The Brain is Stupid, the Gut is Smart (If You Train Your Gut, Your Brain Gets Better #QuoteNote81)

The quote I would like to introduce today is this!

This is more of a book title than a quote. Although the first edition was published in 2012, it is a super popular book that had already reached its 20th printing by the time I got my copy of the 2013 edition. Looking at the Amazon ratings as of 2025, it has a 4.2-star rating with 590 reviews, which shows that it is a book that has been read for many years.

This expression contains the message that while modern people tend to think about things only with their "brains," the "gut" actually plays a very important role in our physical and mental health, and we should realize its intelligence. It uses a straightforward, easy-to-understand phrasing to express that the brain and gut are closely linked, such as how mental distress affects the state of the gut, and conversely, how improving the state of the gut can improve one's mental state.

The person who said it

He was a Japanese doctor and a medical scientist. He specialized in parasitology, tropical medicine, and infectious immunology, and served as a professor emeritus at Tokyo Medical and Dental University. He passed away on May 14, 2021, at the age of 81, but his research and awareness-raising activities had a major impact on health consciousness in Japan.

Background and areas of expertise

Born in 1939 in northeastern China (formerly Manchuria), he graduated from the Faculty of Medicine at Tokyo Medical and Dental University, majored in parasitology at the University of Tokyo Graduate School, and obtained a medical doctorate. He also had research experience at the University of Texas and served as a professor at Kanazawa Medical University, Nagasaki University School of Medicine, Tokyo Medical and Dental University Graduate School, and the Open University of Japan.
As a member of the Japan-US Medical Cooperation Conference, while conducting immune research on malaria and filaria, he left behind many academic achievements, such as the "discovery of allergens within parasites" and the "elucidation of the transmission route of the ATL virus."

Activities for the general public and main arguments

Mr. Fujita also focused on activities to convey his expertise in an easy-to-understand way to the general public. In particular, he wrote numerous books on the importance of the intestinal environment and spoke about it in the media.

• As represented by the phrase "The brain is stupid, the gut is smart," he spread the concept of brain-gut correlation and communicated that the gut has a major influence on immune function and mental state.

• He proposed the "hygiene hypothesis," which links the causes of allergies and hay fever to changes in immune balance due to the eradication of parasites, and as part of that research, he was also known for having raised a tapeworm inside his own body for 15 years.

• He emphasized the importance of intestinal bacteria and provided concrete advice on diet and lifestyle habits to improve the intestinal environment.

Main books

He wrote numerous books, many of which explain health, immunity, and the intestinal environment to the general public. Representative books include the following:

• "The Brain is Stupid, the Gut is Smart"

• "Habits That Ruin Your Gut, Habits That Train It"

• "90% of Human Life Depends on the Gut"

• "If You Want to Lose Weight, Reduce 'Fat Bacteria' in Your Gut!"

• "Immunity"

• "The Laughing Parasite"

The brain is foolish, the gut is wise

The expression "the brain is foolish, the gut is wise" is a metaphorical phrase used to explain the concept of the gut-brain axis (the correlation between the brain and the gut), which has been gaining attention in recent years, in a way that is easy for the general public to understand.

What is the gut-brain axis?

The gut-brain axis refers to a state where the brain and gut are closely influencing each other. Previously, the brain was considered the command center of the body and the gut was thought of as merely a digestive organ, but recent research has revealed that the gut has an independent nervous system and plays a role as important as the brain.

The meaning of "the brain is foolish"

This refers to the aspect that the brain is sometimes easily swayed by emotions and stress, and cannot always make rational judgments. For example, feeling stomach pain or losing your appetite when you feel anxious or stressed is because negative signals are being sent from the brain to the gut.

Furthermore, in modern society, mental stress stemming from information overload and complex human relationships can exhaust the brain and have a negative impact on its functions.

To temporarily escape reality from stress, people may binge-eat unhealthy junk food, or become addicted to things like smartphone gacha games, gambling, or drugs.

The meaning of "the gut is wise"

In contrast to "the brain is foolish," the phrase "the gut is wise" refers to the following excellent functions of the gut:

• Possesses its own nervous system called the "second brain": The gut contains the second-largest number of nerve cells after the brain and has an enteric nervous system that functions independently. This allows it to autonomously control complex processes such as digestion, absorption, and excretion without instructions from the brain.

• The cornerstone of immune function: About 70% of the body's immune cells are concentrated in the gut, and it plays an important role in protecting the body from pathogens and the like.

• Production of happiness hormones: It is said that about 90% of serotonin (also called the happiness hormone), which is involved in mental stability, is produced in the gut. In other words, the state of the gut greatly affects one's mental state.

• The work of gut bacteria: A diverse range of bacteria live in the gut, and these gut bacteria have a wide range of positive effects on our health, such as synthesizing vitamins, fermenting dietary fiber, and regulating immune function. When the gut environment is in good condition, these beneficial effects are maximized.

