Sex-Specific Association Between Body Mass Index and Mortality After Heart Transplantation: A Nationwide KOTRY Cohort Study

Background Body mass index has paradoxical associations with outcomes in advanced heart failure, but whether its relationship with survival after heart transplantation differs by sex remains uncertain. Methods We analyzed 1,059 adult heart transplant recipients enrolled in the Korean Organ Transplantation Registry from 2014 through 2023. Body mass index was assessed using Asian-specific BMI categories and 3-knot restricted cubic splines. The primary outcome was all-cause mortality. Multivariable Cox models adjusted for prespecified clinical characteristics and pretransplant support. Sex differences in the body mass index–mortality association were evaluated using restricted cubic spline modeling. Results The cohort included 304 female patients, and 217 deaths occurred during a median follow-up of 3.6 years. The body mass index–mortality association differed by sex in the pooled spline model (P for interaction=0.041). In male patients, the association was nonlinear and U-shaped (P for overall association=0.031; P for nonlinearity=0.017). Underweight was associated with higher mortality than normal weight (adjusted hazard ratio, 1.97; 95% confidence interval, 1.17–3.33). In female patients, the fitted association was inverse, without evidence of nonlinearity (P for overall association=0.030; P for nonlinearity=0.302). Obesity class I was associated with lower mortality than normal weight (adjusted hazard ratio, 0.24; 95% confidence interval, 0.08–0.73), although this estimate was based on few events. Conclusions The association between body mass index and mortality after heart transplantation may differ by sex. The apparent lower mortality among female patients with obesity class I is hypothesis-generating and requires external validation.