Background: Incidence of early-onset colorectal cancer (EOCRC; e.g., diagnosed before age 50) in the US has increased substantially since the 1990s but the underlying reasons remain unclear. Methods: We examined the ecologic associations between dietary factors and EOCRC incidence in adults aged 25-49 during 1977-2016 in the US, using negative binomial regression models, accounting for age, period, and race. The models also incorporated an age-mean centering (AMC) approach to address potential confounding by age. We stratified the analysis by sex and computed incidence rate ratio (IRR) for each study factor. Study factor data (for 18 variables) came from repeated national surveys; EOCRC incidence data came from the Surveillance Epidemiology, and End Results Program. Results: Results suggest that confounding by age on the association with EOCRC likely existed for certain study factors (e.g., calcium intake), and that AMC can alleviate the confounding. EOCRC incidence was positively associated with smoking (IRR [95% CI]: 1.17 [1.10-1.24] for men; 1.15 [1.09-1.21] for women) and alcohol consumption (IRR [95% CI]: 1.08 [1.04-1.12] for men; 1.08 [1.04-1.11] for women). No strong associations were found for most other study factors (e.g., fiber and calcium). Conclusions: Alcohol consumption was positively associated with EOCRC and has increased among young adults since the 1980s, which may have contributed to the EOCRC incidence increases since the 1990s. The AMC approach may help alleviate age confounding in similar ecologic analyses. Impact: Increases in alcohol consumption may have contributed to the recent increases in colorectal cancer incidence among young adults.

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