Exploring More Than 1 Kilograms of Microorganisms in the Human Intestine: A Symbiotic Relationship


  1. Papua New Guineans Boast Muscular Physiques Despite Sweet Potatoes Being Their Staple Food

  2. Humans Can Even Live Like Gorillas?!

  3. There Are No “Bad Microbes”

  4. To Have a Diverse Diet Is to Live with Diverse Microorganisms

Have you ever considered that we coexist with microorganisms and bacteria? It might sound astonishing, yet it’s the reality of our symbiotic relationship with these tiny beings.
Microorganisms are ubiquitous—floating in the air, drifting in water, residing in soil, and even covering our body surfaces. Among these diverse habitats, the human intestine stands out as a bustling hub of microbial life.
What roles do these invisible intestinal bacteria play? To unravel this mystery, we sought insights from Professor Masahiro Umezaki of the Graduate School of Medicine, the University of Tokyo.
Professor Umezaki’s fascination with intestinal microorganisms began during encounters with the inhabitants of Papua New Guinea’s highlands. Since then, he has delved into the study of these microbes through the lens of human ecology. However, he humbly admits, “There is still so much we do not know.”
These unseen, enigmatic microorganisms hold a multitude of secrets. Yet, one certainty remains: contemplating our symbiosis with them offers a crucial perspective on embracing diversity.

Photo of the exterior of the building where Professor Umezaki's laboratory is located.

Papua New Guineans Boast Muscular Physiques Despite Sweet Potatoes Being Their Staple Food

Why did you decide to focus your research on intestinal bacteria?


My specialty is human ecology, the study of what defines us as humans and how we live. During my early anthropological research days in Papua New Guinea, I was struck by the differences in body structures of the local people, despite their shared ancestry. Their primary sustenance is sweet potatoes, with a significantly lower protein intake compared to the Japanese diet. If we consumed the same fare, we’d likely be thin, yet they exhibit robust, bodybuilder-like physiques. It’s quite perplexing, isn’t it?
While the genetic makeup of different populations certainly plays a role, it seemed unlikely to me that genetics alone could account for such a stark contrast. That’s when I began considering the role of intestinal bacteria.

Are you suggesting that the microorganisms in the bodies of people living in Japan and those in Papua New Guinea function differently?


It’s not so much about the function, but rather the composition of microorganisms within each person’s body that differs greatly. For example, New Guineans produce little to no lactobacilli, while harboring bacteria that are less abundant in Japanese individuals.
However, the full extent of this diversity remains elusive. Many bacteria have been identified by name, yet their specific functions remain a mystery. There are even more unnamed species. The collective weight of intestinal bacteria in an adult male’s body is roughly 1 to 1.5 kilograms, equivalent to that of the brain. It’s quite substantial.
In our research, we’ve been transporting fecal samples from Papua New Guineans back to Japan and partnering with a professor from the Faculty of Agriculture to analyze the microorganisms they host. When we introduced these microorganisms from their feces to laboratory rats, even in a protein-deficient state, the rats didn’t lose much weight.
This outcome suggests that the function of intestinal bacteria plays a significant role in the robust physique observed in Papua New Guinea highlanders.
Often, high school textbooks describe the large intestine’s function as “absorbing water.” However, its role extends far beyond that. It serves as a habitat for bacteria with diverse functions, such as breaking down dietary fiber and synthesizing vitamins. Through further investigation, we may even discover microorganisms with immunity to certain diseases.

Photo of a tank used to bring back collected fecal matter from overseas.
A tank used to bring back collected fecal matter from overseas. It is constructed to keep the inside cool. With the tag, "MUST RIDE".

Humans Can Even Live Like Gorillas?!

What is it that causes such a difference in intestinal bacteria between people living in Japan and those in the highlands of Papua New Guinea?


To be honest, it’s still unclear. As a researcher, this uncertainty can sometimes feel quite “romantic”! (laughs)

Photo of Professor Umezaki speaking in his lab, his arms crossed.

So, you believe that ongoing research could lead to significant discoveries?


To give you a sense of the possibilities of my research, let me share a broader perspective.
Consider gorillas, herbivores, and fellow primates to humans. Their staple diet consists of wild plants, lacking in protein. Despite that, gorillas boast large, powerful bodies. Why then, can’t humans consume wild plants like gorillas, despite being of the same primate species? I suspect that humans once had this ability.
Wild plants contain phytochemicals, toxic substances meant to deter predators. However, herbivores possess intestinal bacteria capable of detoxifying these substances. Humans, with a few exceptions, cannot consume wild plants raw like herbivores.
The history of Homo sapiens spans approximately 200,000 years, roughly 10,000 generations if we consider 20 years as one generation. If we trace back 10,000 of your own mother’s mothers, it leads to the birth of Homo sapiens. Agriculture, however, only began about 10,000 years ago. This suggests that for 190,000 years before agriculture, people lived in the wild.

With the advent of agriculture, we gained access to rice and vegetables, and following the Industrial Revolution, meat and eggs became more readily available, but these were not part of our original diet. Considering this history, the period when we survived on wild plants and animals was longer. Our modern diet is more peculiar, and in understanding our survival through those times, we cannot overlook the role of intestinal bacteria.

