Abstract Body (Do not enter title and authors here): Background
BMI is widely used to assess adiposity and cardiometabolic risk, yet its utility varies by race/ethnicity. Alternative indices such as Body Roundness Index (BRI) and A Body Shape Index (ABSI) may offer improved risk stratification, but their utility remains unclear in the ethnically diverse U.S. population.

Research Question
We evaluated associations of BMI, BRI, and ABSI with prevalent ASCVD and all-cause mortality, and determine whether these associations vary by race/ethnicity.

Methods
Adults from the National Health and Nutrition Examination Survey cycles 2011-2018 (N = 21,294; weighted 226.4 million) were analyzed. BMI was categorized per WHO cutoffs (obesity ≥30 kg/m², underweight <18.5 kg/m², ≥27.5 kg/m² for obese Asian adults). BRI and ABSI were divided into quartiles, and the interquartile range was set as reference. Survey weighted regression models were employed to estimate the odds of prevalent ASCVD and the hazards of mortality. Subgroup analyses tested for racial/ethnic differences.

Results
Obesity as defined by BMI, BRI, and ABSI were all associated with higher odds of ASCVD (aOR 1.58 [1.36-1.83]; 1.69 [1.43-1.99]; 1.66 [1.39-1.98] respectively). Underweight BMI was associated with increased mortality risk (aHR 3.18 [2.04-4.95]), while obesity showed no significant association. However, a high ABSI 1.41 [1.08-1.82] and low BRI 1.44 [1.04-1.98] were associated with mortality in the overall population. Subgroup analyses revealed significant ethnic differences that persisted after comprehensive multivariable adjustment (Figure). Among White adults, underweight BMI and high ABSI predicted mortality. In Asian adults, high BRI conferred greatest risk, while underweight BMI was inversely associated with mortality. In Black adults, higher BMI was associated with lower mortality, and a low BRI was associated with higher risk. For Hispanic adults, high ABSI was the only significant mortality predictor.

Conclusion
Our findings suggest that BMI, BRI, and ABSI have differential predictive value for mortality across racial/ethnic groups. While BMI alone may underestimate risk, BRI and ABSI offer nuanced risk stratification, particularly in non-White populations. These results underscore the need for racial/ethnic conscious anthropometric risk assessments in clinical and public health settings.