A body shape index and sarcopenia risk in middle-aged and older adults: evidence from the China Health and Retirement Longitudinal Study.

BACKGROUND: The A body shape index (ABSI) has been associated with various diseases, and existing research suggests a link with sarcopenia. However, systematic evaluations focusing on Asian populations-particularly middle-aged and older adults in China-remain insufficient, and the specific association mechanisms and predictive value have yet to be fully elucidated. This study aimed to examine this relationship utilizing data from the China Health and Retirement Longitudinal Study (CHARLS) database. METHODS: A total of 1873 participants aged ≥ 45 years with complete data from the 2015 CHARLS were included. Eligibility criteria required the availability of: (1) parameters necessary for ABSI calculation, namely, waist circumference (WC), height, and weight; (2) sarcopenia assessment components (muscle mass, grip strength (GS), and physical performance); and (3) all relevant covariates. Sarcopenia was diagnosed according to AWGS2019 guidelines, defined as low muscle mass (appendicular skeletal muscle mass (ASM)/height2 < 7.0 kg/m2 for male, < 5.7 kg/m2 for female) in combination with either low GS (< 28 kg for male, < 18 kg for female) or impaired physical performance (gait speed < 1 m/s or chair stand time > 12 s). Three progressively adjusted weighted logistic regression models were constructed: Model 1 (unadjusted), Model 2 (adjusted for demographic variables, including age, sex, and marital status), and Model 3 (fully adjusted for demographic, lifestyle, and clinical variables, such as physical activity, chronic diseases, and depression). Predictive performance was assessed using receiver operating characteristic (ROC) curve analysis, and a K-nearest neighbors (KNN) algorithm was applied to determine variable importance. RESULTS: The cross-sectional analysis demonstrated significant differences in ABSI between individuals with sarcopenia (n = 329) and those without (n = 1544), with mean ABSI values of 4.59 and 3.71, respectively. The prevalence of sarcopenia was approximately 17.57% in the study population. All three models consistently indicated a significant association between ABSI and sarcopenia: Model 1 (odds ratio = 5.85, 95% confidence interval: 4.73-7.33), Model 2 (odds ratio = 5.34, 95% confidence interval: 4.23-6.83), and Model 3 (odds ratio = 5.22, 95% confidence interval: 4.10-6.74). The ROC analysis revealed that the fully adjusted model yielded an area under the curve of 0.871, compared to 0.760 for the ABSI-only model. In the KNN model, ABSI ranked highest in feature importance. CONCLUSIONS: ABSI is strongly associated with sarcopenia prevalence and functions as an independent indicator, suggesting its potential applicability in sarcopenia screening strategies.