Physical Inactivity and Obesity in Relation to Tumor Site and Age Onset for CRC on the Rising Incidence of Early-onset Colon Cancer and Early-onset Rectal Cancer

Overall incidence of colorectal cancer (CRC) has declined in recent decades; however, the incidence of early-onset CRC, particularly early-onset rectal cancer, has been increasing steadily since the mid-1990s among younger adults. The factors underlying observed increases in early-onset rectal cancer remain unclear. This study assessed the association of physical activity and obesity with early-onset rectal cancer relative to early-onset colon and later-onset rectal cancers. This study aims to build on existing research and provide a greater understanding as to the distinct epidemiology of early-onset rectal cancer.Methods: With a study population derived from the population-based Advanced Colorectal Cancer of Serrated Subtype (ACCESS) Study and the Seattle site of the Colon Cancer Family Registry (SCCFR), we utilized multivariate regression models to assess the association of physical activity levels and obesity with colorectal tumor site and age at onset via comparisons across three case groups: early-onset rectal cancer vs. early-onset colon cancer and early-onset rectal cancer vs. later-onset rectal cancer. Results: In this study of 1199 participants, including 362 patients with early-onset rectal cancer, 570 patients with early-onset colon cancer, no significant associations were found between physical activity levels and tumor site among those with early-onset CRC. Individuals with early-onset CRC and a BMI over 30kg/m2 were significantly less likely to have tumors located in the rectum (OR = 0.6, p-value = 0.01). When comparing participants with early-onset rectal cancer to those with later-onset rectal cancer (N = 267), a suggestively inverse association was detected for physical activity levels; however, this pattern was not statistically significant. No other significant associations were observed between the three CRC cancer groups. Conclusions: Findings from this study indicated that physical activity levels and obesity were unlikely to be contributing factors for the emerging trend of early-onset rectal cancer incidence, suggesting a need for future research on other potential risk factors associated with CRC tumor site and age at onset, specifically with early-onset rectal cancer.