Have you ever heard the term “physician-scientist”? It refers to someone who not only treats patients as a doctor but also conducts their own research—a true dual talent in the world of medicine. Prof. Hayashi is one such physician-scientist. She’s also a university professor guiding students and a mother to one child, juggling four demanding roles with remarkable dedication.

Prof. Hayashi’s journey into medicine began with an experience when she was around twelve years old.

“When my grandmother was diagnosed with stomach cancer, I saw how sincere and attentive her doctor was with each of their patients. That left a deep impression on me. I realized then that while being a physician carries immense responsibility, it is also an incredibly meaningful profession,” says Prof. Hayashi. She went on to study at the Keio University School of Medicine, where she discovered that while clinical care is noble, life-saving work, she might also have a genuine interest in research.

“I came to enjoy learning about the unknown and seeing my world expand as a result. Of all the mysteries, life itself fascinated me the most, especially the human body and how little we truly understand about it. That curiosity drove me,” she reflects.

Prof. Hayashi says she feels fortunate to be in a setting where she can pursue both clinical work and research. “Basic research informed by the unique perspective of clinicians is vital to advancing the medical sciences and healthcare. While it’s incredibly demanding to specialize in both areas—especially with the high standards expected—the work is deeply rewarding,” she says.

Prof. Hayashi specializes in nephrology, but what led her to choose this path?

“After graduating from the School of Medicine, I spent two years as a junior resident, gaining experience in a department of my choosing. I chose internal medicine, believing it was key to understanding human life and the body. After that, I had to choose a specialty and train further to become a certified specialist. At first, I was considering neurology, but near the end of my residency, I studied renal pathology—and was captivated by the kidney’s complexity and beauty.”

The kidneys contain many glomeruli—tiny structures that filter and purify the blood. The resulting primary urine passes through the renal tubules and eventually reaches the collecting ducts before being excreted via the bladder. But filtration is just one of the kidney’s many functions.

“The kidney is made up of many different types of cells. Its structure is not only complex but also remarkably beautiful.

It does far more than just produce urine—it regulates fluid and mineral balance, purifies the blood, secretes hormones, and supports many essential bodily functions. I was deeply drawn to the kidney’s complexity, elegance, and essential role in maintaining the body’s overall balance. That’s what led me to choose nephrology as my specialty.”

Prof. Hayashi and her team are currently working to clarify the relationship between chronic kidney disease and aging.

“Chronic kidney disease is a condition in which kidney function gradually declines. The kidneys are often called ‘silent organs’ because they rarely show symptoms until the disease is quite advanced. Many people know that chronic kidney disease can eventually lead to dialysis. But what's especially concerning is that it increases the risk of cardiovascular complications, like stroke and heart attack, well before any symptoms appear. Chronic kidney disease on its own is also linked to a higher risk of death from all causes, including cancer, stroke, and heart disease.”

Even more intriguing is the connection between kidney health and aging.

“It was long thought that aging causes kidney function to decline, leading to chronic kidney disease. However, recent findings suggest that the reverse may also be true: poor kidney function may actually accelerate aging. In other words, chronic kidney disease might be a major driver of aging throughout the body. We’re working to uncover why this happens and what mechanisms link kidney disease to the aging process. If we can understand these mechanisms, it may help with treating kidney disease and developing new therapies for aging-related conditions. That hope and curiosity is what drives our research,” she says.

What makes Prof. Hayashi’s research unique is its focus on DNA repair mechanisms and epigenomic changes in kidney cells, seeking to clarify how chronic kidney disease progresses at the molecular level.

“DNA damage and changes to the epigenome are closely tied to aging. In our bodies, DNA is constantly damaged and repaired in a continuous cycle. However, this repair mechanism becomes less effective as the body ages, and the damage accumulates, further driving the aging process.

The kidney is made up of many types of cells, and the effects on the body differ greatly depending on which cells suffer DNA damage. For example, experiments have shown that changes in DNA methylation—a chemical modification—can alter immune and metabolic functions, dramatically changing the severity of complications.”

The question of why kidney dysfunction leads to complications and accelerates aging has long remained unanswered. Prof. Hayashi began to suspect that DNA damage might be the missing link,

and she recalls feeling genuinely excited when she hit upon this hypothesis.

Could Prof. Hayashi’s research make chronic kidney disease a treatable or even curable condition?