Photo of a yellow mask from an ethnic group in Papua New Guinea hanging on a whiteboard support in the lab. The mask has a long elliptical shape vertically and is covered in patterns. The mouth is red and shaped like it is sticking out its tongue.

Does this mean humans originally possessed the ability to thrive in the natural world?


With a balanced diet now the norm in this world of abundance, it’s possible that humans have lost this ability, or perhaps we are simply unaware of it. While I don’t aim to equate humans with gorillas, people in Laos subsist on wild plants, and even in Papua New Guinea, plant-based diets prevail.
If one were to adopt a lifestyle in Papua New Guinea tailored to the local diet, they would likely lack sufficient protein. For instance, if I were to live this way, I would struggle to heal wounds and could eventually fall ill or even succumb to malnutrition. However, with prolonged adherence to the local lifestyle, the body might adapt, mimicking the gut bacteria of the locals. Depending on what is considered “natural,” one might possess an ability to live closer to the wild.

Photo of sundry items and a painting in the lab from an ethnic group in Papua New Guinea. A portion of the painting is obscured by shadows due to light shining in through the window.

There Are No “Bad Microbes”

But aren’t there many microorganisms and bacteria that are harmful to humans?


That’s an anthropocentric notion. I understand it’s challenging to be aware of bacteria since we can’t see them. However, if we view them in terms of the relationship between humans and other organisms, it leads to the idea that it’s acceptable to eliminate other organisms for human survival. Even though some pests are ecologically necessary, categorizing them all as “bad” is perilous. It would result in a loss of diversity.

If we eliminate harmful organisms, we inadvertently eliminate beneficial ones, leading to the collapse of the ecosystem. Bacteria operate similarly. While many would consider them “bad,” they actually provide us with protection. Yet, we unknowingly eradicate various bacteria. In Japan, livestock are kept in complete isolation for hygiene purposes. While this is correct for hygiene, it also means that the exchange of bacteria between pigs and humans, which naturally occurred when they lived together, has been lost. I believe many bacteria have become extinct because they’ve been deprived of their natural habitat. We might feel saddened by the reduction of cute animals, but we don’t quite have the same emotional response to the death of invisible bacteria.

This isn’t to say we shouldn’t wash our hands or use antibiotics. However, prioritizing only human concerns can distort an ecosystem.

Regarding bacteria, those responsible for cholera, dysentery, and typhoid fever were once labeled as “bad.” Yet, the discovery of symbiotic relationships between humans and these bacteria is shifting perspectives. There’s room to question whether living in an excessively “clean environment” is truly beneficial. Perhaps it’s time to consider the diversity of microorganisms alongside the diversity of visible life forms.

Photo of a window in the lab. Countless traces made by someone’s fingers on the window are faintly visible.

To Have a Diverse Diet Is to Live with Diverse Microorganisms

What kind of symbiotic relationship should we have with microorganisms?


Herbivores don’t store nutrients to “become food for carnivores,” do they?
The microorganisms in our large intestine thrive by consuming the leftovers that pass through the stomach into our intestines. We aren’t consciously trying to “nourish” these intestinal microorganisms.
Throughout our extensive history, humans have consumed a variety of foods across different regions. I hypothesize that bacteria in a mutually beneficial relationship have endured. Essentially, the diversity of intestinal bacteria can be partially maintained by the diversity of human diets.
Though we are said to have a diverse global diet, most of us reside in urban areas. Chinese, Japanese, and American cuisines—while distinct—are not vastly different in terms of carbohydrates, proteins, fats, and nutrients.
People truly consume a wide range of foods. While I’ve only directly experienced the diet in Papua New Guinea, some African pastoralists subsist solely on dairy products like milk and yogurt. In contrast, the Dirashe people of Ethiopia have diets rich in alcohol. Despite their seemingly imbalanced diets from our modern perspective, they thrive.
The guideline of “how many grams of protein should be consumed per day” is established without considering the role of intestinal bacteria. Certainly, increased protein intake has contributed to a much longer human lifespan, which once averaged around 30 years. However, there are individuals whose diets fall outside these standards.
What kinds of intestinal bacteria do they possess, and how do these bacteria function? As we gather case studies, we may uncover how intestinal bacteria have influenced the survival of the human race.
Today, Japan’s diet has become standardized, but historically, people likely relied on locally sourced foods, each region with its distinct cuisine. Many are surprised to learn that Papua New Guineans primarily consume sweet potatoes as their staple food, yet our diets were once similar.
We often perceive the world through the lens of the majority, but we require this broader view encompassing time and space. While my current research focuses on modern Papua New Guinea, I aim to contemplate the future of human society from various perspectives—examining past human diets, the associated microorganisms, and the environments in which they thrived.

Photo of the trees visible outside the lab window. The deep green trees and the almost pink tree whose leaves have fallen overlap to create a beautiful contrast.

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We all live with microorganisms and they support our lives.
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