The senior gut and the rookie brain

In the history of evolution, the formation of the gut is far older than the formation of the brain, and it is an organ positioned at the root of life activities.

The gut is considered to have had its prototype exist from the early stages of life billions of years ago as the most fundamental organ for life to take in food and maintain life activities. The origin of the digestive system is extremely old.

At the point when animals were born and adopted the strategy of "digestion" to obtain energy by preying on other organisms, an organ to make that possible was necessary. In the process of evolving from single-celled organisms to multicellular organisms, the "gut (digestive tract)" was formed first as the basic structure for digesting and absorbing nutrients taken into the body.

Of course, about 7 million years ago (though there are various theories), when humanity is said to have diverged from apes, our bodies were already equipped with the same gut that we have today.

Next, regarding the brain: specifically, the human brain was about the same size as a chimpanzee's (about 350cc) at the stage of the australopithecine, which diverged from apes about 7 million years ago. It rapidly increased in size to about 1000cc at the stage of Homo habilis about 2 million years ago, and then to about 1350cc in modern humans (Homo sapiens).

In this way, compared to the gut, which has a career spanning hundreds of millions of years, the human brain has a history of only a few million years, and it has evolved in stages at a rapid pace. The human brain's evolution has not been able to keep up with the rapidly developing culture.

In the hunter-gatherer era, like the Stone Age, there was no agricultural technology, so the smartest strategy was to eat as much as possible whenever you found a fruit-bearing tree. The moment you find fruit (more precisely, the moment the expectation that there 'might' be fruit reaches its maximum), a pleasure substance called dopamine is released in the brain, creating an urge that says, 'More! I need more!'

The brains of us living in the modern age operate on the same system. That is why we get hooked on smartphone gacha games, binge-eat junk food and sweets, or watch porn videos night after night only to realize that an incredible amount of time has passed. This is a state where the brain is being hacked by dopamine, and it is precisely why it is said that 'the brain is stupid.'

On the other hand, the gut is not swayed by short-term pleasure. The massive amounts of junk food and sweets consumed due to the brain's runaway behavior are nothing more than a nuisance to the gut, which thinks, 'That stupid brain did it again...' The gut works with the gut bacteria that make up its interior to diligently carry out digestion. In some cases, it informs us of our body's condition appropriately through diarrhea or constipation.

In reality, the gut is the senior, yet for some reason, it might be said to be in a pitiful position where it is forced to do heavy labor by the whims of the newest recruit... However, whether or not to force heavy labor on the gut can be controlled by changing our eating habits through our own will.

Personal Experience: My Anxiety Disorder Disappeared After I Cared for My Gut

As for my own experience, perhaps because I was originally a sensitive type, a transfer to a new department at work became a trigger that led to something like an anxiety disorder. I was constantly anxious for no clear reason, had heartburn, and occasionally experienced unexplained heart palpitations. I also had chronic fatigue and felt sleepy no matter how much I slept. I think that inflammation was occurring in various places in my body, and my immune system was running wild, causing fatigue to accumulate from within.

To alleviate that stress, I unconsciously started binge-eating more than enough for dinner, and I also drank alcohol frequently.

I tried meditation, exercise, and methods like writing down my anxieties on paper, but none of them were very effective...

Looking back now that I have fully recovered, the biggest deciding factor was improving my eating habits. I stopped binge-eating at dinner and intentionally reduced my carbohydrate intake.

• I halved the amount of white rice in my bowl and stopped eating until I was full in the first place.

• I started eating a proper breakfast and eating natto as much as possible.

• I stopped drinking coffee, which irritates the stomach and intestines.

I think these three things were particularly good. I am still continuing them. In the end, my anxiety disorder was resolved by improving my eating habits. By caring for my 'gut' instead of my 'brain,' my 'brain' also got better.

In fact, the sleepiness that persisted no matter how much I slept has disappeared as if it were a lie; I now wake up naturally after 7 hours of sleep and can function during the day without feeling sleepy. I am also able to work quite positively on programming studies and restarting my blog, which I have wanted to do for a long time (mostly during morning activities and on the train during my commute).

I feel like I am operating at about three times the productivity compared to when I had an anxiety disorder. And above all, what makes me happy is that I can stay in a positive mood no matter what I am doing.

I believe this is because my gut is in a healthy state, allowing it to produce enough serotonin, a hormone related to happiness. Surprisingly, most of all hormones are actually produced in the gut, not the brain. For mental illnesses like anxiety disorders and depression, which have been increasing rapidly in recent years, there have been many observed cases where consuming things that are good for the body is actually more effective for recovery than treatments like medication or meditation that target the head or mental state.

Summary

This time, I introduced the phrase The brain is foolish, the gut is wise!

It is a phrase that teaches us that the gut is extremely important for our health, and that understanding and caring for its functions leads to maintaining physical and mental well-being.

Since improving the gut environment can potentially lead to stress reduction and mental stability, the importance of reviewing our daily diet and lifestyle habits is once again gaining attention.

📢Announcement

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