Bariatric Surgery and Risk of Obesity-Related Cancer by Age and Sex

Importance: Obesity is associated with elevated risks for multiple cancers. How bariatric surgery influences these risks across age and sex is insufficiently understood. Objective: To evaluate the association between bariatric surgery and obesity-related cancer (breast, endometrial, ovarian, esophageal adenocarcinoma, liver, pancreatic, colon, rectal, kidney, and multiple myeloma) and whether this association varies by age and sex. Design, Setting, and Participants: This population-based cohort study within the Nordic Obesity Surgery Cohort (NordOSCo) included individuals aged 18 to 59 years with an obesity record in Denmark, Finland, or Sweden from 1992 to 2023. Prospectively collected data on exposure, outcomes, and confounders were obtained from well-maintained national registries. Follow-up continued until cancer diagnosis, death, emigration, or December 31, 2023. These data were analyzed from December 2025 to April 2026. Exposure: Bariatric surgery vs no surgery among individuals with obesity. Main Outcomes and Measures: The primary outcome was incidence of obesity-related cancers. Risks were compared using crude and multivariable-adjusted incidence rate ratios (aIRR) with 95% CIs derived from Poisson regression adjusted for age, sex, calendar year, country, and obesity- and lifestyle-related comorbidities. Effect modification by age group (18-39 vs 40-59 years) and sex (female vs male) was evaluated using interaction terms. Results: Among 784 105 individuals with obesity (569 266 women [73%] and 214 839 men [27%]), 99 614 underwent bariatric surgery. Surgery was associated with a lower risk of obesity-related cancer overall (aIRR, 0.81; 95% CI, 0.77-0.85). The magnitude of risk reduction did not differ by age group (individuals aged 18 to 39 years: aIRR, 0.80; 95% CI, 0.72-0.88 and individuals aged 40 to 59 years: aIRR, 0.81; 95% CI, 0.78-0.86) (P for interaction = .88). Associations for non-sex-specific cancers were similar in men and women (P for interaction = .50). For individual cancers, the strongest inverse associations were observed for liver cancer (aIRR, 0.49; 95% CI, 0.34-0.70), endometrial cancer (aIRR, 0.51; 95% CI, 0.45-0.59), and ovarian cancer (aIRR, 0.73; 95% CI, 0.60-0.87). In contrast, pancreatic cancer risk was increased after surgery, particularly among younger adults (aIRR, 2.21; 95% CI, 1.44-3.39; P = .01). Conclusions and Relevance: In this study, bariatric surgery was associated with a lower long-term risk of obesity-related cancers, with similar associations across age groups and sexes. These findings suggest that the long-term oncologic benefits of bariatric surgery apply broadly across age groups and sexes, supporting equitable access to bariatric surgery for patients with obesity.