Doubled risk of colorectal polyp occurrence in adults under 50 with a high-level mildly elevated body mass index trajectory: a retrospective cohort study based on latent class mixed modeling

In recent years, the incidence of early-onset colorectal cancer (diagnosed before the age of 50 years) has been increasing. As a modifiable factor, obesity may be associated with the occurrence of colorectal polyps in adults under 50. Since body mass index (BMI) measured at a single time point is insufficient to reflect an individual’s long-term body weight status and its dynamic changes, this study aims to investigate the relationship between longitudinal BMI trajectories and the risk of colorectal polyps in this population, and to further analyze the relationship between different BMI trajectories and the occurrence of adenomatous polyps.

A retrospective cohort study was conducted on the adults younger than 50 years who met the eligibility criteria and underwent at least 2 colonoscopic examinations at Digestive Endoscopy Center of our hospital between January 2016 and February 2025. Based on BMI data at 3 time points (T0: 2 consecutive calendar years prior to the calendar year of the first colonoscopy; T1: the calendar year of the first colonoscopy; T2: 2 consecutive calendar years following the calendar year of the first colonoscopy), latent class mixed model (LCMM) was used to construct BMI trajectories. The optimal model was determined by comprehensively considering the Bayesian information criterion, average posterior probability, entropy, and the sample size proportion of each trajectory group. Log-rank test was employed to compare the cumulative incidence of colorectal polyps among the trajectory groups in the optimal model. Cox proportional hazards regression model was utilized to estimate hazard ratios (HRs) and 95% confidence intervals (95%CIs) for colorectal polyps in each BMI trajectory group. Firth’s penalized maximum likelihood logistic regression was applied to analyze the association between BMI trajectory grouping and adenomatous polyps.

A total of 603 participants were finally included, comprising 258 males (42.79%) and 345 females (57.21%). Based on LCMM, a 3-group linear trajectory model was ultimately selected as the optimal model: low-level stable group, middle-level stable group, and high-level mildly elevated group. The low-level stable group included 243 participants (40.30%), with mean BMI values of 21.11, 20.93, and 21.06 kg/m² at the 3 time points, respectively, indicating an overall stable BMI. The middle-level stable group included 272 participants (45.11%), with mean BMI values of 25.08, 25.24, and 25.32 kg/m² at the 3 time points, respectively, indicating an overall stable BMI. The high-level mildly elevated group included 88 participants (14.59%), with mean BMI values of 30.20, 30.51, and 30.77 kg/m² at the 3 time points, respectively, indicating an overall trend of mild elevation in BMI. Log-rank test showed statistically significant differences in the cumulative incidence of colorectal polyps among the 3 BMI trajectory groups (χ² =58.219, P<0.001). Cox proportional hazards regression analysis showed that, after adjusting for confounding factors, compared with the low-level stable group, the risk of colorectal polyps was increased in both the middle-level stable group (HR=2.90, 95%CI: 1.67 to 5.01, P<0.001) and the high-level mildly elevated group (HR=6.01, 95%CI: 3.26 to 11.10, P<0.001). In addition, compared with the middle-level stable group, the high-level mildly elevated group had an approximately 1.08-fold increased risk of colorectal polyps (HR=2.08, 95%CI: 1.35 to 3.20, P<0.001). Among 121 patients with newly developed polyps, 63 (52.07%) had adenomatous polyps. Firth’s penalized maximum likelihood logistic regression showed that, after adjusting for age, sex, smoking history, drinking history, and other factors, the high-level mildly elevated group had a significantly increased likelihood of adenomatous polyps compared with the low-level stable group (OR=3.77, 95%CI: 1.11 to 13.83, P=0.033).

Higher BMI trajectory levels are independently associated with an increased risk of colorectal polyps in adults younger than 50 years. In particular, the high-level mildly elevated trajectory may also be associated with adenomatous polyps. Dynamic monitoring of BMI changes and strengthened body weight management may contribute to risk identification and prevention of colorectal polyps and adenomatous lesions.