Why are more young people under 50 being diagnosed with colorectal cancer? That’s the question researchers across the world are racing to answer. Now, a major international study published in Nature offers a new lead: a DNA-damaging toxin called colibactin, produced by certain gut bacteria, may play a key role — and our diets could be fueling it.
Colorectal cancer is historically a disease of aging. But in recent decades, cases have been rising sharply in people under 50 — even in their 30s or younger. The Nature study, led by researchers at UC San Diego and the University of Cambridge, analyzed the DNA of 981 colorectal tumors from patients across 11 countries. They found a distinctive fingerprint of damage left by colibactin, a toxin produced by certain strains of E. coli and related bacteria.
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Crucially, the study showed that patients under 40 were 3.3 times more likely to have tumors with this colibactin-associated mutation signature than those over 70.
“This mutational footprint is like a historical record — it tells us these patients were exposed to colibactin, likely early in life,” said Professor Ludmil Alexandrov, senior author of the study and researcher at UC San Diego. “This could be the smoking gun behind the rise in early-onset colorectal cancer.”
Alexandrov believes many of these mutations were acquired during childhood. “The development of colorectal cancer in individuals with colibactin-associated tumors may begin as early as age 10,” he said in a press release, “and manifest as cancer by the time the individual is in their 40s.”
What Role Does Diet Play?
While the study didn’t examine diet directly, scientists widely agree that food plays a critical role in shaping the gut microbiome — the community of trillions of bacteria in our digestive tract. Some of these bacteria are protective. Others, like colibactin-producing E. coli, can be harmful.
According to Dr. David Scott, Director of Cancer Grand Challenges at Cancer Research UK, “It’s unclear how the exposure originates, but we suspect that a combination of factors — including diet — may intersect during a crucial phase in the development of the gut microbiome.”
A Western-style diet, high in saturated fats, processed foods, and red meat, and low in fiber, has been shown to promote inflammation and support the growth of harmful bacteria — while displacing beneficial species. Fiber, by contrast, helps fuel microbes that produce short-chain fatty acids, which protect the colon lining and reduce cancer risk.
In a 2015 study comparing diets in rural Africa and the U.S., researchers led by Dr. Stephen O’Keefe found that a high-fiber, plant-based diet drastically reduced cancer biomarkers — within just two weeks. The rural African diet shifted the microbiome in ways that suppressed inflammation and DNA damage.
Colibactin and the “Unseen Exposure” in Childhood
What makes colibactin so dangerous is that it doesn’t just irritate the gut — it directly alters DNA. This is significant because it means the carcinogenic process may begin decades before diagnosis.
“We’ve identified that a subset of early-onset colorectal cancer patients have had their cancer caused, at least in part, by past exposure to bacteria that produce colibactin,” said Professor Serena Nik-Zainal, co-lead author from the University of Cambridge.
Importantly, these colibactin mutations were more common in countries with higher rates of early-onset colorectal cancer — including the U.S. and U.K. — than in nations with more traditional diets.
The findings raise the possibility of new screening tools — including stool tests to detect high-risk bacterial strains, or even microbial risk profiling in children and adolescents. But prevention may start with something more accessible: what we feed ourselves, and our children.
“In the future, we might be able to identify children carrying colibactin-producing bacteria and take steps early to reduce their cancer risk,” Alexandrov said.
Until then, experts suggest a return to the basics: more plants, less processed meat, and nurturing a diverse, healthy gut microbiome. In an age where chronic disease often begins unseen, diet remains one of our most powerful — and modifiable — tools.
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