Illuminating the Brain-Gut Axis: Insights from Japan’s Leading IBS Researcher

On some level, human beings have always sensed a connection between the mind and the organs of digestion. English speakers talk about a gut feeling and butterflies in their stomach. In Japanese, consuming anger is referred to as hara no mushi (a bug in one’s gut), and wrenching grief as danchō no omoi (feeling gutted). Yet it is only fairly recently that science has begun to illuminate the mechanisms behind this link. Professor Fukudo Shin, a leading expert on irritable bowel syndrome, is among those pursuing the implications of the brain-gut connection for human health.

“For a long time,” Fukudo says, “medical science treated the brain as the most noble and lofty organ, while viewing the gut, down in the lower gastrointestinal tract, as a peripheral organ for the final digestion and excretion of waste. But we’ve learned that there is close bidirectional communication between the brain and the digestive system, especially the gut.” Anything that disrupts or distorts that communication—from emotional stress to bacterial imbalances in the gut—can lead to a variety of physical and psychological ailments.

“We need to rethink our basic assumptions about the relationship between the mind and the gastrointestinal tract,” says Fukudo.

Professor Fukudo Shin of the Tōhoku University Hospital. (Courtesy Fukudo Shin)

IBS as a Brain-Gut Disorder

The brain and the gut communicate through the endocrine system and the autonomic nervous system, which includes the sympathetic, parasympathetic, and enteric (intestinal) nervous systems. The vagus nerve, part of the parasympathetic nervous system, is probably the most important link between the gut and the brain. How psychosocial stress can affect the transmission of these brain-gut signals is a key focus of Fukudo’s research.

Fukudo was among the first to conceptualize irritable bowel syndrome, or IBS, as a disorder of brain-gut interaction. IBS patients suffer from intermittent gastrointestinal (GI) distress in the form of diarrhea, constipation, bloating, and abdominal pain, which can seriously impact their quality of life. Yet clinical examinations and tests typically reveal no severe inflammation or other intestinal abnormalities.

“IBS presents in the gut,” says Fukudo, “but it is also a stress-related disorder. It occurs predominantly in advanced industrial societies. In Japan, it’s estimated that about one in ten people suffers from IBS.”

Not surprisingly, Fukudo has found a higher incidence of depression and anxiety among subjects with IBS or IBS symptoms than among those without GI complaints. According to him, IBS patients also tend to share certain personality traits, such as the tendency to brood or obsess. Moreover, they frequently have trouble verbalizing their emotions, which can cause stress to build up. Stress triggers the secretion of hormones that can increase GI motility and visceral sensitivity, leading to IBS symptoms.

The Second Brain and the Microbiota

While Fukudo’s work has centered on the role of psychosocial stress in IBS and other disorders, he is quick to point out that the brain-gut interaction is no one-way street. The gut is far from a submissive underling, acting obediently on the brain’s orders; it can make its own decisions. About 90% of the signals transmitted by the vagus nerve run from the gut to the brain. The state of the gut, including its microbiota, can affect our moods, preferences, and behavior.

The latest science indicates that the diverse microorganisms inhabiting the gut play an integral role in the communication that goes on between the gastrointestinal tract and the brain. Indeed, so important is their function that many now speak of the “microbiota-gut-brain axis.”

Our bodies are a close-knit, symbiotic community of human cells and microorganisms. Over the past two decades, dramatic progress in DNA has allowed scientists to get a better handle on the quantity and variety of organisms that make up the human microbiome. The average adult body teems with more than 100 trillion microbes representing roughly 1,000 types of bacteria. About 90% of those are found in the cells and mucous membranes of the intestines. As infants, we inherit some of our mother’s microbiota when we come into the world, via the birth canal, and ingest more through our mother’s milk. Our gut flora continue to diversify over the next few years, stabilizing by the time we reach about three years of age.

Aided by gut microbiota, the gut produces a variety of chemicals known to affect mood and cognition as well as gut-brain communication. In fact, more than 90% of the body’s serotonin—an important neurotransmitter and mood regulator—is secreted in the gut with the help of microbes. Through signals sent to the brain, our gut flora influences what foods we crave and how much we eat. Recent research suggests that when certain beneficial bacteria proliferate in the gut, the brain produces more of the “happy hormone” oxytocin. In this way, our intestinal microbiota can affect our emotions, our sensitivity to pain, and even our social behavior.

Equipped with its own nervous system, the gut is clearly much more than a digestive organ. Columbia University Professor Michael Gershon has famously called the enteric nervous system “the second brain.” Fukudo submits that from an evolutionary standpoint, the gut is actually the first brain. “When multicellular organisms appeared on earth a few hundred million years ago, the first organ to evolve was the gut,” he notes. Gradually, a nervous system developed around that primitive gastrointestinal tract. The hydra is a living example of an animal that functions with no brain, just a digestive tube surrounded by a neural network.

Fukudo thinks it likely that chemical imbalances in the gut, including those caused by changes in the microbiota, can trigger inflammation and even morphological changes in the brain. Studies have implicated gut imbalances in a wide range of health conditions, from allergies and asthma to autism, Parkinson’s disease, and Alzheimer’s disease.

Hydractiniidae. (© Pixta)

Maintaining a Healthy Brain-Gut Axis

How, then, does one maintain a healthy brain-gut axis?

Starting with the gut, Fukudo recommends a diet high in fiber and low in refined carbohydrates and additives, with an emphasis on fruit and vegetables in season and fermented foods. He has personally seen cases where a change in diet turned things around completely.

Many drugs can upset the intestines’ natural microbial balance. Fukudo warns against the overuse of antibiotics in particular.

He also stresses the importance of eating a good breakfast each day to support regularity. “I know there are lots of children who skip breakfast so they won’t have to move their bowels while they’re in school. Our teachers need to foster a healthier attitude toward natural bodily functions like bowel movements, so children won’t feel embarrassed by them.” Other habits that support a healthy brain-gut axis are adequate sleep, sufficient down time, and regular exercise.

Based on his own experience, Fukudo believes the most important thing one can do to prevent and alleviate IBS is to recognize stress in everyday life and talk to others about it instead of bottling it up inside. From this standpoint, cognitive behavioral therapy holds real promise as a tool for alleviating GI symptoms and improving overall quality of life. Ultimately, an integrated mind-body treatment may make the most sense for a disorder of the brain-gut axis.

The symptoms of IBS are real and sometimes debilitating. But until recently, sufferers could expect little in the way of sympathy or understanding, let alone an effective treatment. Today, there is new hope for IBS and many other hard-to-treat conditions, thanks to dramatic advances in the science of brain-gut interaction.

Further Reading

• Collen, Alanna. 10% Human: How Your Body’s Microbes Hold the Key to Health and Happiness. Harper, 2015.
• Fukudo, Shin. Naizō kankaku—Nō to chō no fushigi na kankei (Visceral Feelings: The Mysterious Brain-Gut Connection). NHK Books, 2007.
• Mayer, Emeran. The Mind-Gut Connection: How the Hidden Conversation Within Our Bodies Impacts Our Mood, Our Choices, and Our Overall Health. Harper Wave, 2016.

(Originally published in Japanese. Banner illustration © Pixta.)