteria in our guts may secrete substances that influence gut-brain signaling, and this, in turn, could affect how strongly someone feels a fluttery stomach.

"A healthy, diverse microbiome may buffer overactive stress responses, while alterations in microbiome composition can heighten them," Cryan suggested. The interactions between the microbiome and the gut-brain axis are still a relatively new area of research, and scientists are still determining how specific microbes influence gut-brain signaling.

Two-way road

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. This umbrella term covers GI conditions that cause significant symptoms and impact quality of life but don't always trigger clear, measurable changes in the digestive tract. These include conditions like irritable bowel syndrome (IBS), in which a variety of symptoms, such as abdominal pain, diarrhea or constipation, occur without a clear cause; or functional dyspepsia, which involves stomachaches that occur during or after eating, without a known cause.

DGBIs are thought to be characterized by ongoing disturbances in communication between the gut and the brain, as well as shifts in the gut microbiome and immune function, in some cases. 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, to help break this cycle.

"Far from being 'just in your head,' emotional experiences are woven through your viscera," Cryan said. "Understanding this connection reminds us that mental and digestive health are inseparable."