Obesity increases colorectal cancer risk while normal weight is associated with lower riskHigher body mass index linked to earlier colorectal cancer risk

The incidence of obesity and early-onset colorectal cancer (CRC) has rapidly increased in recent decades. We estimated CRC risk differences by body mass index (BMI) and derived corresponding BMI-specific CRC screening initiation ages across multiple countries, to help refine the timing of CRC prevention. We based our analyses on population-level CRC incidence data from the GLOBOCAN database (2022) and risk estimates from a meta-analysis of six cohort studies. The analysis included adults aged 30 to 60 years from six countries: United States, Canada, Germany, France, United Kingdom, and Italy. Individuals with overweight were used as the reference category, reflecting the average BMI of adults eligible for screening in these countries. Risk-adapted screening initiation ages were defined as the ages at which individuals with obesity reached the same 5-year cumulative CRC risk as those with overweight at ages 45 and 50 (aCR and aCR). Compared with individuals with overweight, those with normal weight had a lower risk of early-onset CRC (OR, 0.76; 95% CI, 0.68-0.84), while those with obesity had higher risk (OR, 1.42; 95% CI, 1.04-1.95). Across countries, individuals with obesity reached the aCR at age 41 to 42 years and the aCR at age 46 to 47 years. These findings indicate that individuals with obesity reached CRC screening thresholds up to 3-4 years earlier than those at average risk. BMI may offer a practical and scalable marker to inform personalized CRC screening initiation, with the potential to enhance early detection if integrated into existing screening programs.