Nonlinear and linear associations of body roundness index with all-cause and cardiovascular mortality in US adults with cardiovascular-kidney-metabolic syndrome

The body roundness index (BRI) is a body measurement tool that may offer advantages in tracking visceral fat compared to conventional measurements like body mass index (BMI). However, the association between BRI and mortality risk in patients with cardiovascular-kidney-metabolic (CKM) syndrome remains underexplored. This study examined the association between BRI and all-cause and cardiovascular disease (CVD) mortality among US adults with CKM syndrome. We analyzed data from 34,883 US adults with CKM syndrome from the National Health and Nutrition Examination Survey (NHANES). Mortality data were linked to the National Death Index. The study employed multivariable Cox proportional hazards models, Fine-Gray proportional subdistribution hazards model, restricted cubic splines, and threshold effect analyses to determine how BRI relates to mortality outcomes. External validation was performed using the China Health and Retirement Longitudinal Study (CHARLS) database. During a median follow-up of 110.0 months, 4,236 deaths occurred, including 1,223 from CVD. After full adjustment for demographic, lifestyle, and clinical covariates, a nonlinear U-shaped association was identified between BRI and all-cause mortality, with an inflection point at BRI = 4.251. Below this point, each unit increase in BRI was associated with a lower risk of all-cause mortality (hazard ratio (HR): 0.85, 95% confidence interval (CI) 0.76 – 0.95, P = 0.004), while above it, the risk increased (HR: 1.03, 95% CI 1.01 – 1.05, P = 0.017). In contrast, a linear positive association was found between BRI and CVD mortality (subdistribution hazard ratio (sHR): 1.04, 95% CI 1.01 – 1.08, P = 0.012). Subgroup analyses revealed stronger associations for all-cause mortality in males and individuals aged < 65 years, and for CVD mortality in younger adults and those with CKM stage 2. External validation using the CHARLS database confirmed a non-linear (L-shaped) association between BRI and all-cause mortality, reinforcing the robustness of our primary findings. BRI exhibits a U-shaped association with all-cause mortality and a linear association with CVD mortality in adults with CKM syndrome. BRI may serve as a novel, reliable, and non-invasive tool to improve risk stratification and guide clinical intervention in CKM syndrome.