Baseline and Cumulative Exposure to the Hemoglobin A1c–Systolic Blood Pressure Index × Waist-to-Height Ratio and Risk of New-Onset Cardiometabolic Multimorbidity in Middle-Aged and Older Chinese Adults: A Nationwide Prospective Cohort Study

Background Cardiometabolic multimorbidity (CMM), the coexistence of at least two cardiometabolic diseases in the same individual, is a growing public health challenge in aging populations, yet simple composite indicators simultaneously capturing glycemic status, blood pressure, and central obesity remain scarce. We investigated the associations of baseline and cumulative exposure to the hemoglobin A1c–systolic blood pressure index (HSI) combined with waist-to-height ratio (WHtR) with new-onset CMM among middle-aged and older Chinese adults. Methods This prospective cohort study included 6,279 participants free of CMM at baseline from the China Health and Retirement Longitudinal Study (2011–2020). HSI×WHtR was calculated, and cumulative exposure (cuHSI×WHtR) was derived from repeated measurements weighted by time intervals. K-means clustering identified longitudinal trajectories of HSI×WHtR. Cox proportional hazards models, restricted cubic spline (RCS) analyses, subgroup analyses, and Fine-Gray competing risk models were applied. Results During a median follow-up of approximately 9 years, 410 incident CMM cases occurred. Each 1-SD increment in baseline HSI×WHtR was associated with a 57% higher risk (HR 1.57, 95% CI 1.43–1.72), and the corresponding HR for cuHSI×WHtR was 1.83 (95% CI 1.67–2.00). Three trajectory patterns were identified (low-stable, moderate, and high-increasing); the high-increasing group had an approximately 4-fold higher risk (HR 4.00, 95% CI 2.88–5.56). RCS analyses indicated nonlinear dose–response associations (P for nonlinearity < 0.001). Conclusions Higher baseline and cumulative HSI×WHtR were independently associated with increased risk of new-onset CMM, with the strongest risk observed among individuals with high-increasing trajectories. Joint monitoring of glucose–blood pressure status and central obesity may facilitate early CMM risk stratification.