Why Do You Feel Butterflies in Your Stomach?

Before you walked onstage.

Before you opened the message. Before you said the first word, before the test landed on your desk, before you knocked on that door — something happened in your stomach. A hollowness. A flutter. Like something small was circling in there, looking for a way out.

You called it nerves. You were right. But the mechanism is stranger and more interesting than the word implies.

Your gut doesn’t just respond to anxiety. In a real sense, it participates in it.

The Second Brain You Never Think About

In 1998, neuroscientist Michael Gershon at Columbia University published a book called The Second Brain. The title was not metaphor.

Embedded in the walls of your gastrointestinal tract — from your esophagus down through your intestines — is a dense meshwork of neurons called the enteric nervous system. It contains somewhere between 100 million and 500 million nerve cells. More neurons than your spinal cord.

This system operates largely independently. It regulates digestion, controls the muscular contractions that move food through your gut, monitors the chemical environment of your intestines, and communicates constantly with your brain. And critically: it does all of this without waiting for instructions from above.

The enteric nervous system was probably the first nervous system. Long before animals had brains, they had guts. The gut’s neural architecture is ancient in a way the cerebral cortex isn’t.

The Vagus Nerve, Backwards

Most people imagine the brain-gut relationship running one way: the brain sends orders, the gut obeys.

The anatomy is almost the opposite.

The vagus nerve — the tenth cranial nerve, the longest in the body — connects the brainstem to the gut, heart, lungs, and most major organs. But roughly 80 percent of the fibers in the vagus nerve run upward: from the gut to the brain, not the other way around.

Your gut is not primarily receiving messages. It is primarily sending them.

What it reports shapes your emotional state in ways that researchers are still mapping. Gut signals influence mood, stress response, and even decision-making. The gut is not just a digestive organ with some nerves in it. It is an active participant in your inner life.

What Actually Happens When You’re Nervous

When your brain detects a threat — a test, a performance, a social risk — it activates the sympathetic nervous system: the fight-or-flight cascade. Adrenaline releases. Cortisol follows. Your heart rate goes up. Blood pressure rises. Muscle groups throughout your body receive increased blood flow.

And blood is diverted away from your gastrointestinal tract.

Digestion is metabolically expensive. In a moment of threat, your body makes a calculation: whatever is happening in your intestines can wait. Resources go to your limbs, your heart, your lungs — everything that helps you run or fight.

When blood flow to the gut drops and the enteric nervous system registers the shift, gut motility changes. Contractions become irregular. The normal rhythmic movement of your intestines falters. That hollow, fluttery feeling — that’s your gut contracting and releasing in a pattern it doesn’t usually run.

The butterflies are real. You are feeling your intestines.

Serotonin Started in Your Gut

Here is a fact that reorients things: approximately 90 percent of the serotonin in your body is produced in your gut, not your brain.

Serotonin is usually associated with mood. It’s the molecule that antidepressants target. But its original job, evolutionarily, was to regulate gut motility — to help coordinate the muscular contractions of digestion. The enteric nervous system runs on it.

When stress floods the gut with signals, serotonin release is disrupted. The normal coordination breaks down. Gut contractions become erratic.

This is also why stress and gastrointestinal problems are so tightly linked. Irritable bowel syndrome is partly a disorder of the gut-brain axis — a loop in which stress triggers gut symptoms, and gut symptoms amplify stress. The mechanism that gives you butterflies before a performance is the same mechanism, running hotter and longer than it was designed to.

Why Your Body Does This

Cannon described the fight-or-flight response in 1915, but the logic runs deeper than survival.

Suppressing appetite and halting digestion in a moment of acute threat is rational. If a predator is nearby, finishing your lunch is not the priority. The body has one question: can you survive the next few minutes? Everything else — digestion, immune function, reproduction — goes into standby.

The problem is that your nervous system cannot reliably distinguish between a predator and a job interview. Both register as threat. Both activate the same cascade. The butterflies you feel before something important are a version of the same response that helped your ancestors sprint from things that wanted to kill them.

Your gut treated the presentation like a lion. That’s not a flaw in the system. That’s the system working exactly as designed — for an environment that no longer exists.

The Loop That Feeds Itself

The gut doesn’t just receive the stress signal and generate symptoms. It sends the signal back.

Via the vagus nerve, gut distress reaches the brainstem and the insular cortex — the region involved in interoception, in your awareness of what’s happening inside your body. You feel the butterflies. You interpret them as evidence that the thing you’re nervous about is truly dangerous. That interpretation deepens the anxiety. Deeper anxiety produces more gut disruption.

This is why nervousness can feel like it escalates as the moment approaches. The gut and brain are running a loop that amplifies itself, each side responding to the other.

Understanding the loop doesn’t break it. But it makes the sensation legible. The butterflies aren’t a warning that something will go wrong. They’re the body preparing for something it judges to be significant.

You feel it before you understand what you’re feeling.

That happens because your gut reports first. The enteric nervous system, the vagus nerve sending messages upward, the serotonin disruption, the altered motility — it all registers as sensation before your brain has assembled a story about it. The hollowness arrives before the thought.

That’s not weakness. That’s the architecture. A hundred million neurons doing exactly what they were built to do — keeping you ready for whatever comes next.

They’ve been doing this since before you had a brain to worry with.

Michael Gershon, The Second Brain, 1998. Mark Gershon and Sidney Tack, “The Serotonin Signaling System,” Gastroenterology, 2007. Walter Cannon, Bodily Changes in Pain, Hunger, Fear and Rage, 1915. John Cryan and Ted Dinan, “Mind-altering microorganisms: the impact of the gut microbiota on brain and behaviour,” Nature Reviews Neuroscience, 2012. Martin Farmer and Beverley Randall, “Why is the gut sometimes called the ‘second brain’?” Scientific American Mind, 2015.