Obesity and Cancer Risk in Young Adults in Korea

BACKGROUND: Obesity and early-onset cancer are both increasing among young adults, yet evidence remains limited. OBJECTIVE: To assess associations between obesity and early-onset cancer risk in young adults. DESIGN: Nationwide population-based cohort study. SETTING: Korean National Health Insurance Service database, 2009 to 2021. PATIENTS: 6 313 338 adults aged 20 to 39 years without prior cancer, from a national health screening program. MEASUREMENTS: Body mass index (BMI) was categorized into 5 groups (underweight to class II obesity) according to Asian-specific criteria. The outcome was incident cancer. RESULTS: During a mean follow-up of 11.2 years, 151 893 cancer cases occurred. Ten-year cumulative incidence ranged from 1.84% (95% CI, 1.83% to 1.86%) in the normal-weight group to 2.24% (CI, 2.22% to 2.27%) in class I obesity and 2.62% (CI, 2.56% to 2.68%) in class II obesity, with absolute risk differences of 0.40% (CI, 0.39% to 0.41%) and 0.78% (CI, 0.73% to 0.82%). The relative association was more pronounced in men, and absolute risk differences were greater in women. In combined BMI-waist circumference analyses, people with class I obesity based on BMI and with abdominal obesity had higher 10-year cumulative incidence (2.45% [CI, 2.41% to 2.50%]) than those without abdominal obesity (2.13% [CI, 2.10% to 2.16%]), exceeding that of those with class II obesity without abdominal obesity (2.20% [CI, 2.03% to 2.36%]). The highest cumulative incidence was observed for thyroid, endometrial, and ovarian cancer. Obesity was not associated with higher risk for breast cancer diagnosed by age 50 years. LIMITATIONS: Observational design; nonrepresentative sample limited to screening participants; and unmeasured confounding, including smoking burden. CONCLUSION: Obesity in young Korean adults, measured by BMI and waist circumference, is associated with higher early-onset cancer risk persisting into midlife, with patterns varying by cancer type. Early weight management may be important for prevention. PRIMARY FUNDING SOURCE: None.