Just before an important presentation.
On the train to an exam.
Before meeting someone for the first time.
Your stomach had felt perfectly normal—then, suddenly, it begins to hurt.
“Why now, of all times?”
You did not eat anything unusual.
You do not remember getting cold.
You are simply nervous.
So why does the tension arising in your mind appear in a completely different part of your body?
The brain and the gut do not function independently. Information is constantly traveling between them, largely outside our conscious awareness.
The brain and the gut are having a conversation.
The human body is fascinating, isn’t it?
The Gut Is More Than a Tube for Food
We tend to think of the gut as a place that digests food and absorbs nutrients.
That is true—but it does not fully explain the gut’s complexity.
Within the walls of the gastrointestinal tract lies an extensive neural network known as the enteric nervous system, or ENS.
The enteric nervous system locally regulates many digestive functions, including intestinal movement and secretion. It is sufficiently autonomous to carry out certain reflex activities even when separated from direct input from the brain and spinal cord.
However, this does not mean that the gut operates entirely independently of the brain.
The brain and the gut exchange information in both directions through the nervous, endocrine, and immune systems.
This relationship is commonly known as the gut–brain axis.
What Happens to the Body When We Feel Nervous?
Imagine that you are about to speak in front of a large audience.
Your heart begins to beat faster.
Your palms become sweaty.
Your mouth feels dry.
Your breathing changes.
And for some people, the stomach begins to hurt.
These may appear to be separate reactions, but they are all connected to the body’s stress response.
When we experience stress, several systems become involved, including the autonomic nervous system and the hypothalamic–pituitary–adrenal axis, commonly called the HPA axis.
Their effects can extend to the gastrointestinal tract.
Research has shown that stress may alter gastrointestinal movement, secretion, sensitivity, and barrier function.
In other words, nervousness does not occur only inside the mind.
The entire body responds.
Why Does the Body React to a Presentation?
There is something curious about this response.
There is no wild animal waiting in the presentation room.
An exam venue is not usually a place that threatens our physical survival.
Even so, psychological thoughts such as:
“What if I fail?”
“I cannot afford to make a mistake.”
“Everyone is watching me.”
can change our heart rate, sweating, breathing, and gastrointestinal sensations.
The body has systems that continually adjust its internal state in response to changes inside and outside us.
Those same systems also respond to the nonphysical stresses of modern life.
For the body, what matters is not only what is objectively happening.
How the brain interprets the situation matters too.
Stress Does Not Affect the Stomach and Intestines in the Same Way
It is slightly misleading to say only that stress “activates the digestive system.”
The gastrointestinal tract is long, and different regions may respond differently.
Studies have reported that stress-related signaling can suppress movement in the stomach while increasing movement in the colon.
This may help explain why different people experience different symptoms:
A heavy feeling in the stomach.
Loss of appetite.
Abdominal pain.
A sudden need to use the bathroom.
Not everyone develops stomach pain when nervous. The way stress appears in the body varies from person to person.
“What If It Hurts Again?” Can Become a New Source of Stress
This is where the gut–brain axis becomes especially interesting.
Suppose your stomach once began to hurt before an important event.
Before the next event, you may start thinking:
“What if my stomach hurts again?”
That anxiety draws your attention toward sensations in the abdomen.
A small movement in the gut may make you think:
“It might be starting.”
Then you become even more nervous.
The communication does not travel only in one direction:
Brain → Gut
Sensory information from the gut also travels back:
Gut → Brain
The brain influences the gut, and sensations from the gut influence the brain.
This exchange continues inside the body, often without our awareness.
Is the Gut Really a “Second Brain”?
Health articles often describe the gut as the “second brain.”
The enteric nervous system is certainly complex. It can locally regulate gastrointestinal movement, secretion, and other digestive functions.
However, the phrase “second brain” should not be interpreted literally.
The gut does not think, understand language, or make decisions in the same way as the brain inside our head.
The more important point is that the human body is not a simple machine in which the brain issues every command in a one-way direction.
The gut has its own neural network, which interacts with the brain, the autonomic nervous system, the immune system, and the endocrine system.
