A Data-Driven Approach to Cardiometabolic Risk Stratification: Development of the Adiposity-Fitness Imbalance Index Using a National Chilean Dataset

The increasing prevalence of adolescent obesity and declining physical fitness highlights the need for integrative, non-invasive tools to identify central-adiposity–related cardiometabolic risk early. This study aimed to develop and analytically evaluate the adiposity–fitness imbalance (AFI) index and to examine its association with an anthropometric proxy of cardiometabolic risk (waist-to-height ratio > 0.50) in a nationally representative sample of Chilean adolescents. This cross-sectional study analyzed data from 7852 students from the Chilean National Physical Fitness Assessment System (SIMCE-EF). The AFI index was calculated as the difference between standardized adiposity and fitness components. Logistic and robust linear regression models were used. Higher standing long jump (OR = 0.69, 95% CI 0.65–0.74), push-ups (OR = 0.76, 95% CI 0.71–0.80), sit-ups (OR = 0.81, 95% CI 0.77–0.85), and VO2max (OR = 0.82, 95% CI 0.75–0.89) were associated with lower odds of elevated WHtR (all p < 0.001), and a small protective association was also observed for flexibility (OR = 0.93, 95% CI 0.88–0.99, p = 0.016). Each one-standard-deviation increase in the AFI index was associated with a substantially higher odds of elevated WHtR (OR = 26.74, 95% CI 22.57–31.68, p < 0.001). In a sensitivity analysis that removed WHtR from the adiposity pillar, to avoid component–outcome overlap, the AFI index remained strongly associated with the outcome (OR per 1 SD = 14.60, 95% CI 12.77–16.70), with internal-validation discrimination of AUC = 0.93. The AFI index may represent a practical and scalable tool for early screening of central-adiposity–related risk in adolescents.