“If we can clarify the mechanisms connecting kidney DNA damage and aging, one potential strategy would be to develop treatments that block the factors driving disease progression. Another approach would be to identify people at high risk of kidney disease or aging through diagnostic testing and use that data to guide early treatment. That said, the research is still in its early stages, and there’s a long way to go. “Even if we can’t solve everything at once, I hope to find at least one treatment that helps a certain group of patients by the end of my career,” she says.

Professor Hayashi leads a busy life. “Research, more often than not, doesn’t go as planned,” she admits. When asked what keeps her motivated, she doesn’t hesitate to answer.

“I’ve always loved being hands-on with research. It’s exciting when a hypothesis you’ve developed turns out to be correct. But even when the results are unexpected, asking, ‘Why?’ and digging deeper is just as rewarding. “

These days, she oversees the entire team and lab, so she doesn’t have as many opportunities to do hands-on research as she would like.

Even so, she says working with talented early-career researchers is incredibly stimulating, and it is a joy to watch them grow. “What makes me happiest is seeing them develop a passion for research, become outstanding physician-scientists, and go on to thrive in their careers.”

One of the values Prof. Hayashi holds dear is maintaining a sense of curiosity for learning.

“The medical sciences and health care are constantly evolving, so it’s essential to maintain a lifelong commitment to learning. At Keio, we have a philosophy called hangaku-hankyo—learning while teaching, teaching while learning—which reflects the idea that teachers and students alike are always learning. I try to always hold on to that mindset.”

She also reflected on who might be naturally suited to work as a researcher or physician.

“Researchers are driven by curiosity—a strong desire to explore and make sense of the unknown. Since research doesn’t always produce results, perseverance and a healthy dose of optimism are essential. Doctors, on the other hand, work directly with patients—so it’s important that you genuinely like engaging with people. On top of that, you need to be ready to take on serious responsibility and carry it with conviction. “

Professor Hayashi also shared her perspective on choosing a career, drawing from her own experience.

“Whatever career you choose, work will take up a huge part of your life, so it’s important to find something you’re truly interested in. It’s also crucial to see how long you can hold on to that initial spark—the feeling of ‘this is what I want to be doing.’ Ideally, you’ll find work that you care deeply about and can pursue with lasting passion. And while work is certainly important, I believe that unless you’re healthy and happy, you won’t be able to do your best work.”

Prof. Hayashi has spent her career at the Keio University School of Medicine, where she studied, trained, and continues her work in medical care and research. She reflected on what she values most about learning in this unique environment.

“One major appeal is the chance to be surrounded by professors who are leaders in their fields.

Another is that many of my classmates were highly motivated and had a clear sense of purpose. Being surrounded by peers who challenged and supported one another was an incredible environment to grow in,” she says.

Reflecting on life after graduation, Prof. Hayashi noted, “Many people assume that medical students go on to become physicians in hospitals or clinics. But that’s not the only option. Some go into research, others launch startups based on their findings, and others pursue careers outside of medicine. There are many possible paths. Many also go on to become physician-scientists, of course. “Keio is a university with a supportive environment that helps students achieve whatever they set their sights on,” she emphasizes.

For example, Keio offers support for entrepreneurship through its Startup Community Program, venture capital partnerships, and the Center for Research and Incubation (CRIK) at Shinanomachi Campus. Study abroad opportunities are also extensive.

“Keio fosters the kind of people who can lead the future of medicine and healthcare from every angle. That, to me, is what makes the School of Medicine truly exceptional. “

Kaori Hayashi
Professor, Department of Internal Medicine (Nephrology, Endocrinology, and Metabolism)

Keio University School of Medicine
Kaori Hayashi graduated from the Keio University School of Medicine in 2004. She completed her postgraduate residency at the National Center for Global Health and Medicine and, in 2006, joined the Department of Internal Medicine at the Keio University School of Medicine. From 2007 to 2011, she earned her Ph.D. in Medicine at the Keio University Graduate School of Medicine. She served as Assistant Professor at the Keio University Hospital’s Center for Preventive Medicine, and later in the Department of Internal Medicine (Nephrology, Endocrinology, and Metabolism). After holding a position as Senior Assistant Professor, she was appointed to her current role as professor in 2023.

*All affiliations and titles listed are those at the time of the interview.