Association between weight-adjusted waist and incidence of low back pain: evidence from the China Health and Retirement Longitudinal Study

The weight-adjusted waist index (WWI), a novel adiposity metric, provides precise assessment of abdominal fat accumulation. This longitudinal study aims to examine the association between WWI and incident low back pain (LBP) risk in middle-aged and older Chinese adults. Data from the China Health and Retirement Longitudinal Study (CHARLS) (2011–2020) were utilized. Cox proportional hazards models were employed to assess WWI’s association with incident LBP, while restricted cubic splines (RCS) modeled their potential nonlinear relationship. Kaplan-Meier curves were generated to estimate LBP-free probabilities across WWI strata. Sensitivity analyses included multiple imputation and subgroup analyses. A total of 3,912 individuals were included in the study. After full adjustment for covariates, each 1-unit increase in WWI within the highest WWI quintile was associated with an 18.6% elevated risk of incident LBP (HR = 1.186, 95% CI: 1.057–1.331). Further analysis revealed a J-shaped association between WWI and incident LBP risk, whereby each 1-unit increment above 10.56 corresponded to a significant 9.6% risk increase (HR = 1.096, 95% CI: 1.026–1.171). A lower probability of remaining LBP-free was observed in the highest WWI quintile group (Q5) relative to other groups (p = 0.00018). Additionally, no statistically significant interaction effects were observed across subgroups (all interaction P > 0.05). In the Q5, WWI demonstrated an independent association with incident LBP. A critical risk threshold of WWI = 10.56 was established, suggesting the potential of WWI as a screening tool.