Early-Onset Colorectal Cancer in Australia: Environmental, Microbial, and Policy Implications.

BACKGROUND: Early-onset colorectal cancer (EOCRC; age <50 years) is rising in Australia despite improving outcomes in older adults. EOCRC shows a strong birth cohort effect, disproportionate growth in left-sided and rectal tumours, and more frequent stage III-IV presentation. Most cases occur without a family history, suggesting that environmental and biological pressures are accelerating carcinogenesis in otherwise average-risk hosts.

SUMMARY: Traditional risk factors such as obesity, metabolic syndrome, sedentary behaviour, alcohol, and smoking likely contribute through insulin resistance, chronic inflammation, and insulin-like growth factor 1-mediated signalling, but they do not fully explain the recent acceleration or distal predominance of EOCRC. Hereditary syndromes account for only a minority of cases, and tumour driver mutation patterns broadly resemble those of later-onset colorectal cancer, supporting earlier triggering rather than novel genetics. Emerging evidence implicates gut dysbiosis and exposures that disrupt mucosal defences or cause direct DNA damage. Colibactin-producing Escherichia coli may induce distinctive mutational signatures enriched in early and distal tumours. Microplastics and plasticisers may impair barrier function and promote low-grade inflammation, while per- and polyfluoroalkyl substances and related endocrine-disrupting chemicals are linked to metabolic and immune perturbation and altered bile acid biology. Antibiotic exposure, particularly early in life, may reduce microbial diversity and favour pathobionts. Inflammatory phenotypes, including inflammatory bowel disease, provide an additional model of inflammation-driven carcinogenesis relevant to EOCRC.

KEY MESSAGES: EOCRC in Australia is a growing clinical and public health challenge that cannot be explained by inherited predisposition alone. A unifying exposome model may help integrate dietary, microbial, inflammatory, and environmental drivers of risk. Clinicians should promote earlier participation in the National Bowel Cancer Screening Program, including the 45-49 opt-in pathway, expedite investigation of rectal bleeding, altered bowel habit, and iron deficiency anaemia in younger adults, and embed lifestyle counselling into routine and survivorship care. Research priorities include prospective cohorts with early-life exposure data, integrated exposomics, microbiome profiling, and mutational signature analysis to clarify modifiable drivers and guide prevention.