Association of longitudinal A Body Shape Index trajectories with metabolic syndrome in older adults: findings from the CHARLS and ELSA cohorts

Background: A Body Shape Index is a composite anthropometric measure that captures abdominal adiposity independent of body mass index. While higher values have been associated with cardiometabolic risk in cross-sectional studies, the longitudinal relationship between distinct trajectories of this index and metabolic syndrome remains unclear, particularly among older adults. This study aimed to identify longitudinal trajectory groups of A Body Shape Index and evaluate their associations with metabolic syndrome in older adults from two nationally representative cohorts. Methods: Data were drawn from the China Health and Retirement Longitudinal Study (Waves 1 to 3, 2011 to 2015; n = 811) and the English Longitudinal Study of Ageing (Waves 2 to 6, 2004 to 2012; n = 2,317), including participants aged 65 years and above. A Body Shape Index was calculated at three measurement waves in each cohort. K-means clustering was applied to standardized values to identify trajectory groups, with the optimal number of clusters determined by the Calinski-Harabasz index. Metabolic syndrome was defined according to the 2009 harmonized criteria. Multivariable logistic regression estimated odds ratios and 95% confidence intervals across three sequential models. Restricted cubic splines assessed the dose-response relationship between cumulative A Body Shape Index and metabolic syndrome. Subgroup analyses with interaction tests were performed. Results: Three trajectory groups were identified in both cohorts: Stable-Low, Stable-Medium, and Stable-High. In fully adjusted models, compared with the Stable-Low group, the Stable-High group had significantly higher odds of metabolic syndrome in both cohorts (China: odds ratio 2.25, 95% confidence interval 1.38 to 3.67; England: odds ratio 2.16, 95% confidence interval 1.60 to 2.93), with significant linear trends (both P for trend < 0.001). Restricted cubic spline analyses confirmed a linear positive dose-response relationship, with no evidence of nonlinearity. These associations were consistent across subgroups defined by age, sex, education, marital status, lifestyle factors, and residential location, with no significant interactions detected. Conclusions: Higher longitudinal trajectory groups of A Body Shape Index were independently associated with increased odds of metabolic syndrome in older Chinese and English adults. Longitudinal monitoring of this index may complement single-timepoint assessment for metabolic risk identification in aging populations.