A 'smoking gun' in the gut: Unpacking the surge in early-onset bowel cancer

“Compared to people born in the 1960s, people born in the early 2000s have a six-to-seven-fold increased likelihood of getting colorectal cancer (bowel cancer), which is huge.” – Associate Professor Daniel Buchanan.

Once thought of as an "old person’s disease," bowel cancer is now hitting younger Australians at alarming rates. Those aged 25–49 are warned to pay attention to their bodies, as early-onset bowel cancer becomes one of the deadliest cancers for this age group. And Australia is at the forefront of this troubling trend.

Associate Professor Daniel Buchanan, who leads the Colorectal Oncogenomics Lab at the University of Melbourne, is determined to find out why.

“My work focuses on understanding the causes of early-onset bowel cancer,” he says. “In the past, we’ve focused on genetic causes, that's really important for being able to identify people that are at risk, so we can give them increased surveillance and clinical management.”

But genetics account for only 20 per cent of early-onset cases, leaving 80 per cent of diagnoses with no clear answers. A/Prof. Buchanan’s team is now turning to the gut microbiome—home to trillions of bacteria—for answers.

“We know that since about the 1950s and 1960s, we've become more sedentary in our work and home life. And there’s been big changes in our diet through increasing sugar and processed meat, so that pointed towards something being different in our gut microbiome,” explains A/Prof. Buchanan.

His team’s research led to a startling discovery: a species of gut bacteria that produces a toxin called colibactin. This toxin can damage DNA in the bowel, potentially triggering cancer.

“We’ve found a smoking gun,” says A/Prof. Buchanan. “We now have the tools to be able to determine which early-onset bowel cancers were caused by colibactin-related DNA damage.”

The exciting part? Now this bacterium can be identified, it can be eradicated.

“The challenge is understanding when the exposure happens,” he adds. “Is it in early childhood, adolescence, or closer to when the cancer develops? That’s what we need to figure out to design strategies to eradicate it and prevent ongoing infection.”

A global phenomenon, but Australia leads the pack.

According to the Victorian Cancer Registry , early-onset bowel cancer incidence has surged by an alarming 65 per cent since 2005. It is now the most common cancer in young men and the third most common in young women, trailing only breast and thyroid cancers.

“Australia leads the world with the greatest rise in people developing early-onset bowel cancer,” says A/Prof. Buchanan.

This trend contrasts sharply with older Australians, who have seen a steady decline in bowel cancer rates thanks to screening programs.

But younger adults fall outside the recommended age range for routine screening, leaving many cases undetected until the disease has advanced. Symptoms such as blood in the stool or abdominal pain are often ignored or dismissed because bowel cancer is still seen as an ‘older person’s disease’, compounding the problem.

“It’s critical for both patients and GPs to take persistent symptoms seriously,” warns A/Prof. Buchanan. “Delaying investigation can have a real impact on early detection versus later-stage disease and your chance of surviving".

Bowel cancer is now the leading cause of cancer-related death in Victorians aged 25-49.

Not an older person’s disease anymore

For James, a clinical nurse specialist, being diagnosed with bowel cancer at 31 came as a devastating blow.

“We’re told it’s an old person’s disease, but clearly, it isn’t,” he says. “It felt like a betrayal—I thought I was doing everything right to live a healthy life.”

James underwent surgery to remove 25 centimetres of his bowel and 60 lymph nodes, followed by six gruelling months of chemotherapy.

“Some days, I felt so sick from the chemo I felt like I was dying in the process of trying to live,” he recalls.

While a cancer diagnosis at any age is distressing, for those diagnosed at a younger age, the impact can be especially severe.

“There's the physical part of it, which is lifelong, it’s ongoing. Then there's the financial aspect. And there's the psychological aspect to it, and that's probably the biggest thing that we've got from working with consumers involved with our research is just the constant anxiety around it,” said A/Prof. Buchanan.

As part of one of these consumer advisory groups, James is sharing his perspective as both a health professional and as an early-onset bowel cancer survivor. After finishing his treatment, James lived for a long time with the fear of it returning. It was only around the third year of being cancer-free, that he noticed a change in himself.

“All of a sudden, I could allow myself to hope that I could survive this and that I might be lucky enough to live a full and active life now,” he said.

Prevention and early detection has to start early.

A/Prof. Buchanan’s work identifying the role of colibactin-producing bacteria in the development of bowel cancer holds many answers, but until more is discovered about the time and level of exposure, understanding what the risk factors are is critical for prevention and early detection.

“Understanding the role of a healthy diet, healthy lifestyle is important. Even from early childhood it's this kind of accumulating evidence that these risk factors early on in life are what might be contributing to an earlier diagnosis,” says A/Prof Buchanan. His work points towards the modern Western diet—a cocktail of processed and sugary foods—in creating a dangerous environment in the gut.

As a researcher working in this space, what he has learned has influenced his own life too; “put it this way, I don't buy my kids fast food anymore”.

While his research continues, for now, A/Prof Buchanan says; “the main thing we can do in the short term is raise awareness of the signs and symptoms of early-onset bowel cancer and not ignore them.”

Adopting a healthy diet and lifestyle from childhood is key. This isn’t just about preventing cancer later in life – these factors are contributing to cancer at younger ages.

Don’t ignore the symptoms – blood in the stool, pain, changes in bowel habits. The earlier we detect bowel cancer, the better the outcomes.