Body Roundness Index and Body Shape Index as Predictors for All‐Cause Mortality Beyond Body Mass Index: Findings From a National Cohort Study

Objective: Body mass index (BMI) has been criticised for its inability to differentiate between fat and nonfat tissues and to reflect body fat distribution. Body Roundness Index (BRI) and A Body Shape Index (ABSI) are novel indices addressing these limitations, yet their mortality risk-stratification utility remains understudied in Asian populations. Methods: In this retrospective cohort study using a nationally representative Japanese claims database (2014-2022), we analysed 778,812 individuals who underwent healthcare checkups. Anthropometric indices were categorised into five groups based on restricted cubic spline curve-derived cutoffs. Multivariate Cox proportional hazards models, adjusted for demographic factors, lifestyle variables and comorbidities, assessed associations between these categorical variables and all-cause mortality. Results: Among participants (mean [standard deviation] age of 62.8 [9.6] years and 445,250 [57.2%] women), 14,690 deaths occurred during a median [interquartile range] follow-up of 4.53 [3.28-6.23] years. While BMI and BRI showed U-shaped relationships with all-cause mortality, ABSI demonstrated a J-shaped relationship. Significant differences in mortality risk compared with the reference category were observed in three categories for BMI, four categories for BRI and four categories for ABSI. Conclusions: BRI and ABSI identified mortality risk differences across more categories than BMI, indicating that these indices may provide additional insights beyond BMI for mortality risk assessment.