A 38-year-old with no family history of cancer, no major risk factors, and a healthy lifestyle gets a colorectal cancer diagnosis — a scenario doctors say they’re seeing more and more often, and one that doesn’t fit the old assumption that this is a disease of aging. Colorectal cancer is now the leading cause of cancer death in men under 50 in the U.S., and researchers have spent years trying to figure out why. A study published in Nature this year offers one of the most concrete leads yet: a toxin produced by certain strains of E. coli that may be leaving a mutational fingerprint in the colon decades before cancer develops.
Key Takeaways
- Colorectal, breast, pancreatic, and gastric cancers are all rising among adults under 50, with overall early-onset cancer incidence increasing roughly 1-2% per year since the mid-1990s.
- A 2025 Nature study analyzing nearly 1,000 bowel cancer genomes across 11 countries found a mutation pattern linked to colibactin — a toxin made by certain gut bacteria — was 3.3 times more common in patients diagnosed before age 40 than in those diagnosed in their 70s.
- This is an association, not proof of cause — the study’s own researchers say it isn’t yet known whether colibactin actually causes colorectal cancer, and stress more research is needed.
- Researchers believe multiple factors are likely involved together, including diet, obesity, antibiotic use, and other still-unidentified environmental exposures — not colibactin alone.
- Because of this trend, screening guidelines have already changed: the American Cancer Society now recommends colorectal cancer screening starting at age 45 instead of 50 for people at average risk.
What the new research found
An international team analyzed 981 bowel cancer genomes from patients across 11 countries, looking for a specific mutational signature linked to colibactin, a toxin produced by certain strains of E. coli that live in the gut. That mutation pattern showed up 3.3 times more often in people diagnosed with bowel cancer before age 40 than in those diagnosed in their 70s or later. Because this kind of mutation signature acts like a genetic timestamp, and earlier research has shown colibactin-linked mutations tend to occur within the first decade of life, the pattern suggests some early-onset patients may have picked up a cancer-driving mutation in childhood — decades before any cancer actually develops.
Why this matters, but isn’t the whole story
It’s worth being direct about what this study does and doesn’t show. It found an association between a mutation pattern and early-onset cases — it did not prove that colibactin exposure causes colorectal cancer. As one of the researchers involved, Dr. David Scott, put it, the findings are “not conclusive, and more research will be needed.” The team’s next steps include longer-term studies tracking colibactin exposure in children directly, rather than reconstructing it after the fact from tumor genomes.
Researchers are also clear that colibactin is very unlikely to be acting alone. A parallel body of research points to a mix of contributing factors behind the broader rise in early-onset cancers: diets higher in processed foods and lower in fiber, rising obesity rates, antibiotic overuse disrupting the gut microbiome, and hormonal factors in some cancers. A July 2024 study in The Lancet Public Health found incidence rates had increased in 17 of 34 cancer types studied among younger generations, with breast, pancreatic, gastric, and colorectal cancers among those climbing fastest — a pattern too broad to pin on any single cause.
The bigger trend behind the headline
This isn’t a one-study story — it’s part of a broader, well-documented shift. According to Yale Medicine, overall cancer incidence in adults under 50 rose by roughly 1-2% per year between 1995 and 2020, and colorectal cancer has become the leading cause of cancer death in men under 50 specifically. Researchers studying the trend, including Dr. Veda Giri, have been candid that the field doesn’t have a full answer yet — some patients developing early-onset cancers have no identifiable risk factors at all, which points to contributing causes that haven’t been identified.
What’s actually changed because of this
The rise in early-onset colorectal cancer has already led to a real, practical shift: the American Cancer Society now recommends people at average risk start colorectal cancer screening at 45 instead of the previous standard of 50. If you’re under 45, the most useful things you can do are know your family history, pay attention to persistent changes in bowel habits or unexplained symptoms rather than dismissing them because of your age, and talk to a doctor about your personal risk factors — especially since, as researchers note, some people diagnosed young have none of the traditional risk factors at all.
Sources: Cancer Research UK, “E. coli toxin could be linked to rising rates of bowel cancer in younger adults”; Yale Medicine, “What to Know About Rising Rates of ‘Early-Onset’ Cancer”; American Cancer Society, colorectal cancer screening guidelines.
This article is for informational purposes and isn’t a substitute for medical advice. Talk to your doctor about your personal cancer risk factors and appropriate screening schedule.