The Relationship between Surrogate Markers of Insulin Resistance and Occurrence of Colorectal Adenoma in Individuals under 50 Years Old: A Single-Center Retrospective Cross-Sectional Study

(1) Background: Young-onset colorectal adenomas (YOAs) are precursors to early-onset colorectal cancer, a growing concern among individuals under 50 years old. This study investigated the association between surrogate markers of insulin resistance (IR) and YOAs occurrence. (2) Methods: A retrospective cross-sectional analysis was conducted on 4467 individuals aged 20 to 49 years who underwent their first screening colonoscopy at Dong-A University Hospital from 2018 to 2022. IR was assessed using the triglyceride-glucose (TyG) index, triglyceride-to-high-density lipoprotein cholesterol ratio (TG/HDL-C), and metabolic score for insulin resistance (METS-IR). (3) Results: Individuals with YOAs exhibited significantly higher median TyG index (8.51 ± 0.71 vs. 8.32 ± 0.61, <i>p</i> < 0.001), TG/HDL-C ratio (2.78 ± 3.05 vs. 2.12 ± 1.85, <i>p</i> < 0.001), and METS-IR (35.72 ± 8.37 vs. 33.44 ± 9.11, <i>p</i> < 0.001) values than controls. The adjusted odds ratios for YOAs were 1.064 (95% CI: 1.22-2.23, <i>p</i> = 0.021) for the TyG index, 1.067 (95% CI: 1.031-1.105, <i>p</i> < 0.001) for the TG/HDL-C ratio, and 1.011 (95% CI: 1.002-1.021, <i>p</i> = 0.023) for METS-IR values, indicating a strong association between higher IR marker values and the presence of YOAs. (4) Conclusions: Elevated IR marker values are strongly associated with the occurrence of YOAs in individuals under 50 years old.