Gut Microbes Are Also Part of the Conversation
In recent years, the discussion has expanded beyond the brain and the gut.
Researchers are also paying close attention to the gut microbiota—the vast community of microorganisms living in the digestive tract.
This has led to increasing research into what is known as the microbiota–gut–brain axis.
Scientists are investigating the complex relationships between gut microorganisms and the host’s nervous, immune, metabolic, and endocrine systems.
However, caution is necessary.
Research has not established simple conclusions such as:
“Changing your gut bacteria will automatically change your mood.”
“Eating this particular food will eliminate stress.”
The relationship between the brain, the gut, and the microbiota is extremely complex and remains an active field of research.
Why Do We Associate Feelings with the Gut?
Let us step briefly away from science and look at language.
English includes expressions such as:
“I have a gut feeling.”
“That was a gut reaction.”
“It takes guts.”
Japanese also contains many expressions that connect emotions and decisions with the abdomen:
Hara ga tatsu—literally, “the belly rises,” meaning to become angry.
Hara o kimeru—“to decide with the belly,” meaning to make up one’s mind.
Hara o watte hanasu—“to open the belly and talk,” meaning to speak openly and honestly.
These expressions are not scientific evidence of the gut–brain axis.
Still, it is fascinating that people connected emotions with sensations in the abdomen long before science described the mechanisms involved.
When angry.
When frightened.
When nervous.
Humans could feel changes occurring inside their bodies long before we had the term “gut–brain axis.”
Humans Can React to Things That Have Not Happened Yet
In HUE #01, we explored the question: “Why can’t we sleep even when we are tired?”
Stress and arousal played central roles there too.
The same structure appears in the relationship between nervousness and stomach pain.
Humans possess the ability to imagine a future that has not yet happened.
Tomorrow’s presentation.
Next week’s exam.
Next month’s project.
Future finances.
Personal relationships.
The mind can travel into the future.
The body, however, does not always respond by saying:
“That is only your imagination, so it does not concern me.”
Thinking about something that has not yet occurred can change your heart rate.
It can make you sweat.
It can keep you awake.
It can change the condition of your stomach and intestines.
This may be one of the ironies of being human: our highly developed imagination allows us to respond physically to events that exist only in the future.
The Body May Know Before Words Do
“Are you nervous?”
Sometimes we answer:
“No, not really.”
But our hands are sweating.
Our heart is beating slightly faster.
Our breathing is shallow.
And our stomach feels unsettled.
Even if the conscious mind has not yet acknowledged the tension, the body may already be responding.
Thinking about health is not only about preventing illness.
Understanding how the body works is also a way of understanding what kind of creature a human being is.
The Human Body Is Fascinating, Isn’t It?
You become nervous—and your stomach begins to hurt.
At first, it may feel as if your body is refusing to obey you.
But look inside, and you find the brain.
The nerves.
The gut.
Hormones.
The immune system.
They do not exist as separate, isolated parts.
They are constantly exchanging information and creating a single body together.
So the next time your stomach feels unsettled before an important event, it may help to remember:
Tension that begins in the mind is traveling through the body.
And information from the body is returning to the brain.
The brain and the gut are quietly talking today, just as they always do.
The human body is fascinating, isn’t it?
References
- Spencer NJ, Hu H. “Enteric nervous system: sensory transduction, neural circuits and gastrointestinal motility.” Nature Reviews Gastroenterology & Hepatology, 2020.
- Furness JB, et al. “The enteric nervous system.” Physiological Reviews, 2023.
- Geng S, et al. “The impact of acute and chronic stress on gastrointestinal physiology and function: a microbiota-gut-brain axis perspective.” The Journal of Physiology, 2023.
- Taché Y, Bonaz B. “Corticotropin-releasing factor receptors and stress-related alterations of gut motor function.”
This article provides general scientific information about health and the human body. It is not intended to offer an individual diagnosis or medical treatment. Consider consulting a healthcare professional if abdominal pain is severe, persistent, recurrent, or accompanied by blood in the stool, fever, vomiting, or other concerning symptoms.
