The Phone Call the Gut Makes to the Brain

On a day the belly is uneasy, the mood sinks too and small things set you on edge. Conversely, when the stomach is at ease, the mind is oddly relaxed. In my clinic I often see this common connection. And the direction of this connection is often the opposite of what we usually think.

Not from the Brain to the Gut, but from the Gut to the Brain

We usually think "stress harms the gut." It is the picture of the brain above and the gut below taking orders. Half of it is right. But when you look into the large telephone line joining the brain and the gut — the vagus nerve — most of the signals traveling back and forth go up from the gut to the brain. The call the gut makes to the brain is far thicker than the call the brain makes to the gut.

That is, the gut is not merely a lower floor that only takes orders. The gut ceaselessly reports its own state upward — whether it is at ease right now, what has come in, whether the wall is uneasy. The brain receives that report and sets the mood of the whole body. When a signal of "all is well here" comes up from the gut, the brain loosens; when a signal of "it's noisy here" comes up, the brain tilts quietly toward a guarded posture.

That Is Why the Mind Rides on the Gut

Knowing this direction, a day when an uneasy belly makes the mood sink reads differently. It is not that the mind is weak, but that the report the gut sends upward has changed.

When the gut wall is uneasy, when the environment inside the gut is disturbed, or when the gut is always tense, the content of the call going up changes. The brain receives that changed report and tunes the body toward being more sensitive and more easily startled. Sleep grows light, small stimuli draw large reactions, and the mind tightens for no reason. On the surface it looks like a problem of the mind, but if you trace back up the telephone line, the start is sometimes the gut. What Part 5 of this book calls "the parts are normal but the coordination between them wavers" holds here too.

An Endoscope Looks at the Shape of the Gut

Tests that look at the gut — endoscopy or blood tests — generally look at the shape of the gut and inflammation. But "what signal that gut is sending to the brain" is not an item those tests measure. That is why "the endoscopy is clean, but my stomach is always sensitive and my mind is uneasy too" happens. The shape of the gut and the content of the call the gut makes upward are different stories.

So for such patients I do not merely soothe the mind or look only at the gut; I look together at the telephone line between them. Easing the gut helps calm the mind, and conversely, relieving tension can settle the gut. Because the two directions are joined on one line.

But if there is black stool or blood-mixed stool, weight loss for no reason, or abdominal pain enough to wake you at night — that is a signal beyond this story. A test to check the gut comes first.

We tend to think the mind governs the body. But between the gut and the brain, the call rising from below to above is thicker.

That the mind sinks on a day the belly is uneasy is not from weak willpower, but may be the body's report that the content of the call the gut made to the brain has changed.

References

• About 80% of the vagus nerve runs in the gut→brain direction (afferent), and gut signals influence mood and arousal — Vagus Nerve as Modulator of the Brain–Gut Axis, 2018
• Serotonin made in the gut stimulates vagal afferent nerves and is relayed to the brain — Vagus Nerve and Serotonin in the Gut–Brain Axis, 2025

Written by Dr. Heo Ji-young (PhD in Korean Medicine Pathology, Kyung Hee University · former Research Professor of Herbology, Kyung Hee University)