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

Umezaki

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.

Umezaki

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.

Umezaki

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

Umezaki

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.

Umezaki

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.

Umezaki

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.

Umezaki

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.