Background: The incidence of early-onset colorectal cancer (CRC) has risen significantly over the past several decades, yet the etiologic drivers of this trend remain only partially understood. Established risk factors—including hereditary syndromes, germline APC mutations, dietary and alcohol exposures, low socioeconomic status, male sex, sedentary lifestyle, and emerging concerns such as microplastic ingestion—account for only a subset of cases. Despite extensive research into these known contributors, the accelerating rise in CRC among younger adults suggests additional, under-recognized influences.
Objective: This project aims to synthesize current evidence on well-characterized risk factors for early-onset CRC while also exploring potential emerging or hypothesized contributors that remain poorly defined in the literature.
Methods: A narrative review framework was used to contrast established CRC risk factors with emerging hypotheses. Known contributors were identified from epidemiologic and genomics-based studies. Potential but understudied factors—such as environmental radiation exposure, industrial byproducts, recycled water contamination, oncogenic viral influences (e.g., HPV), and speculative concerns including bioterrorism-related exposures—were categorized based on plausibility and current evidence gaps.
Results: While well-established risk factors explain some of the rise in early-onset CRC, substantial gaps persist. Environmental exposures, radiation sources, evolving viral epidemiology, and novel contaminants may represent important but insufficiently investigated contributors. Current data are largely indirect or theoretical, underscoring a need for systematic research.
Conclusion: Early-onset CRC likely results from a multifactorial interplay of genetic susceptibility, lifestyle factors, and environmental or emerging exposures. Highlighting both what is known and what remains uncertain may guide future research priorities, improve early detection strategies, and encourage broader investigation into unconventional or newly emerging risk factors.